Download Chemistry Review 2 answer key

Chemistry Review 2
TEACHER ANSWER KEY
June 12, 2011
'see explanation below'
1. Base your answer on the information below.
The catalytic converter in an automobile changes harmful gases produced during fuel combustion to less harmful exhaust gases. In the catalytic
converter, nitrogen dioxide reacts with carbon monoxide to produce nitrogen and carbon dioxide. In addition, some carbon monoxide reacts with
oxygen, producing carbon dioxide in the converter. These reactions are represented by the balanced equations below.
Reaction 1:
2NO2(g) + 4CO(g) → N2(g) + 4CO2(g) + 1198.4 kJ
Reaction 2:
2CO(g) + O2(g) → 2CO2(g) + 566.0 kJ
Determine the oxidation number of carbon in each carbon compound in reaction 2. Your response must include both the sign and value of each
oxidation number. [1]
In any compound, the sum of the oxidation numbers must total zero. The oxidation number of O is –2 in both CO and CO2. It follows that the
oxidation number of C in CO is +2 and the oxidation number of C in CO2 is +4. [1 point]
'see explanation below'
2. Base your answer on the information below.
Rust on an automobile door contains Fe2O3(s). The balanced equation representing one of the reactions between iron in the door of the automobile
and oxygen in the atmosphere is given below.
4Fe(s) + 3O2(g) → 2Fe2O3(s)
Identify the type of chemical reaction represented by this equation. [1]
The reaction shown in this question can be classified as synthesis, redox, or oxidation. Any of these responses are acceptable for credit.
'see explanation below'
3. Base your answer on the information below.
Electroplating is an electrolytic process used to coat metal objects with a more expensive and less reactive metal. The accompanying diagram shows
an electroplating cell that includes a battery connected to a silver bar and a metal spoon. The bar and spoon are submerged in AgNO 3(aq).
Explain the purpose of the battery in this cell. [1]
The electrolysis reaction is not a spontaneous reaction. Therefore, an external source of energy (the battery) is needed to drive the reaction.
'see explanation below'
4. Base your answer on the information below.
In a laboratory investigation, a student constructs a voltaic cell with iron and copper electrodes. Another student constructs a voltaic cell with zinc
and iron electrodes. Testing the cells during operation enables the students to write the balanced ionic equations below.
Cell with iron and copper electrodes:
Cu2+(aq) + Fe(s) → Cu(s) + Fe2+(aq)
Cell with zinc and iron electrodes:
Fe2+(aq) + Zn(s) → Fe(s) + Zn2+(aq)
State evidence from the balanced equation for the cell with iron and copper electrodes that indicates the reaction in the cell is an oxidation-reduction
reaction. [1]
In an oxidation-reduction reaction, changes in oxidation states are observed. In this reaction, Cu changes from Cu2+ to Cu0, and Fe changes from Fe0
to Fe2+. [1 point]
'see explanation below'
5. Base your answer on the information below.
In a laboratory investigation, a student constructs a voltaic cell with iron and copper electrodes. Another student constructs a voltaic cell with zinc
and iron electrodes. Testing the cells during operation enables the students to write the balanced ionic equations below.
Cell with iron and copper electrodes:
Cu2+(aq) + Fe(s) → Cu(s) + Fe2+(aq)
Cell with zinc and iron electrodes:
Fe2+(aq) + Zn(s) → Fe(s) + Zn2+(aq)
Identify the particles transferred between Fe2+ and Zn during the reaction in the cell with zinc and iron electrodes. [1]
In oxidation-reduction reactions, electrons are transferred between the particles that are oxidized (Zn) and the particles that are reduced (Fe2+). [1
point]
'see explanation below'
6. Base your answer on the information below.
In a laboratory investigation, a student constructs a voltaic cell with iron and copper electrodes. Another student constructs a voltaic cell with zinc
and iron electrodes. Testing the cells during operation enables the students to write the balanced ionic equations below.
Cell with iron and copper electrodes:
Cu2+(aq) + Fe(s) → Cu(s) + Fe2+(aq)
Cell with zinc and iron electrodes:
Fe2+(aq) + Zn(s) → Fe(s) + Zn2+(aq)
Write a balanced half-reaction equation for the reduction that takes place in the cell with zinc and iron electrodes. [1]
In a reduction half-reaction, electrons are gained by the particle that is reduced:
Fe2+ + 2e− &rarr Fe
[1 point]
'see explanation below'
7. Base your answer on the information below.
In a laboratory investigation, a student constructs a voltaic cell with iron and copper electrodes. Another student constructs a voltaic cell with zinc
and iron electrodes. Testing the cells during operation enables the students to write the balanced ionic equations below.
Cell with iron and copper electrodes:
Cu2+(aq) + Fe(s) → Cu(s) + Fe2+(aq)
Cell with zinc and iron electrodes:
Fe2+(aq) + Zn(s) → Fe(s) + Zn2+(aq)
State the relative activity of the three metals used in these two voltaic cells. [1]
Use Reference Table J to determine the relative activity of the three metals, which is shown below in decreasing order:
Zn > Fe > Cu [1 point]
'see explanation below'
8. Base your answer on the accompanying information.
The accompanying diagram represents an operating voltaic cell at 298 K and 1.0 atmosphere in a laboratory investigation. The reaction occurring in
the cell is represented by the balanced ionic equation shown.
Identify the anode in this cell. [1]
The anode is the electrode where oxidation occurs. The reaction given under the diagram indicates that Ni(s) is oxidized to Ni2+(aq) while Ag+(aq) is
reduced to Ag(s). Therefore, the nickel electrode is the anode in this cell. [1 point]
'see explanation below'
9. Base your answer on the accompanying information.
The accompanying diagram represents an operating voltaic cell at 298 K and 1.0 atmosphere in a laboratory investigation. The reaction occurring in
the cell is represented by the balanced ionic equation shown.
Determine the total number of moles of Ni2+(aq) ions produced when 4.0 moles of Ag+(aq) ions completely react in this cell. [1]
According to the reaction, 2 moles of Ag+(aq) ions produce 1 mole of Ni2+(aq) ions. Use dimensional analysis (the factor-label method) to solve this
problem: [1 point]
'see explanation below'
10. Base your answer on the accompanying information.
The accompanying diagram represents an operating voltaic cell at 298 K and 1.0 atmosphere in a laboratory investigation. The reaction occurring in
the cell is represented by the balanced ionic equation shown.
Write a balanced half-reaction equation for the reduction that occurs in this cell. [1]
When reduction occurs, electrons are gained. The reduction half-reaction is as follows:
2Ag+(aq) + 2e– → 2Ag(s)
[1 point]
'see explanation below'
11. Base your answer on the information below.
Two sources of copper are cuprite, which has the IUPAC name copper(I) oxide, and malachite, which has the formula Cu 2CO3(OH)2. Copper is used
in home wiring and electric motors because it has good electrical conductivity. Other uses of copper not related to its electrical conductivity include
coins, plumbing, roofing, and cooking pans. Aluminum is also used for cooking pans.
At room temperature, the electrical conductivity of a copper wire is 1.6 times greater than an aluminum wire with the same length and cross-sectional
area. At room temperature, the heat conductivity of copper is 1.8 times greater than the heat conductivity of aluminum. At STP, the density of copper
is 3.3 times greater than the density of aluminum.
Determine the oxidation number of oxygen in the carbonate ion found in malachite. [1]
The oxidation state of oxygen in compounds is almost always –2. (The exception is oxygen in peroxides, where the oxidation state is –1.) [1 point]
'see explanation below'
12. Given the reaction shown:
a Write the balanced oxidation half-reaction for this oxidation-reduction reaction. [1]
b What is the oxidation number of oxygen in Al2O3? [1]
'see explanation below'
13. Given the reaction: Cl2 + 2 HBr --> Br2 + 2 HCl
On a separate sheet of paper write a correctly balanced reduction half-reaction for this equation. [1]
Reduction is the gain of electrons. The half-reaction must be a balanced equation that shows atoms, ions, and electrons. Acceptable responses
include:
Cl2 + 2e- --> 2ClCl2 --> 2Cl- - 2eCl + e- --> Cl[1 point]
'see explanation below'
14. Base your answer on the information shown, which describes the smelting of iron ore, and on your knowledge of chemistry.
In the smelting of iron ore, Fe2O3 is reduced in a blast furnace at high temperature by a reaction with carbon monoxide. Crushed limestone, CaCO 3, is
also added to the mixture to remove impurities in the ore. The carbon monoxide is formed by the oxidation of carbon (coke), as shown in the reaction
shown:
2 C + O2 --> 2 CO + energy
Liquid iron flows from the bottom of the blast furnace and is processed into different allays of iron.
What is the oxidation number of carbon in CaCO3? [1]
The sum of the oxidation numbers in a compound must equal zero. Since calcium is present as a 2+ ion, its oxidation number is +2. Each of the three
oxygen atoms has an oxidation number of -2, accounting for a total of -6. In order for the sum to equal zero, the single carbon atom must have an
oxidation number of +4. [1 point]
'see explanation below'
15. Base your answer on the diagram (see image), which represents a voltaic cell at 298 K and 1 atm.
In which half-cell will oxidation occur when switch S is closed? [1]
Oxidation is the loss of electrons. According to the equation given under the diagram, Zn atoms form Zn 2+ ions, so the oxidation half-reaction is Zn -> Zn2+ + 2e-. Therefore, oxidation occurs in half-cell 2. [1 point]
'see explanation below'
16. Base your answer on the diagram (see image), which represents a voltaic cell at 298 K and 1 atm.
Write the balanced half-reaction equation that will occur in half-cell 1 when switch S is closed. [1]
Reduction occurs in half-cell 1. The balanced half-reaction is:
Pb2+ + 2e- --> Pb
[1 point]
'see explanation below'
17. Base your answer on the diagram (see image), which represents a voltaic cell at 298 K and 1 atm.
Describe the direction of electron flow between the electrodes when switch S is closed. [1]
When the switch is closed, the electrons lost by the Zn electrode travel through the wire and enter the Pb electrode. Therefore, the electron flow is
from the Zn electrode to the Pb electrode. [1 point]
'see explanation below'
18. Base your answer to the question on the information below.
Element X is a solid metal that reacts with chlorine to form a water-soluble binary compound.
The binary compound consists of element X and chlorine in a 1:2 molar ratio. What is the oxidation number of element X in this compound?
Use the Periodic Table of the Elements. The formula of the compound is XCl2. The (negative) oxidation number of the chloride ion is -1. Since the
compound is neutral, the sum of the oxidation numbers must equal 0:
'see explanation below'
19. Base your answer to the question on the accompanying diagram and balanced equation, which represent the electrolysis of molten NaCl.
When the switch is closed, which electrode will attract the sodium ions?
Sodium ions are positive, and they will be attracted to the negative electrode. [1 point]
'see explanation below'
20. Base your answer to the question on the accompanying diagram and balanced equation, which represent the electrolysis of molten NaCl.
What is the purpose of the battery in this electrolytic cell?
The battery provides the energy needed for the nonspontaneous reaction (electrolysis of molten NaCl) to occur. [1 point]
'see explanation below'
21. Base your answer to the question on the accompanying diagram and balanced equation, which represent the electrolysis of molten NaCl.
Write the balanced half-reaction for the reduction that occurs in this electrolytic cell.
Reduction involves a gain of electrons. Na+ gains an electron to produce Na. Either of the two half-reactions given below is acceptable:
Na+ + e- --> Na 2Na+ + 2e- --> 2Na
[1 point]
'see explanation below'
22. What is the oxidation number of nitrogen in NO(g)? [1]
Refer to the Periodic Table of the Elements. The oxidation number of oxygen is –2. Since the sum of the oxidation numbers in the compound (NO)
must equal 0, the oxidation number of the nitrogen must be +2.
'see explanation below'
23. Base your answer on the information below.
Aluminum is one of the most abundant metals in Earth's crust. The aluminum compound found in bauxite ore is Al2O3. Over one hundred years ago,
it was difficult and expensive to isolate aluminum from bauxite ore. In 1886, a brother and sister team, Charles and Julia Hall, found that molten
(melted) cryolite, Na3AlF6, would dissolve bauxite ore. Electrolysis of the resulting mixture caused the aluminum ions in the Al 2O3 to be reduced to
molten aluminum metal. This less expensive process is known as the Hall process.
Write the balanced half-reaction equation for the reduction of Al3+ to Al.
When a species is reduced, it gains electrons. The correct half-reaction is:
Al3+ + 3e- --> Al
'see explanation below'
24. Base your answer on the information below.
Aluminum is one of the most abundant metals in Earth's crust. The aluminum compound found in bauxite ore is Al2O3. Over one hundred years ago,
it was difficult and expensive to isolate aluminum from bauxite ore. In 1886, a brother and sister team, Charles and Julia Hall, found that molten
(melted) cryolite, Na3AlF6, would dissolve bauxite ore. Electrolysis of the resulting mixture caused the aluminum ions in the Al 2O3 to be reduced to
molten aluminum metal. This less expensive process is known as the Hall process.
Explain, in terms of electrical energy, how the operation of a voltaic cell differs from the operation of an electrolytic cell used in the Hall process.
Include both the voltaic cell and the electrolytic cell in your answer.
A voltaic cell produces electrical energy by using a spontaneous redox reaction. An electrolytic cell uses electrical energy to drive a redox
reaction that is not spontaneous.
'see explanation below'
25. Base your answer on the information below.
A flashlight can be powered by a rechargeable nickel-cadmium battery. In the battery, the anode is Cd(s) and the cathode is NiO2(s). The unbalanced
equation below represents the reaction that occurs as the battery produces electricity. When a nickel-cadmium battery is recharged, the reverse
reaction occurs (see accompanying equation).
Determine the change in oxidation number for the element that makes up the anode in the reaction that produces electricity. [1]
Oxidation occurs at the anode and is accompanied by an increase in the oxidation number of the element being oxidized. The Cd(s) must be the
anode because its oxidation number changes from 0 to +2 as the Cd(s) forms Cd(OH)2(s). [1 point]
'see explanation below'
26. Base your answer on the information below.
A flashlight can be powered by a rechargeable nickel-cadmium battery. In the battery, the anode is Cd(s) and the cathode is NiO2(s). The unbalanced
equation below represents the reaction that occurs as the battery produces electricity. When a nickel-cadmium battery is recharged, the reverse
reaction occurs (see accompanying equation).
Explain why Cd would be above Ni if placed on Table J. [1]
Reference Table J lists metallic substances in (descending) order of their ability to be oxidized. Since Cd(s) is oxidized and Ni 4+ (in NiO2) is reduced,
Cd must be more active than Ni and would lie above it on Reference Table J. [1 point]
'see explanation below'
27. Base your answer on the information below.
A voltaic cell with magnesium and copper electrodes is shown in the accompanying diagram. The copper electrode has a mass of 15.0 grams.
When the switch is closed, the reaction in the cell begins. The balanced ionic equation for the reaction in the cell is shown below the cell diagram.
After several hours, the copper electrode is removed, rinsed with water, and dried. At this time, the mass of the copper electrode is greater than 15.0
grams.
State the direction of electron flow through the wire between the electrodes when the switch is closed. [1]
In the half cell on the left, Mg is oxidized to Mg2+ by losing 2 electrons. The electrons flow from the Mg electrode through the wire into the Cu
electrode.
[1 point]
'see explanation below'
28. Base your answer on the information below.
A voltaic cell with magnesium and copper electrodes is shown in the accompanying diagram. The copper electrode has a mass of 15.0 grams.
When the switch is closed, the reaction in the cell begins. The balanced ionic equation for the reaction in the cell is shown below the cell diagram.
After several hours, the copper electrode is removed, rinsed with water, and dried. At this time, the mass of the copper electrode is greater than 15.0
grams.
State the purpose of the salt bridge in this cell. [1]
The salt bridge allows the passage of ions into the half cells and ensures that the half cells remain electrically neutral.
[1 point]
'see explanation below'
29. Base your answer on the information below.
A voltaic cell with magnesium and copper electrodes is shown in the accompanying diagram. The copper electrode has a mass of 15.0 grams.
When the switch is closed, the reaction in the cell begins. The balanced ionic equation for the reaction in the cell is shown below the cell diagram.
After several hours, the copper electrode is removed, rinsed with water, and dried. At this time, the mass of the copper electrode is greater than 15.0
grams.
Explain, the terms of copper ions and copper atoms, why the mass of the copper electrode increases as the cell operates. Your response must include
information about both copper ions and copper atoms. [1]
During the operation of the cell, Cu2+ ions are reduced to Cu atoms and are deposited on the Cu electrode. This causes the mass of the Cu electrode to
increase.
[1 point]
'see explanation below'
30. Base your answer on the following redox reaction, which occurs spontaneously in an electrochemical cell.
Zn + Cr3+ --> Zn2+ + Cr
In the spaces provided, or a separate sheet of paper, balance the equation using the smallest whole-number coefficients. [1]
'see explanation below'
31. Base your answer on the following redox reaction, which occurs spontaneously in an electrochemical cell.
Zn + Cr3+ --> Zn2+ + Cr
Which species loses electrons and which species gains electrons? [1]
'see explanation below'
32. Base your answer on the following redox reaction, which occurs spontaneously in an electrochemical cell.
Zn + Cr3+ --> Zn2+ + Cr
Which half-reaction occurs at the cathode? [1]
'see explanation below'
33. Base your answer on the following redox reaction, which occurs spontaneously in an electrochemical cell.
Zn + Cr3+ --> Zn2+ + Cr
State what happens to the number of protons in a Zn atom when it changes to Zn 2+ as the redox reaction occurs. [1]
'see explanation below'
34. Base your answer on the information below.
Two chemistry students each combine a different metal with hydrochloric acid. Student A uses zinc, and hydrogen gas is readily produced. Student B
uses copper, and no hydrogen gas is produced.
State one chemical reason for the different results of students A and B. [1]
'see explanation below'
35. Base your answer on the information below.
Two chemistry students each combine a different metal with hydrochloric acid. Student A uses zinc, and hydrogen gas is readily produced. Student B
uses copper, and no hydrogen gas is produced.
Using Reference Table J, identify another metal that will react with hydrochloric acid to yield hydrogen gas. [1]
'see explanation below'
36. State one difference between voltaic cells and electrolytic cells. Include information about both types of cells in your answer. [1]
'see explanation below'
37. Base your answer on the information and equation below.
Human blood contains dissolved carbonic acid, H2CO3, in equilibrium with carbon dioxide and water. The equilibrium system is shown.
What is the oxidation number of carbon in H2CO3(aq)? [1]
In combination, both H atoms have an oxidation number of +1 and all three O atoms have an oxidation number of -2. The sum of all the oxidation
numbers in this neutral compound must add to 0:
(2) x (+1) + (3) x (-2) + X = 0
X = +4
[1 point]
'see explanation below'
38. A plan is being developed for an experiment to test the effect of concentrated strong acids on a metal surface protected by various coatings. Some
safety precautions would be the wearing of chemical safety goggles, an apron, and gloves. State one additional safety precaution that should be
included in the plan. [1]
A number of precautions can be used as acceptable answers:




The test should be performed under a fume hood.
Avoid spilling the acid.
In case of an acid spill, spread powdered NaHCO3 over the spill.
Avoid wearing clothing, such as open-toed shoes, that would not prevent spilled acid from penetrating the skin.
[1 point]
'see explanation below'
39. Base your answer on the accompanying diagram of a voltaic cell and the balanced ionic equation shown.
What is the total number of moles of electrons needed to completely reduce 6.0 moles of Ni 2+(aq) ions?
In order for 1 mole of Ni2+(aq) to be reduced to Ni(s), 2 moles of electrons are needed. The reduction half-reaction is:
Ni2+(aq) + 2e- --> Ni(s)
The solution to the problem is (see image):
'see explanation below'
40. Base your answer on the accompanying diagram of a voltaic cell and the balanced ionic equation shown.
Explain the function of the salt bridge in the voltaic cell.
The salt bridge completes the electric circuit by permitting ions to flow into the half-cells. As a result, the half-cells remain electrically neutral.
'see explanation below'
41. Base your answer on the balanced equation below.
Fe(s) + 2HNO3(aq) --> Fe(NO3)2(aq) + H2(g)
Explain, using information from Reference Table J, why this reaction is spontaneous.
In this reaction, Fe is oxidized to Fe2+ while 2H+ is reduced to H2. In order for the reaction to be spontaneous, Fe must be more active than H. In
other words, Fe must be higher than H in Reference Table J, which it is.
'see explanation below'
42. Base your answer on the information below.
Aluminum is one of the most abundant metals in Earth's crust. The aluminum compound found in bauxite ore is Al2O3. Over one hundred years ago,
it was difficult and expensive to isolate aluminum from bauxite ore. In 1886, a brother and sister team, Charles and Julia Hall, found that molten
(melted) cryolite, Na3AlF6, would dissolve bauxite ore. Electrolysis of the resulting mixture caused the aluminum ions in the Al 2O3 to be reduced to
molten aluminum metal. This less expensive process is known as the Hall process.
Write the oxidation state for each of the elements in cryolite.
Refer to the Periodic Table of the Elements. Na is located in Group 1; its oxidation state is +1. Al is located in Group 13; its oxidation state is +3. F is
located in Group 17; its oxidation state is -1.
'see explanation below'
43. Base your answer on the following redox reaction, which occurs spontaneously in an electrochemical cell.
Zn + Cr3+ --> Zn2+ + Cr
Write the half-reaction for the reduction that occurs. [1]
'see explanation below'
44. Base your answer on the following redox reaction, which occurs spontaneously in an electrochemical cell.
Zn + Cr3+ --> Zn2+ + Cr
Write the half-reaction for the oxidation that occurs. [1]
'see explanation below'
45. Base your answer on the information below.
Elements with atomic numbers 112 and 114 have been produced and their IUPAC names are pending approval. However, an element that would be
put between these two elements on the Periodic Table has not yet been produced. If produced, this element will be identified by the symbol Uut until
an IUPAC name is approved.
In the space on the answer sheet, or, if taken online, on a separate piece of paper, draw a Lewis electron-dot diagram for an atom of Uut. [1]
Refer to the Periodic Table of the Elements. The element Uut, when discovered, will be placed into Group 13 of the Periodic Table. Lewis electrondot diagrams for an atom of this element (or any other element in Group 13) are shown in the accompanying diagram:
Note that either of the diagrams are acceptable for credit.
'see explanation below'
46. Base your answer on the information below.
Rust on an automobile door contains Fe2O3(s). The balanced equation representing one of the reactions between iron in the door of the automobile
and oxygen in the atmosphere is given below.
4Fe(s) + 3O2(g) → 2Fe2O3(s)
Write the IUPAC name for Fe2O3. [1]
Since the iron in Fe2O3 has an oxidation state of +3, the IUPAC name is iron(III) oxide.
'see explanation below'
47. Describe one appropriate laboratory test that can be used to determine the malleability of a solid sample of an element at room temperature. [1]
Malleability is the property by which a metal can be shaped. Flattening the metal by striking it with a hammer or bending the metal by hand are ways
to demonstrate malleability. [1 point]
'see explanation below'
48. Base your answer on the information below.
A piece of magnesium ribbon is reacted with excess hydrochloric acid to produce aqueous magnesium chloride and hydrogen gas. The volume of the
dry hydrogen gas produced is 45.6 milliliters. The temperature of the gas is 293 K, and the pressure is 99.5 kilopascals.
Identify the type of bond between the atoms in a molecule of the gas produced in this laboratory investigation. [1]
A hydrogen molecule has a nonpolar covalent bond between the hydrogen atoms. [1 point]
'see explanation below'
49. Base your answer on the information below.
A portable propane-fueled lantern contains a mesh silk bag coated with metal hydroxides. The primary metal hydroxide is yttrium hydroxide. When
the silk bag is installed, it is ignited and burned away, leaving the metal hydroxide coating. The coating forms metal oxides that glow brightly when
heated to a high temperature. During a test, a propane lantern is operated for three hours and consumes 5.0 moles of propane from the lantern's tank.
The balanced equation below represents the combustion of propane.
C3H8 + 5O2 → 3CO2 + 4H2O + energy
Write the formula for the primary metal hydroxide used in the lantern. [1]
Use the Periodic Table of the Elements and Reference Tables E and S. Yttrium (Y, atomic number = 39) has an oxidation state of +3. The oxidation
state of the hydroxide ion is –1. Therefore, the formula of the primary metal hydroxide is Y(OH)3.[1 point]
'see explanation below'
50. Base your answer on the information below.
When a person perspires (sweats), the body loses many sodium ions and potassium ions. The evaporation of sweat cools the skin.
After a strenuous workout, people often quench their thirst with sports drinks that contain NaCl and KCl. A single 250.-gram serving of one sports
drink contains 0.055 gram of sodium ions.
In the space on the answer sheet or on a separate piece of paper, draw a Lewis electron-dot diagram for one of the positive ions lost by the body as a
person perspires. [1]
When Na or K becomes a positive ion, the single electron in the valence shell is transferred. Acceptable Lewis electron-dot diagrams are shown
below:
In order to receive credit, no dots (electrons) can appear in the diagram. [1 point]
'see explanation below'
51. Base your answer on the information below and the accompanying table.
Bond energy is the amount of energy required to break a chemical bond. The table gives a formula and the carbon-nitrogen bond energy for selected
nitrogen compounds.
Describe, in terms of electrons, the type of bonding between the carbon atom and the nitrogen atom in a molecule of methanamine. [1]
The structural formula shows that carbon and nitrogen share a pair of electrons, which means the bond between the atoms is covalent. [1 point]
'see explanation below'
52. Base your answer on the information below and the accompanying table.
Bond energy is the amount of energy required to break a chemical bond. The table gives a formula and the carbon-nitrogen bond energy for selected
nitrogen compounds.
Identify the noble gas that has atoms in the ground state with the same electron configuration as the nitrogen in a molecule of isocyanic acid. [1]
The nitrogen atom in the molecule of isocyanic acid has 8 valence electrons. Use the Periodic Table of the Elements. Since nitrogen is found in
Period 2, its electron configuration is the same as that of an atom of the noble gas neon (Ne, element 10). [1 point]
'see explanation below'
53. Base your answer on the information below and the accompanying table.
Bond energy is the amount of energy required to break a chemical bond. The table gives a formula and the carbon-nitrogen bond energy for selected
nitrogen compounds.
Explain, in terms of charge distribution, why a molecule of hydrogen cyanide is polar. [1]
A molecule of hydrogen cyanide is polar because the molecule has a charge distribution that is not symmetrical. [1 point]
'see explanation below'
54. Draw a correct Lewis electron-dot structure for each of the following.
a An atom of hydrogen [1]
b An atom of nitrogen [1]
c A molecule of ammonia (NH3) [1]
'see explanation below'
55. Explain, in terms of subatomic particles, why the radius of a chloride ion is larger than the radius of a chlorine atom. [1]
When an electron is added to a chlorine atom to form a chloride ion, the additional repulsion among the valence electrons leads to an increase in size.
Therefore, a chloride ion has a larger radius than a chlorine atom. [1 point]
'see explanation below'
56. Explain, in terms of valence electrons, why the bonding in magnesium oxide, MgO, is similar to the bonding in barium chloride, BaCl2. [1]
Both MgO and BaCl2 are ionic compounds. In both cases, the valence electrons are transferred from the metal (Mg and Ba) to the nonmetal (O and
Cl). [1 point]
'see explanation below'
57. On a separate sheet of paper, draw an electron-dot diagram for each of the following substances:
a calcium oxide (an ionic compound) [1]
b hydrogen bromide [1]
c carbon dioxide [1]
'see explanation below'
58. Base your answer on the article below, the Reference Tables for Physical Setting/Chemistry, and your knowledge of chemistry.
In the 1920s, paint used to inscribe the numbers on watch dials was composed of a luminescent (glow-in-the-dark) mixture. The powdered-paint base
was a mixture of radium salts and zinc sulfide. As the paint was mixed, the powdered base became airborne and drifted throughout the workroom
causing the contents of the workroom, including the painters' clothes and bodies, to glow in the dark.
The paint is luminescent because radiation from the radium salts strikes a scintillator. A scintillator is a material that emits visible light in response to
ionizing radiation. In watchdial paint, zinc sulfide acts as the scintillator.
Radium present in the radium salts decomposes spontaneously, emitting alpha particles. These particles can cause damage to the body when they
enter human tissue. Alpha particles are especially harmful to the blood, liver, lungs, and spleen because they can alter genetic information in the cells.
Radium can be deposited in the bones because it substitutes for calcium.
On a separate sheet of paper explain why zinc sulfide is used in luminescent paint. [1]
The zinc sulfide is a scintillator, that is, a substance that emits visible light when exposed to ionizing radiation. [1 point]
'see explanation below'
59. Base your answer on the electronegativity values and atomic numbers of fluorine, chlorine, bromine, and iodine that are listed on Reference Table
S.
Explain, in terms of electronegativity, why the H--F bond is expected to be more polar than the H--I bond. [1]
Bond polarity is determined by the difference in electronegativities. Since the electronegativity difference for H--F (1.9) is greater than the
electronegativity difference of H--I (0.6), the H--F bond is more polar than the H--I bond. [1 point]
'see explanation below'
60. Base your answer on the information shown.
Potassium ions are essential to human health. The movement of dissolved potassium ions, K+(aq), in and out of a nerve cell allows that cell to
transmit an electrical impulse.
What is the total number of electrons in a potassium ion? [1]
See Reference Table S or refer to the Periodic Table of the Elements. The atomic number of potassium (K) is 19. An atom of potassium has 19
protons and 19 electrons. A potassium ion, K+, has one less electron than the atom. Therefore, a K+ ion contains 18 electrons. [1 point]
'see explanation below'
61. Base your answer on the information shown.
Potassium ions are essential to human health. The movement of dissolved potassium ions, K+(aq), in and out of a nerve cell allows that cell to
transmit an electrical impulse.
Explain, in terms of atomic structure, why a potassium ion is smaller than a potassium atom. [1]
When an atom of potassium loses an electron to form a K+ ion, the electron configuration changes from 2-8-8-1 to 2-8-8. The loss of an electron shell
results in a smaller particle. [1 point]
'see explanation below'
62. Base your answer on the information shown.
Potassium ions are essential to human health. The movement of dissolved potassium ions, K+(aq), in and out of a nerve cell allows that cell to
transmit an electrical impulse.
What property of potassium ions allows them to transmit an electrical impulse? [1]
K+ ions are charged particles. Moreover, when dissolved in water, they are free to move through the solution. Both of these factors contribute to the
ability of K+ ions to conduct electrical impulses. [1 point]
'see explanation below'
63. In the space provided or on a separate piece of paper, draw a Lewis electron-dot diagram for a molecule of phosphorus trichloride, PCl3.
In the Lewis electron-dot diagram for PCl3, P is the central atom and it shares a single pair of electrons with each chlorine atom. Every atom in the
molecule has a total of 8 electrons. See the accompanying diagram:
'see explanation below'
64. Base your answer on the information below.
A metal, M, was obtained from a compound in a rock sample. Experiments have determined that the element is a member of Group 2 on the Periodic
Table of the Elements.
Explain, in terms of electrons, why element M is a good conductor of electricity. [1]
The elements of Group 2 are metallic, and metals have mobile valence electrons. These freely moving electrons account for the good electrical
conductivity of metals. [1 point]
'see explanation below'
65. Base your answer on the information below.
A metal, M, was obtained from a compound in a rock sample. Experiments have determined that the element is a member of Group 2 on the Periodic
Table of the Elements.
Explain why the radius of a positive ion of element M is smaller than the radius of an atom of element M. [1]
The elements of Group 2 form 2+ ions by losing their 2 valence electrons. This loss results in an ion that is smaller than its parent atom. [1 point]
'see explanation below'
66. Base your answer on the information below.
A metal, M, was obtained from a compound in a rock sample. Experiments have determined that the element is a member of Group 2 on the Periodic
Table of the Elements.
Using the symbol M for the element, write the chemical formula for the compound that forms when element M reacts with iodine. [1]
Iodine forms a 1- ion. The correct formula is MI2. [1 point]
'see explanation below'
67. Base your answer on the information below.
A safe level of fluoride ions is added to many public drinking water supplies. Fluoride ions have been found to help prevent tooth decay. Another
common source of fluoride ions is toothpaste. One of the fluoride compounds used in toothpaste is tin(II) fluoride.
A town located downstream from a chemical plant was concerned about fluoride ions from the plant leaking into its drinking water. According to the
Environmental Protection Agency, the fluoride ion concentration in drinking water cannot exceed 4 ppm. The town hired a chemist to analyze its
water. The chemist determined that a 175-gram sample of the town's water contains 0.000 250 gram of fluoride ions.
In the box provided (print this page), draw a Lewis electron-dot diagram for a fluoride ion. [1]
The fluoride ion has a charge of 1- and has 8 valence electrons:
Note that credit is awarded even if the brackets are omitted. [1 point]
'see explanation below'
68. Base your answer on the balanced equation below.
2Na(s) + Cl2(g) --> 2NaCl(s)
On a separate piece of paper, draw a Lewis electron-dot diagram for a molecule of chlorine, Cl2.
The Lewis electron-dot diagram for a molecule of Cl2 shows two chlorine atoms sharing a single pair of electrons. Each chlorine atom in the
molecule has a total of eight electrons. Two possible diagrams are shown:
'see explanation below'
69. Base your answer on the balanced equation below.
2Na(s) + Cl2(g) --> 2NaCl(s)
Explain, in terms of electrons, why the bonding in NaCl is ionic.
NaCl is ionic because the sodium atom transfers its single valence electron to the chlorine atom. As a result, Na + and Cl- ions, each having 8 valence
electrons, are formed. The accompanying diagram illustrates the transfer of the valence electron:
'see explanation below'
70. Base your answer on the information below.
Aluminum is one of the most abundant metals in Earth's crust. The aluminum compound found in bauxite ore is Al2O3. Over one hundred years ago,
it was difficult and expensive to isolate aluminum from bauxite ore. In 1886, a brother and sister team, Charles and Julia Hall, found that molten
(melted) cryolite, Na3AlF6, would dissolve bauxite ore. Electrolysis of the resulting mixture caused the aluminum ions in the Al2O3 to be reduced to
molten aluminum metal. This less expensive process is known as the Hall process.
Explain, in terms of ions, why molten cryolite conducts electricity.
A molten salt such as cryolite conducts electricity because the salt is an ionic compound. The melting process allows the ions to move freely
throughout the liquid.
'see explanation below'
71. Explain, in terms of electronegativity, why a P–Cl bond in a molecule of PCl5 is more polar than a P–S bond in a molecule of P2S5. [1]
The difference in the electronegativities of two bonded atoms determines the polarity of the bond. Use Reference Table S. The electronegativity
difference between P and Cl is 1.0 (3.2 - 2.2), while the difference between P and S is 0.4 (2.6 - 2.2). Therefore, a P-Cl bond is more polar than a P-S
bond.
[1 point]
'see explanation below'
72. A 1.00-mole sample of neon gas occupies a volume of 24.4 liters at 298 K and 101.3 kilopascals. In the space provided or on a separate piece of
paper, calculate the density of this sample. Your response must include both a correct numerical setup and the calculated results. [2]
Density is the ratio of mass to volume. Refer to the Periodic Table of the Elements. The mass of 1.00 mole of neon is 20.179 grams. Use the density
equation on Reference Table T:
d=m/V
=20.179 g/24.4 L
=0.827 g/L
Note: One point is awarded for a correct numerical setup, and one point is awarded for the correct answer (or an answer consistent with your setup).
[2 points]
'see explanation below'
73. Base your answer on the information below.
The hydrocarbon 2-methylpropane reacts with iodine as represented by the balanced equation below. At standard pressure, the boiling point of 2methylpropane is lower than the boiling point of 2-iodo-2-methylpropane.
Explain the difference in the boiling points of 2-methylpropane and 2-iodo-2-methylpropane in terms of both molecular polarity and intermolecular
forces. [2]
The difference in the boiling points of 2-methylpropane and 2-iodo-2-methylpropane are due to:


The molecules of 2-iodo-2-methylpropane being more polar than the molecules of 2-methylpropane, and
The intermolecular forces between 2-iodo-2-methylpropane molecules being stronger than the intermolecular forces between 2methylpropane molecules.
One point is allowed for correctly describing the molecular polarities, and one point is allowed for correctly describing the intermolecular forces. [2
points]
'see explanation below'
74. Base your answer on the information below.
An unlit candle is secured to the bottom of a 200-milliliter glass beaker. Baking soda (sodium hydrogen carbonate) is added around the base of the
candle as shown below. The candle is lit and dilute ethanoic acid is poured down the inside of the beaker. As the acid reacts with the baking soda,
bubbles of CO2 gas form. After a few seconds, the air in the beaker is replaced by 0.20 liter of CO 2 gas, causing the candle flame to go out. The
density of CO2 gas is 1.8 grams per liter at room temperature.
Calculate the mass of the CO2 gas that replaced the air in the beaker. Your response must include both a correct numerical setup and the calculated
result. [2]
The density of the CO2 is 1.8 grams per liter, and the volume of the beaker is 0.20 L (200 mL). Use the density formula on Reference Table T: One
point is awarded for a correct numerical setup, and one point is awarded for a calculation that is consistent with your setup. [2 points]
'see explanation below'
75. What is the total number of electron pairs shared between the carbon atom and one of the oxygen atoms in a carbon dioxide molecule? [1]
The Lewis electron-dot diagram for CO2 is shown in the accompanying diagram:
Each horizontal line between the carbon and the oxygen represents a shared pair of electrons. Two lines are between the carbon atom and one of the
oxygen atoms. Therefore, 2 electron pairs are shared. [1 point]
'see explanation below'
76. In the box shown, or on a separate piece of paper, draw the electron-dot (Lewis) structure of calcium chloride. [2]
'see explanation below'
77. Base your answer on the information below.
Testing of an unknown solid shows that it has the properties listed below.
(1) low melting point
(2) nearly insoluble in water
(3) nonconductor of electricity
(4) relatively soft solid
State the type of bonding that would be expected in the particles of this substance. [1]
'see explanation below'
78. Base your answer on the information below.
Testing of an unknown solid shows that it has the properties listed below.
(1) low melting point
(2) nearly insoluble in water
(3) nonconductor of electricity
(4) relatively soft solid
Explain why the particles of this substance are nonconductors of electricity. [1]
'see explanation below'
79. As a neutral sulfur atom gains two electrons, what happens to the radius of the atom? [1]
'see explanation below'
80. After a neutral sulfur atom gains two electrons, what is the resulting charge of the ion? [1]
'see explanation below'
81. Base your answer on the information below.
The equilibrium equation shown is related to the manufacture of a bleaching solution. In this equation, Cl -(aq) means that chloride ions are
surrounded by water molecules.
Explain, in terms of collision theory, why increasing the concentration of Cl2(g) increases the concentration of OCl-(aq) in this equilibrium system.
[1]
As the concentration of the Cl2(g) increases, the number of effective collisions between the Cl2(g) and the OH–(aq) increases, leading to an increase in
the concentration of OCl–(aq).
'see explanation below'
82. State two methods to increase the rate of a chemical reaction and explain, in terms of particle behavior, how each method increases the reaction
rate. [2]
Any two of the following methods are acceptable for maximum credit.




Increase the temperature. This causes the reacting particles to move faster and collide more frequently.
Increase the concentrations of the reacting particles. This causes an increase in the number of collisions.
Increase the surface area of a solid reactant. This exposes more reacting particles to collisions with other reactants.
Add a catalyst. This provides the reacting particles with an alternate pathway that has a lower activation energy.
One point is awarded for each acceptable response. [2 points]
'see explanation below'
83. On the potential energy diagram on the answer sheet or on a separate piece of paper, draw an arrow to represent the activation energy of the
forward reaction. [1]
The activation energy of the forward reaction is the energy needed to bring the reactants into the activated complex stage (the top of the potential
energy diagram). Refer to the accompanying diagram: [1 point]
'see explanation below'
84. Base your answer on the information below.
A 1.0-gram strip of zinc is reacted with hydrochloric acid in a test tube. The unbalanced equation below represents the reaction.
Zn(s) + HCl(aq) → H2(g) + ZnCl2(aq)
Explain, in terms of collision theory, why using 1.0 gram of powdered zinc, instead of the 1.0-gram strip of zinc, would have increased the rate of the
reaction. [1]
Powdered zinc has a greater surface area than that of a zinc strip. Since more particles of Zn in the powder are exposed to the HCl(aq), the number of
effective collisions would be greater for the powder. [1 point]
'see explanation below'
85. Base your answer on the information below.
Heat is added to a sample of liquid water, starting at 80.oC, until the entire sample is a gas at 120.oC. This process, occurring at standard pressure, is
represented by the balanced equation below.
H2O(l) + heat → H2O(g)
On the diagram or on a separate piece of paper, complete the heating curve for this physical change. [1]
As heat is added to liquid water, its temperature rises until it reaches the boiling point. The temperature then remains constant until all of the liquid
water has changed to gas. From that point on, the temperature rises once again. An example of an acceptable heating curve is shown in the
accompanying diagram: [1 point]
'see explanation below'
86. Base your answer on the information below.
The catalytic converter in an automobile changes harmful gases produced during fuel combustion to less harmful exhaust gases. In the catalytic
converter, nitrogen dioxide reacts with carbon monoxide to produce nitrogen and carbon dioxide. In addition, some carbon monoxide reacts with
oxygen, producing carbon dioxide in the converter. These reactions are represented by the balanced equations below.
Reaction 1:
2NO2(g) + 4CO(g) → N2(g) + 4CO2(g) + 1198.4 kJ
Reaction 2:
2CO(g) + O2(g) → 2CO2(g) + 566.0 kJ
The potential energy diagram (see image) represents reaction 1 without a catalyst. On the same diagram or a separate piece of paper, draw a dashed
line to indicate how potential energy changes when the reaction is catalyzed in the converter. [1]
Catalysts speed reactions by lowering their activation energies. The dashed curve in the accompanying potential energy diagram represents the effect
of a catalyst on the reaction. [1 point]
'see explanation below'
87. Given the equation: . . . (see image)
a Name the type of reaction this equation represents. [1]
b Explain, in terms of particle behavior, why entropy is increasing during this reaction. [1]
'see explanation below'
88. A student wishes to determine how the rate of reaction of magnesium strips with hydrochloric acid, HCl(aq), varies as a function of temperature
of the HCl(aq). Give two additional factors, other than the temperature, that could affect the rate of reaction and must be held constant during the
experiment. [2]
'see explanation below'
89. The equation for the saturated solution equilibrium of potassium nitrate (KNO3) is shown.
aOn a separate sheet of paper, diagram the products. Use the key provided. Indicate the exact arrangement of the particles you diagram. [2]
b Compare the rate of dissolving KNO3 with the rate of recrystallization of KNO3 for the saturated solution. [1]
'see explanation below'
90. On a separate sheet of paper explain how a catalyst may increase the rate of a chemical reaction. [1]
Catalysts increase the rate of a reaction by lowering the activation energy of the reaction. Other acceptable responses include (1) providing an
alternate reaction pathway and (2) forming a different activated complex with a lower activation energy. [1 point]
'see explanation below'
91. On the set of axes provided (print out this page), sketch the potential energy diagram for an endothermic chemical reaction that shows the
activation energy and the potential energy of the reactants and the potential energy of the products. [2]
Note: One point is allowed for showing that the peak (the activated complex) is the highest point in the curve, and one point is allowed for showing
that the end of the curve is higher than the beginning of the curve. [2 points]
'see explanation below'
92. Given the equation for the dissolving of sodium chloride in water:
On a separate piece of paper describe what happens to entropy during this dissolving process. [1]
As the solid NaCl dissolves in water, the dissolved ions are distributed more randomly than they were in the crystal. That is, the entropy of the
system increases. [1 point]
'see explanation below'
93. Base your answer on the information.
Given the equilibrium equation at 298 K:
Describe, in terms of LeChatelier's principle, why an increase in temperature increases the solubility of KNO3. [1]
The forward reaction (the dissolving of KNO3) is endothermic. According to LeChatelier's principle, an increase in temperature favors the
endothermic reaction. Therefore, increasing the temperature increases the solubility of KNO 3. [1 point]
'see explanation below'
94. Base your answer on the information shown, which describes the smelting of iron ore, and on your knowledge of chemistry.
In the smelting of iron ore, Fe2O3 is reduced in a blast furnace at high temperature by a reaction with carbon monoxide. Crushed limestone, CaCO 3, is
also added to the mixture to remove impurities in the ore. The carbon monoxide is formed by the oxidation of carbon (coke), as shown in the reaction
shown:
2 C + O2 --> 2 CO + energy
Liquid iron flows from the bottom of the blast furnace and is processed into different allays of iron.
Using the set of axes provided (print this page), sketch a potential energy diagram for the reaction of carbon and oxygen that produces carbon
monoxide. [1]
The reaction between carbon and oxygen is exothermic. Refer to the potential energy diagram shown (see image):
[1 point]
'see explanation below'
95. Base your answer to the question on the accompanying potential energy diagram.
What is the heat of reaction for the forward reaction?
Refer to the diagram accompanying this question. The heat of reaction is the difference between the potential energy of the reactants, located at 40 kJ,
and the potential energy of the products, located at 120 kJ:
'see explanation below'
96. Base your answer to the question on the accompanying potential energy diagram.
What is the activation energy for the forward reaction with the catalyst?
Refer to the diagram accompanying this question. The activation energy of the forward catalyzed reaction is the difference between the potential
energy of the reactants and the peak of the dotted curve, located at 140 kJ:
'see explanation below'
97. Base your answer to the question on the accompanying potential energy diagram.
Explain, in terms of the function of a catalys, why the curves on the potential energy diagram for the catalyzed and uncatalyzed reactions are
different.
A catalyst provides an alternate pathway for the reaction. This pathway has a lower activation energy than that of the uncatalyzed reaction. [1 point]
'see explanation below'
98. Base your answer on the information below.
An investigation was conducted to study the effect of the concentration of a reactant on the total time needed to complete a chemical reaction. Four
trials of the same reaction were performed. In each trial the initial concentration of the reactant was different. The time needed for the chemical
reaction to be completed was measured. The data for each of the four trials are shown in the accompanying table.
In a different experiment involving the same reaction, it was found that an increase in temperature increased the rate of the reaction. Explain this
result in terms of collision theory. [1]
An increase in temperature raises the average kinetic energy of the particles. As a result, there will be an increase in the number of effective collisions
between the reacting particles. [1 point]
'see explanation below'
99. Base your answer on the reaction represented by the balanced equation below.
2H2(g) + O2(g) → 2H2O(l )+ 571.6 kJ
On the axes provided, or, if taken online, on a separate piece of paper, draw a potential energy diagram for the reaction represented by this equation.
[1]
An unlabeled potential energy diagram is shown. Note that the potential energy of the product (the flat portion on the right side of the diagram) is
lower than the potential energy of the reactants (the flat portion on the left side of the diagram).
'see explanation below'
100. Base your answer on the reaction represented by the balanced equation below.
2H2(g) + O2(g) → 2H2O(l )+ 571.6 kJ
Explain why the entropy of the system decreases as the reaction proceeds. [1]
Entropy is a measure of disorder. The following conditions lead to a decrease in the entropy of the system.



The product is a liquid, while the reactants are gases.
The product consists of one substance, while the reactants consist of two substances.
The product is a more complex molecule than either of the two reactant molecules.
'see explanation below'
101. Base your answer on the information below:
Given the reaction at equilibrium: (see accompanying diagram)
Explain, in terms of Le Chatelier's principle, why the concentration of NH3(g) decreases when the temperature of the equilibrium system increases.
According to Le Chatelier's principle, increasing the temperature favors the endothermic reaction (that is, the reverse reaction). As the system shifts
to the left, the concentration of NH3(g) will decrease.
'see explanation below'
102. Base your answer on the information below.
Given the reaction at equilibrium (see accompanying diagram):
Explain, in terms of Le Chatelier's principle, why the equilibrium shifts to the right to relieve the stress when the pressure on the system is increased
at constant temperature.
According to Le Chatelier's principle, an increase in pressure (that is, a decrease in volume) will shift the equilibrium in a direction that produces
the smaller number of moles. In this equation, the left side has 2 moles (2NO2(g)), and the right side has 1 mole (N2O4(g)). Therefore, the
equilibrium will shift toward the right.
'see explanation below'
103. Base your answer on the information below.
Given the balanced equation for an organic reaction between butane and chlorine that takes place at 300. oC and 101.3 kilopascals:
C4H10 + Cl2 --> C4H9Cl + HCl
Explain, in terms of collision theory, why the rate of the reaction would decrease if the temperature of the reaction mixture was lowered to 200. oC
with pressure remaining unchanged.
A decrease in temperature lowers the average kinetic energy of the particles. As a result, there will be fewer effective collisions among molecules.
'see explanation below'
104. Explain, in terms of collision theory, why the rate of a chemical reaction increases with an increase in temperature. [1]
As the temperature increases, the reactant particles move faster and collide more frequently. This leads to an increase in the reaction rate. Another
acceptable response is that increasing the temperature provides more molecules with the necessary activation energy.
[1 point]
'see explanation below'
105. Base your answer on the information below.
Nitrogen gas, hydrogen gas, and ammonia gas are in equilibrium in a closed container at constant temperature and pressure. The accompanying
equation represents this equilibrium. The accompanying graph shows the initial concentration of each gas, the changes that occur as a result of adding
H2(g) to the system, and the final concentrations when equilibrium is reestablished.
What information on the graph indicates that the system was initially at equilibrium? [1]
We know that the system is initially at equilibrium because the initial concentration of each gas is constant. [1 point]
'see explanation below'
106. Base your answer on the information below.
Nitrogen gas, hydrogen gas, and ammonia gas are in equilibrium in a closed container at constant temperature and pressure. The accompanying
equation represents this equilibrium. The accompanying graph shows the initial concentration of each gas, the changes that occur as a result of adding
H2(g) to the system, and the final concentrations when equilibrium is reestablished.
Explain, in terms of LeChatelier's principle, why the final concentration of NH3(g) is greater than the initial concentration of NH3(g). [1]
The stress of adding H2(g) shifts the reaction to the right, producing more NH3(g). [1 point]
'see explanation below'
107. Base your answer on the information below.
Nitrogen gas, hydrogen gas, and ammonia gas are in equilibrium in a closed container at constant temperature and pressure. The accompanying
equation represents this equilibrium. The accompanying graph shows the initial concentration of each gas, the changes that occur as a result of adding
H2(g) to the system, and the final concentrations when equilibrium is reestablished.
Explain, in terms of collision theory, why the concentration of H2(g) begins to decrease immediately after more H2(g) is added to the system. [1]
The addition of H2(g) causes more collisions between the molecules of H2(g) and N2(g) and increases the rate of the forward reaction. This change
results in the lowering of the H2 concentration before a new equilibrium point is reached. [1 point]
'see explanation below'
108. Base your answer on the information below.
At room temperature, a reaction occurs when KIO3(aq) is mixed with NaHSO3(aq) that contains a small amount of starch. The colorless reaction
mixture turns dark blue after a period of time that depends on the concentration of the reactants.
In a laboratory, 12 drops of a 0.02 M NaHSO3(aq) solution containing starch were placed in each of six test tubes. A different number of drops of
0.02 M KIO3(aq) and enough water to maintain a constant volume were added to each test tube and the time for the dark-blue color to appear was
measured. The data were recorded in the accompanying table.
Identifyone factor, other than the concentration of the reactants, that would affect the rate of this reaction. [1]
Two additional factors that could affect the rate of the reaction are changing the temperature and adding a catalyst. Only one factor need be listed in
order to receive credit. [1 point]
'see explanation below'
109. Base your answer on the information below.
The Solvay process is a multistep industrial process used to produce washing soda, Na2CO3(s). In the last step of the Solvay process, NaHCO3(s) is
heated to 300oC, producing washing soda, water, and carbon dioxide. This reaction is represented by the balanced equation below.
2NaHCO3(s) + heat → Na2CO3(s) + H2O(g) + CO2(g)
State evidence that indicates the entropy of the products is greater than the entropy of the reactant. [1]
Two moles of a single, solid reactant decompose into three moles of products, of which two products are gases. Gases have more entropy than solids.
[1 point]
'see explanation below'
110. Base your answer on the information and diagram shown, which represents the changes in potential energy that occur during the given reaction.
A + B --> C
Does the diagram illustrate an exothermic or an endothermic reaction? State one reason, in terms of energy, to support your answer. [2]
'see explanation below'
111. Base your answer on the information and diagram shown, which represents the changes in potential energy that occur during the given reaction.
A + B --> C
On the diagram provided, or on a separate sheet of paper, draw a dashed line to indicate a potential energy curve for the reaction if a catalyst is added.
[1]
'see explanation below'
112. Given the reaction shown at equilibrium:
a State the effect on the number of moles of N2(g) if the temperature of the system is increased. [1]
b State the effect on the number of moles of H2(g) if the pressure on the system is increased. [1]
c State the effect on the number of moles of NH3(g) if a catalyst is introduced into the reaction system. Explain why this occurs. [2]
'see explanation below'
113. Base your answer on the information below.
A student wishes to investigate how the reaction rate changes with a change in concentration of HCl(aq).
Given the reaction: (see image)
Identify one other variable that might affect the rate and should be held constant during this investigation. [1]
'see explanation below'
114. Base your answer on the information below.
A student wishes to investigate how the reaction rate changes with a change in concentration of HCl(aq).
Given the reaction: (see image)
Describe the effect of increasing the concentration of HCl(aq) on the reaction rate and justify your response in terms of collision theory. [1]
'see explanation below'
115. Base your answer on the information and equation below.
Human blood contains dissolved carbonic acid, H2CO3, in equilibrium with carbon dioxide and water. The equilibrium system is shown.
Explain, using LeChatelier's principle, why decreasing the concentration of CO2 decreases the concentration of H2CO3. [1]
When CO2 is removed from the system, the state of equilibrium is disturbed. As a response to this disturbance, the system shifts to the right in order
to replace some of the CO2. As a result, the concentration of H2CO3 decreases. [1 point]
'see explanation below'
116. Base your answer on the information below:
Given the reaction at equilibrium: (see accompanying diagram)
Copyon a separate piece of paper potential energy diagram shown and complete the diagram for the forward reaction. Be sure your drawing shows
the activation energy and the potential energy of the products.
In this equation, the heat appears on the right side of the equation. It is an exothermic reaction, in which the potential energy of the products is lower
than the potential energy of the reactants. A labeled diagram is shown:
1
117. What is the oxidation number of chromium in K2Cr2O7?
1. +6
3. +7
2. +2
4. +12
2
118. Given the reaction: . . . (see image)
Which equation represents the oxidation that takes place?
4
119. A student collects the materials and equipment below to construct a voltaic cell.






two 250-mL beakers
wire and a switch
one strip of magnesium
one strip of copper
125 mL of 0.20 M Mg(NO3)2(aq)
125 mL of 0.20 M Cu(NO3)2(aq)
Which additional item is required for the construction of the voltaic cell?
1. an anode
3. a cathode
2. a battery
4. a salt bridge
4 In constructing the voltaic cell, provision must be made for the flow of ions in order to complete the circuit and keep the half-cells electrically
neutral. This is accomplished by means of a salt bridge.
Wrong Choices Explained:
(1) The magnesium strip is the anode.
(2) Voltaic cells produce electricity; no battery is needed.
(3) The copper strip is the cathode.
1
120. A voltaic cell spontaneously converts chemical energy to
1. electrical energy
3. mechanical energy
2. geothermal energy
4. nuclear energy
1 A voltaic cell is defined as an electrochemical cell that uses a spontaneous redox reaction to convert chemical energy to electrical energy. A
commercial battery is an example of a voltaic cell.
2
121. Given the balanced equation representing a reaction:
Mg(s) + Ni2+(aq) → Mg2+(aq) + Ni(s)
What is the total number of moles of electrons lost by Mg(s) when 2.0 moles of electrons are gained by Ni 2+(aq)?
1. 1.0 mol
3. 3.0 mol
2. 2.0 mol
4. 4.0 mol
2 Since the equation is balanced for mass and charge, all of the electrons lost by Mg(s) must be gained by the Ni2+(aq). Therefore, if Ni2+(aq) gains
2.0 moles of electrons, Mg(s) must lose 2.0 moles of electrons.
3
122. Which half-reaction correctly represents reduction?
3 In a reduction half-reaction, electrons are gained (i.e., they appear on the left side of the arrow), and the oxidation number of the species must
decrease. Of the choices given, only the half-reaction given in choice (3) meets both criteria.
1
123. Which equation represents an oxidation-reduction reaction?
1. CH4 + 2O2 --> CO2 + 2H2O
3. MgCrO4 + BaCl2 --> MgCl2 + BaCrO4
2. H2SO4 + Ca(OH)2 --> CaSO4 + 2H2O
4. Zn(NO3)2 + Na2CO3 --> 2NaNO3 + ZnCO3
1 In an oxidation-reduction reaction, at least some of the atoms change their oxidation numbers. In choice (1), the oxidation number of C changes
from –4 to +4 and the oxidation number of O changes from 0 to –2.
Wrong Choices Explained:
(2) The oxidation numbers of the atoms remain unchanged: H = +1; S = +6; O = –2; Ca = +2.
(3) The oxidation numbers of the atoms remain unchanged: Mg = +2; Cr = +6; O = –2; Ba = +2; Cl = –1.
(4) The oxidation numbers of the atoms remain unchanged: Zn = +2; N = +5; O = –2; Na = +1; C = +4.
4
124. Given the balanced equation representing a reaction:
2KClO3(s) → 2KCl(s) + 3O2(g)
The oxidation state of chlorine in this reaction changes from
1. -1 to +1
3. +1 to -1
2. -1 to +5
4. +5 to -1
4 KClO3 and KCl are (neutral) compounds. The sum of the oxidation numbers in each neutral compound must equal 0. K is a Group 1 element, its
oxidation number in a compound is always +1. O in combination almost always has the oxidation number –2. When Cl is a simple ion, its oxidation
number is –1 (as in KCl). To find the oxidation number of Cl in KClO3, add the known oxidation numbers and set the sum equal to 0:
K + Cl + O3 = 0
(+1) + x + (3 x (–2)) = 0
x = Cl = +5
The oxidation number of chlorine changes from +5 to –1.
4
125. Given the balanced equation representing a reaction occurring in an electrolytic cell:(see image)
Where is Na(l ) produced in the cell?
1. at the anode, where oxidation occurs
3. at the cathode, where oxidation occurs
2. at the anode, where reduction occurs
4. at the cathode, where reduction occurs
4 In this electrolytic cell, Na+(aq) ions are reduced to Na(l ).The relevant equation is:(see image)
Reduction in any type of electrochemical cell always takes place at the cathode.
3
126. What is the oxidation state of nitrogen in the compound NH4Br?
1. -1
3. -3
2. +2
4. +4
3 Use the Periodic Table of the Elements. The sum of the oxidation states in NH4Br must equal zero since the compound is neutral. Hydrogen
accounts for +4 (4 x (+1)), and bromine accounts for —1. Therefore, the nitrogen must have an oxidation state of —3.
2
127. Which energy conversion occurs during the operation of an electrolytic cell?
1. chemical energy to electrical energy
3. nuclear energy to electrical energy
2. electrical energy to chemical energy
4. electrical energy to nuclear energy
2 An electrolytic cell is a type of electrochemical cell in which an external source of electrical energy is converted to the chemical energy needed to
drive a reduction-oxidation reaction that is not spontaneous.
4
128. Which balanced equation represents a redox reaction?
4 In a redox reaction, electrons are transferred between reacting particles, and the oxidation numbers of the particles change. In choice (4), the
oxidation number of Mg changes from 0 to +2, while the oxidation number of H changes from +1 to 0. (The oxidation numbers of H and Cl do not
change.)
Wrong Choices Explained:
(1) The oxidation numbers of the particles do not change. Throughout the reaction, they are Ag = +1, N = +5, O = –2, Na = +1, and Cl = –1.
(2) The oxidation numbers of the particles do not change. Throughout the reaction, they are H = +1, C = +4, and O = –2.
(3) The oxidation numbers of the particles do not change. Throughout the reaction, they are Na = +1, O = –2, H = +1, and Cl = –1.
1
129. The transfer of which particle is required for a redox reaction to occur?
1. electron
3. neutron
2. ion
4. proton
1
130. In a voltaic cell, chemical energy is converted to
1. electrical energy, spontaneously
3. nuclear energy, spontaneously
2. electrical energy, nonspontaneously
4. nuclear energy, nonspontaneously
1 By definition, a voltaic cell is an electrochemical cell that converts chemical energy to electrical energy spontaneously.
3
131. Which statement is true about oxidation and reduction in an electrochemical cell?
1. Both occur at the anode.
3. Oxidation occurs at the anode and reduction occurs at the cathode.
2. Both occur at the cathode.
4. Oxidation occurs at the cathode and reduction occurs at the anode.
3 By definition, the anode is the electrode at which oxidation occurs, and the cathode is the electrode at which reduction occurs.
4
132. Half-reactions can be written to represent all
1. double-replacement reactions
3. fission and fusion reactions
2. neutralization reactions
4. oxidation and reduction reactions
4 Half-reactions show the transfer (loss or gain) of electrons by a given species and are part of all oxidation and reduction reactions.
2
133. Given the balanced equation representing a redox reaction:
2Al + 3Cu2+ --> 2Al3+ + 3Cu
Which statement is true about this reaction?
1. Each Al loses 2e- and each Cu2+ gains 3e-.
-
2+
-
2. Each Al loses 3e and each Cu gains 2e .
3. Each Al3+ gains 2e- and each Cu loses 3e-.
4. Each Al3+ gains 3e- and each Cu loses 2e-.
2 The oxidation-reduction reaction can be written in terms of its oxidation and reduction half-reactions:
Al --> Al3+ + 3eCu2+ + 2e- --> Cu
Each Al loses 3 electrons, and each Cu2+ gains 2 electrons.
4
134. Which conversion of energy always occurs in a voltaic cell?
1. light energy to chemical energy
3. chemical energy to light energy
2. electrical energy to chemical energy
4. chemical energy to electrical energy
4 A voltaic cell is an electrochemical cell that uses a spontaneous oxidation-reduction reaction to convert chemical energy to electrical energy.
Wrong Choice Explained: (2) The conversion of electrical energy to chemical energy is accomplished by an electrolytic cell.
2
135. Which half-reaction shows conservation of charge?
1. Cu + e- --> Cu+
3. Cu+ --> Cu + e-
2. Cu2+ + 2e- --> Cu
4. Cu2+ --> Cu + 2e-
2 A half-reaction shows conservation of charge if the total charges on each side of the arrow are equal in both sign and magnitude. In choice (2), the
total charge on each side of the arrow is equal to 0.
3
136. Given the balanced equation representing the reaction occurring in a voltaic cell:
Zn(s) + Pb2+(aq) → Zn2+(aq) + Pb(s)
In the completed external circuit, the electrons flow from
1. Pb(s) to Zn(s)
3. Zn(s) to Pb(s)
2. Pb2+(aq) to Zn2+(aq)
4. Zn2+(aq) to Pb2+(aq)
3 In this voltaic cell, each Zn atom is oxidized to a Zn2+ ion by losing two electrons. These electrons flow through the external circuit from the Zn(s)
electrode to the Pb(s) electrode, reducing Pb2+ ions to Pb atoms.
2
137. Which balanced equation represents a redox reaction?
1. CuCO3(s) → CuO(s) + CO2(g)
3. AgNO3(aq) + KCl(aq) → AgCl(s) + KNO3(aq)
2. 2KClO3(s) → 2KCl(s) + 3O2(g)
4. H2SO4(aq) + 2KOH(aq) → K2SO4(aq) + 2H2O(l )
2 A redox reaction is always accompanied by a change in the oxidation numbers of at least two atoms. In choice (2), the oxidation number of Cl
changes from +5 to –1, and the oxidation number of O changes from –2 to 0. Note that K does not change its oxidation number, which remains at +1.
Wrong Choices Explained:
(1) The oxidation numbers of Cu, C, and O remain constant at +2, +4, and –2, respectively.
(3) The oxidation numbers of Ag, N, O, K, and Cl remain constant at +1, +5, –2, +1, and –1, respectively.
(4) The oxidation numbers of H, S, O, and K remain constant at +1, +6, –2, and +1, respectively.
2
138. Given the unbalanced ionic equation:
3Mg +
Fe3+ → 3Mg2+ +
Fe
When this equation is balanced, both Fe3+ and Fe have a coefficient of
1. 1, because a total of 6 electrons is transferred
3. 1, because a total of 3 electrons is transferred
2. 2, because a total of 6 electrons is transferred
4. 2, because a total of 3 electrons is transferred
2 All equations must be balanced for mass (i.e., the number of atoms), for electric charge, and for energy. Since the Mg 2+ ion is associated with the
coefficient 3, the total charge on the right side of the equation is 6+. The Fe2+ ion must be assigned the coefficient 2 in order to have a total charge of
6+ on the right side of the equation. The balanced equation is shown below:
3Mg + 2Fe3+ → 3Mg2+ +2Fe
The half-reactions for this equation show why a total of 6 electrons is transferred:
3Mg → 3Mg2+ + 6e–
2Fe3+ + 6e– → 2Fe
4
139. Which metal is more active than H2?
1. Ag
3. Cu
2. Au
4. Pb
4 Use Reference Table J. Metals that are more active than H2 lie above H2 on the table. Of the choices given, only choice (4), Pb, lies above H2.
3
140. Given the balanced ionic equation representing the reaction in an operating voltaic cell:
Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
The flow of electrons through the external circuit in this cell is from the
1. Cu anode to the Zn cathode
3. Zn anode to the Cu cathode
2. Cu cathode to the Zn anode
4. Zn cathode to the Cu anode
3 According to the ionic equation, Zn is oxidized to Zn2+ and is the anode; Cu2+ is reduced to Cu and is the cathode. The electrons lost by the Zn
electrode pass through the external circuit and enter the Cu electrode.
3
141. In any redox reaction, the substance that undergoes reduction will
1. lose electrons and have a decrease in oxidation number
3. gain electrons and have a decrease in oxidation number
2. lose electrons and have an increase in oxidation number
4. gain electrons and have an increase in oxidation number
3
142. As a Ca atom undergoes oxidation to Ca2+, the number of neutrons in its nucleus
1. decreases
3. remains the same
2. increases
2
143. Which type of reaction occurs when nonmetal atoms become negative nonmetal ions?
1. oxidation
3. substitution
2. reduction
4. condensation
2 A negative ion is formed by gaining one or more electrons. The gaining of electrons is known as reduction.
Wrong Choice Explained:
(1) Oxidation involves the loss of electrons.
1
144. Given the reaction:
Zn(s) + 2 HCl(aq) --> ZnCl2(aq) + H2(g)
Which statement correctly describes what occurs when this reaction takes place in a closed system?
1. Atoms of Zn(s) lose electrons and are oxidized.
3. There is a net loss of mass.
2. Atoms of Zn(s) gain electrons and are reduced.
4. There is a net gain of mass.
1 The oxidation state of Zn in Zn(s) is 0. In ZnCl2(aq), the oxidation state of Zn is +2. Atoms increase their oxidation states by losing electrons, a
process known as oxidation. Therefore, atoms of Zn(s) lose electrons and are oxidized.
Wrong Choices Explained:
(3), (4) Mass is always conserved in a chemical reaction.
1
145. A voltaic cell differs from an electrolytic cell in that in a voltaic cell
1. energy is produced when the reaction occurs
3. both oxidation and reduction occur
2. energy is required for the reaction to occur
4. neither oxidation nor reduction occurs
1 In a voltaic cell, a redox reaction is used to produce electrical energy. In an electrolytic cell, a source of electrical energy is used to drive a redox
reaction.
4
146. What is the purpose of the salt bridge in a voltaic cell?
1. It blocks the flow of electrons.
3. It is a path for the flow of electrons.
2. It blocks the flow of positive and negative ions.
4. It is a path for the flow of positive and negative ions.
4 In a voltaic cell, electrons are transferred through the external circuit (that is, the wire connecting the half-cells). In order to complete the circuit, a
flow of ions must be maintained. In some types of voltaic cells, this is accomplished by means of a salt bridge.
3
147. Which half-reaction correctly represents reduction?
1. Ag --> Ag+ + e-
-
2. F2 --> 2 F + 2e
3. Au3+ + 3e- --> Au
4. Fe2+ + e- --> Fe3+
3 Reduction represents the gain of electrons. In a reduction half-reaction, the product will have a lower oxidation number than that of the reactant,
and the total charge will be the same on both sides of the reaction. Of the choices given, only choice (3), Au 3+ + 3e- --> Au, meets all three criteria.
Wrong Choices Explained:
(1) This half-reaction represents the oxidation of Ag to Ag+.
(2), (4) These half-reactions are incorrectly written.
2
148. In a redox reaction, how does the total number of electrons lost by the oxidized substance compare to the total number of electrons gained by the
reduced substance?
1. The number lost is always greater than the number gained.
3. The number lost is sometimes equal to the number gained.
2. The number lost is always equal to the number gained.
4. The number lost is sometimes less than the number gained.
2 Charge is always conserved in a chemical reaction. The number of electrons lost must equal the number of electrons gained.
2
149. Which reaction is an example of an oxidation-reduction reaction?
1. AgNO3 + KI --> AgI + KNO3
3. 2 KOH + H2SO4 -> K2SO4 + 2 H2O
2. Cu + 2 AgNO3 --> Cu(NO3)2 + 2 Ag
4. Ba(OH)2 + 2 HCl --> BaCl2 + 2 H2O
2 In an oxidation-reduction reaction, the transfer of electrons causes the oxidation number to change. The substance that is oxidized increases its
oxidation number, while the substance that is reduced decreases its oxidation number. In choice (2), Cu is oxidized to Cu 2+ and its oxidation number
increases from 0 to +2, while Ag+ is reduced to Ag and its oxidation number decreases from +1 to 0.
Wrong Choices Explained:
(1), (3), (4) In each of these reactions, none of the elements undergoes a change in oxidation number. Therefore, they are not oxidation-reduction
reactions.
1
150. Which metal reacts spontaneously with a solution containing zinc ions?
1. magnesium
3. copper
2. nickel
4. silver
1 Refer to Reference Table J. Any metal that lies above zinc (Zn) will react spontaneously with Zn 2+ ions. Of the choices given, only choice (1),
magnesium (Mg), lies above Zn.
2
151. Given the balanced ionic equation:
Zn(s) +Cu2+(aq) --> Zn2+(aq) + Cu(s)
Which equation represents the oxidation half-reaction?
1. Zn(s) + 2e- -->Zn2+(aq)
2+
3. Cu2+(aq) --> Cu(s) + 2e-
4. Cu2+(aq) + 2e- --> Cu(s)
2. Zn(s) --> Zn (aq) + 2e
2 Oxidation involves the loss of electrons. As a result, the oxidation number of the substance increases. Of the choices given, only the half-reaction
shown in choice (2) meets both of these criteria.
WRONG CHOICES EXPLAINED:
(1) This equation is an incorrectly written reduction half-reaction.
(3) This equation is an incorrectly written oxidation half-reaction.
(4) This equation is correctly written, but it represents reduction (the gain of electrons).
3
152. Which balanced equation represents a redox reaction?
1. AgNO3 + NaCl --> AgCl + NaNO3
3. CuO + CO --> Cu + CO2
2. BaCl2 + K2CO3 --> BaCO3 + 2KCl
4. HCl + KOH --> KCl + H2O
3 A redox reaction is always accompanied by changes in oxidation numbers. In choice (3), the oxidation number of Cu changes from +2 to 0, and the
oxidation number of C changes from +2 to +4. In all of the other choices, the oxidation numbers remain unchanged.
2
153. Which process occurs at the anode in an electrochemical cell?
1. the loss of protons
3. the gain of protons
2. the loss of electrons
4. the gain of electrons
2 The anode of an electrochemical cell (whether voltaic or electrolytic) is the site where oxidation occurs. Oxidation is defined as the loss of
electrons.
2
154. Which changes occur when Pt2+ is reduced?
1. The Pt2+ gains electrons and its oxidation number increases.
3. The Pt2+ loses electrons and its oxidation number increases.
2+
4. The Pt2+ loses electrons and its oxidation number decreases.
2. The Pt gains electrons and its oxidation number decreases.
2 Reduction involves the gain of one or more electrons and a decrease in the oxidation number of the particle that is reduced. Refer to the reduction
half-reaction for Pt2+ shown below:
Pt2+ + 2e- → Pt
Note that the oxidation number of Pt decreases from 2 to 0.
2
155. Which balanced equation represents an oxidation-reduction reaction?
1. BaCl2 + Na2SO4 --> BaSO4 + 2NaCl
3. CaCO3 --> CaO + CO2
2. C + H2O --> CO + H2
4. Mg(OH)2 + 2HNO3 --> Mg(NO3)2 = 2H2O
2 A redox reaction is always accompanied by changes in oxidation numbers. In choice (2), the oxidation number of C changes from 0 to +2, and the
oxidation number of H changes from +1 to 0. In all of the other choices, the oxidation numbers of the particles remain unchanged.
1
156. Which energy conversion occurs during the operation of a voltaic cell?
1. Chemical energy is spontaneously converted to electrical energy.
3. Electrical energy is spontaneously converted to chemical energy.
2. Chemical energy is converted to electrical energy only when an
4. Electrical energy is converted to chemical energy only when an
external power source is provided.
external power source is provided.
1 A voltaic cell is defined as an electrochemical cell in which a spontaneous redox reaction converts chemical energy into electrical energy.
3
157. Given the balanced ionic equation representing a reaction:
2Al3+(aq) + 3Mg(s) → 3Mg2+(aq) + 2Al(s)
In this reaction, electrons are transferred from
1. Al to Mg2+
3. Mg to Al3+
2. Al3+ to Mg
4. Mg2+ to Al
3 The two half-reactions for this balanced redox equation are:
3Mg(s) → 3Mg2+2Al3+(aq) + 6e- → 2Al(s)
Electrons are lost by Mg(s) and transferred to Al3+(aq).
1
158. Which half-reaction equation represents the reduction of a potassium ion?
1. K+ + e- --> K
-
3. K+ --> K + e-
+
4. K --> K+ + e-
2. K + e --> K
1 When a species is reduced, it gains electrons and its oxidation number decreases. In choice (1), the K+ ion gains an electron and the oxidation
number decreases from +1 to 0.
3
159. Given the balanced equation representing a reaction:
Fe2O3 + 2Al --> Al2O3 + 2Fe
During this reaction, the oxidation number of Fe changes from
1. +2 to 0 as electrons are transferred
3. +3 to 0 as electrons are transferred
2. +2 to 0 as protons are transferred
4. +3 to 0 as protons are transferred
3 This equation represents a redox reaction in which electrons are transferred. In Fe2O3, the oxidation number of oxygen is -2. Therefore, the Fe in
this compound must have an oxidation number of +3. On the right side of the equation, Fe appears as a free element. Its oxidation number is 0.
2
160. Which statement describes one characteristic of an operating electrolytic cell?
1. It produces electrical energy.
3. It uses radioactive nuclides.
2. It requires an external energy source.
4. It undergoes a spontaneous redox reaction.
2 Electrolytic cells are electrochemical cells that use an external source of electrical energy in order to drive a nonspontaneous reduction-oxidation
reaction.
2
161. Which half-reaction equation represents the reduction of an iron(II) ion?
1. Fe2+ --> Fe3+ + e-
3. Fe3++ e- --> Fe2+
2. Fe2+ + 2e- --> Fe
4. Fe --> Fe2+ + 2e-
2 In a reduction half-reaction, electrons are gained and the oxidation number of the reduced substance is lowered. The equation given in choice (2)
indicates that 2 electrons are gained and that the oxidation number of Fe is lowered from +2 to 0.
4
162. In which substance does hydrogen have an oxidation number of zero?
1. LiH
3. H2S
2. H2O
4. H2
3
163. Which procedure requires the use of an external electric current to force a redox reaction to occur?
1. polymerization
3. electrolysis
2. distillation
4. saponification
2
164. Given the balanced equation: . . . (see image)
What is the total number of moles of electrons lost by 2 moles of Al(s)?
1. 1 mole
3. 3 moles
2. 6 moles
4. 9 moles
4
165. Given the reaction: . . . (see image)
As the reaction occurs, what happens to copper?
1. It undergoes reduction and its oxidation number decreases.
3. It undergoes oxidation and its oxidation number decreases.
2. It undergoes reduction and its oxidation number increases.
4. It undergoes oxidation and its oxidation number increases.
2
166. In any redox reaction, a reactant can undergo a decrease in oxidation number by
1. losing electrons, only
3. losing protons, only
2. gaining electrons, only
4. gaining protons, only
3
167. Which is a redox reaction?
2
168. Given the nickel-cadmium battery reaction: . . . (see image)
During the discharge of the battery, Ni3+ ions are
1. reduced, and cadmium metal is reduced
3. oxidized, and cadmium metal is reduced
2. reduced, and cadmium metal is oxidized
4. oxidized, and cadmium metal is oxidized
4
169. Which metal can replace Cr in Cr2O3?
1. nickel
3. copper
2. lead
4. aluminum
1
170. Given the reaction shown:
Which species undergoes oxidation?
1. Mg(s)
3. Cl-(aq)
2. H+(aq)
4. H2(g)
1
171. Which particles are gained and lost during a redox reaction?
1. electrons
3. neutrons
2. protons
4. positrons
4
172. What is the oxidation number of chromium in K2Cr2O7?
1. +12
3. +3
2. +2
4. +6
4
173. Which process requires an external power source?
1. neutralization
3. fermentation
2. synthesis
4. electrolysis
3
174. In which substance does chlorine have an oxidation number of +1?
1. Cl2
3. HClO
2. HCl
4. HClO2
1
175. Which statement is true for any electrochemical cell?
1. Oxidation occurs at the anode, only.
3. Oxidation occurs at both the anode and the cathode.
2. Reduction occurs at the anode, only.
4. Reduction occurs at both the anode and the cathode.
2
176. Given the equation:
1
177. A diagram of a chemical cell and an equation are shown (see image). When the switch is closed, electrons will flow from
1. the Pb(s) to the Cu(s)
3. the Pb2+(aq) to the Pb(s)
2. the Cu(s) to the Pb(s)
4. the CU2+(aq) to the Cu(s)
1
178. Given the reaction that occurs in an electrochemical cell:
Zn(s) + CuSO4(aq) --> ZnSO4(aq) + Cu(s)
During this reaction, the oxidation number of Zn changes from
1. 0 to +2
3. +2 to 0
2. 0 to -2
4. -2 to 0
1 As a free element, Zn(s) has an oxidation number of 0. In a compound such as ZnSO 4, the sum of the oxidation numbers must add to 0. Refer to
Reference Table E. Since the SO4 2¿ ion has an oxidation number of ¿2 (its charge), the Zn in ZnSO 4 must have an oxidation number of +2.
Therefore, the oxidation number of Zn changes from 0 to +2.
2
179. A voltaic cell spontaneously converts
1. electrical energy to chemical energy
3. electrical energy to nuclear energy
2. chemical energy to electrical energy
4. nuclear energy to electrical energy
2 A voltaic cell is a type of electrochemical cell that generates electrical energy by converting the chemical energy obtained from a spontaneous
oxidation-reduction reaction.
2
180. Given the reaction for the corrosion of aluminum:
4 Al + 3 O2 --> 2 Al2O3
Which half-reaction correctly represents the oxidation that occurs?
1. Al + 3e- --> Al3+
3+
-
2. Al --> Al + 3e
3. O2 + 4e- --> 2 O24. O2 --> 2 O2- + 4e-
2 As a free element, Al has an oxidation number of 0. Refer to the Periodic Table of the Elements. When in Al 2O3, the Al has an oxidation number of
+3 since it exists as the Al3+ ion. Al forms a 3+ ion by losing three electrons. The correct oxidation half-reaction is:
Al --> Al3+ + 3e3
181. What is the oxidation state of nitrogen in NaNO2?
1. +1
3. +3
2. +2
4. +4
3 Since NaNO2 is a neutral compound, the sum of the oxidation numbers must add to zero. Na is located in Group 1 and has an oxidation number of
+1. Oxygen is located in Group 16 and has an oxidation number of -2. Since two oxygen atoms are present, the contribution of oxygen is -4. Nitrogen
must have an oxidation number of +3 in order to bring the total to zero.
3
182. Where does oxidation occur in an electrochemical cell?
1. at the cathode in both an electrolytic cell and a voltaic cell
3. at the anode in both an electrolytic cell and a voltaic cell
2. at the cathode in an electrolytic cell and at the anode in a voltaic cell
4. at the anode in an electrolytic cell and at the cathode in a voltaic cell
3 The anode is defined to be the electrode at which oxidation occurs in an electrochemical cell. This definition holds for both voltaic and electrolytic
cells.
1
183. Which expression correctly represents a balanced reduction half-reaction?
4
184. Which component of an electrochemical cell is correctly paired with its function?
1. external conductor -- allows the solutions to mix
3. salt bridge -- allows the solutions to mix
2. external conductor -- permits the migration of ions
4. salt bridge -- permits the migration of ions
2
185. In which substance is the oxidation number of Cl equal to +1?
1. Cl2
3. AlCl3
2. Cl2O
4. HClO2
1
186. Given the reaction: . . . (see image)
Which substance undergoes oxidation?
1. Na
3. H2
2. NaOH
4. H2O
3
187. The potential energy diagram for a chemical reaction is shown in the accompanying graph.
Each interval on the axis labeled "Potential Energy (kJ)" represents 40 kilojoules. What is the heat of reaction?
1. -120 kJ
3. +40 kJ
2. -40 kJ
4. +160 kJ
3 Refer to the graph below:
Note that the difference between the reactants and products (labeled "Heat of Reaction") is one interval high. The magnitude of the heat of reaction is
40 kJ. Since the products have more energy than the reactants, this is an endothermic reaction. The heat of reaction is +40 kJ.
1
188. The activation energy of a chemical reaction can be decreased by the addition of
1. a catalyst
3. electrical energy
2. an indicator
4. thermal energy
1 Catalysts act to lower the activation energy of a reaction by providing an alternate pathway for the reaction.
3
189. Why can an increase in temperature lead to more effective collisions between reactant particles and an increase in the rate of a chemical
reaction?
1. The activation energy of the reaction increases.
3. The number of molecules with sufficient energy to react increases.
2. The activation energy of the reaction decreases.
4. The number of molecules with sufficient energy to react decreases.
3 According to the kinetic-molecular theory (KMT) of matter, the temperature of a sample of matter is a measure of the average kinetic energy of the
particles of the sample. As the temperature increases, the average kinetic energy of the reactant particles increases. This increase produces more
molecules with sufficient energy to react.
3
190. The entropy of a sample of H2O increases as the sample changes from a
1. gas to a liquid
3. liquid to a gas
2. gas to a solid
4. liquid to a solid
3 Entropy is a measure of disorder among a collection of particles. As a liquid changes to a gas, the particles become more randomly distributed and
the entropy of the system increases.
Wrong Choices Explained:
(1), (2), (4) In each of these changes, the particles become less randomly distributed and the entropy in each case decreases.
1
191. Each of four test tubes contains a different concentration of HCl(aq) at 25 oC. A 1-gram cube of Zn is added to each test tube. In which test tube
is the reaction occurring at the fastest rate?
1 The speed of the reaction between Zn(s) and HCl(aq) depends on several factors, including the concentration of the HCl solution. Choice (1), 1 M
HCl(aq), has the highest concentration of HCl and will yield the fastest rate.
2
192. As the temperature of a chemical reaction in the gas phase is increased, the rate of the reaction increases because
1. fewer particle collisions occur
3. the required activation energy increases
2. more effective particle collisions occur
4. the concentration of the reactants increases
2 Increasing the temperature of a chemical reaction increases the average kinetic energy of the reacting particles, resulting in an increase in the
number of particle collisions per unit of time. As the number of particle collisions increases, the number of effective collisions also increases.
4
193. The entropy of a sample of CO2 increases as the CO2 changes from
1. gas to liquid
3. liquid to solid
2. gas to solid
4. solid to gas
4 Entropy is the measure of randomness or disorder in a system. As the CO2 changes from solid to gas, the molecules can assume more random
arrangements. The entropy of the system increases.
3
194. Which two factors must be equal when a chemical reaction reaches equilibrium?
1. the concentration of the reactants and the concentration of the products 3. the rate of the forward reaction and the rate of the reverse reaction
2. the number of reactant particles and the number of product particles
4. the mass of the reactants and the mass of the products
3 Dynamic equilibrium is the state in which the rates of opposing processes are equal. In a chemical reaction, equilibrium occurs when the rate of the
forward reaction equals the rate of the reverse reaction.
3
195. Given the reaction at equilibrium: . . . (see image)
Which change will cause the equilibrium to shift?
1. increase in pressure
3. addition of heat
2. increase in volume
4. addition of a catalyst
2
196. Given the equilibrium at 101.3 kPa: . . . (see image)
At what temperature does this equilibrium occur?
1. 100 K
3. 298 K
2. 273 K
4. 373 K
1
197. Given the accompanying diagram that shows carbon dioxide in an equilibrium system at a temperature of 298 K and a pressure of 1 atm:
Which changes must increase the solubility of the carbon dioxide?
1. increase pressure and decrease temperature
3. decrease pressure and decrease temperature
2. increase pressure and increase temperature
4. decrease pressure and increase temperature
2
198. When a spark is applied to a mixture of hydrogen and oxygen, the gases react explosively. Which potential energy diagram best represents the
reaction?
2
199. Which equation shows an increase in entropy?
4
200. Given the reaction system in a closed container at equilibrium and at a temperature of 298 K: (see image)
The measurable quantities of the gases at equilibrium must be
1. decreasing
3. equal
2. increasing
4. constant
4 When a reaction system reaches equilibrium, the rates of the forward and reverse reactions are equal and the concentrations of the reactants and
products remain constant.
4
201. In each of the four beakers shown in the accompanying image, a 2.0-centimeter strip of magnesium ribbon reacts with 100 milliliters of HCl(aq)
under the conditions shown.
In which beaker will the reaction occur at the fastest rate?
1. A
3. C
2. B
4. D
4 The rate of a reaction depends on the concentration of the reactants and the temperature of the system: the more concentrated the reactant, the faster
the rate; the higher the temperature, the faster the rate. Of the choices given, choice (4), Beaker D, has the highest concentration of HCl and the
highest temperature.
4
202. Note: This question may require the use of the Reference Tables for Physical Setting/Chemistry.
At STP, a sample of which element has the highest entropy?
1. Na(s)
3. Br2(l)
2. Hg(l)
4. F2(g)
4 Entropy is a measure of disorder. A substance in the gas phase has the highest degree of disorder, while a solid has the lowest degree of disorder.
2
203. Which statement best describes how a catalyst increases the rate of a reaction?
1. The catalyst provides an alternate reaction pathway with a higher
activation energy.
3. The catalyst provides the same reaction pathway with a higher
activation energy.
2. The catalyst provides an alternate reaction pathway with a lower
activation energy.
4. The catalyst provides the same reaction pathway with a lower
activation energy.
2 The rate of a reaction can be increased by lowering its activation energy. A catalyst lowers the activation energy by providing an alternative and
more efficient pathway for the reaction.
4
204. A chemical reaction is at equilibrium. Compared to the rate of the forward reaction, the rate of the reverse reaction is
1. faster and more reactant is produced
3. the same and the reaction has stopped
2. faster and more product is produced
4. the same and the reaction continues in both directions
4 By definition, a state of chemical equilibrium exists when the rates of the opposing reactions (forward and reverse) are equal.
1
205. Given the reaction (see image) at equilibrium:
What occurs when the concentration of H2(g) is increased?
1. The rate of the forward reaction increases and the concentration of
N2(g) decreases.
3. The rate of the forward reaction and the concentration of N2(g) both
increase.
2. The rate of the forward reaction decreases and the concentration of
N2(g) increases.
4. The rate of the forward reaction and the concentration of N2(g) both
decrease.
1 According to Le Chatelier's principle, when an equilibrium system is subjected to a stress, the system will shift in order to relieve the stress. When
H2(g) is added to the system, the system shifts to the right in order to remove some of the added H2. As a result, the rate of the forward reaction
increases and thereby decreases the concentration of N2(g).
4
206. Given the accompanying potential energy diagram for a reaction:
Which interval on this diagram represents the difference between the potential energy of the products and the potential energy of the reactants?
1. 1
3. 3
2. 2
4. 4
4 Refer to the labeled accompanying diagram:
The difference between the potential energies of the products and reactants is also known as the heat of reaction.
2
207. Which list of the phases of H2O is arranged in order of increasing entropy?
1. ice, steam, and liquid water
3. steam, liquid water, and ice
2. ice, liquid water, and steam
4. steam, ice, and liquid water
2 Entropy is a measure of disorder. Gases have the most disorder and solids the least. The correct sequence in order of increasing entropy is ice
(solid), water (liquid), steam (gas).
3
208. Which statement must be true when solution equilibrium occurs?
1. The solution is at STP.
3. The concentration of the solution remains constant.
2. The solution is supersaturated.
4. The masses of the dissolved solute and the undissolved solute are
equal.
3 Equilibrium is defined as the state in which the rates of two opposing processes (that is, the forward and reverse processes) are equal. When a
solution reaches equilibrium, the rates of dissolving and crystallization are equal, and the concentration of the solution can no longer change.
4
209. Given the balanced equation representing a reaction:
Zn(s) + 2HCl(aq) → H2(g) + ZnCl2(aq)
Which set of reaction conditions produces H2(g) at the fastest rate?
1. a 1.0-g lump of Zn(s) in 50. mL of 0.5 M HCl(aq) at 20.oC
3. 1.0 g of powdered Zn(s) in 50. mL of 1.0 M HCl(aq) at 20.oC
2. a 1.0-g lump of Zn(s) in 50. mL of 0.5 M HCl(aq) at 30.oC
4. 1.0 g of powdered Zn(s) in 50. mL of 1.0 M HCl(aq) at 30. oC
4 Three factors that affect reaction rate are concentration, surface area, and temperature. As the concentration increases, the rate increases. Therefore,
a 1.0 M HCl solution will produce a faster reaction rate than a 0.5 M HCl solution. As the surface area increases, the rate increases. Therefore,
powdered zinc will produce a faster reaction rate than a lump of zinc. As the temperature increases, the rate increases. Therefore, a temperature of
30oC will produce a faster reaction rate than a temperature of 20 oC.
3
210. Given the equilibrium reaction in a closed system as shown:
What will be the result of an increase in temperature?
1. The equilibrium will shift to the left and [H2] will increase.
3. The equilibrium will shift to the right and [HI] will increase.
2. The equilibrium will shift to the left and [H2] will decrease.
4. The equilibrium will shift to the right and [HI] will decrease.
2
211. Which sample has the lowest entropy?
1
212. According to Table I, which potential energy diagram best represents the reaction that forms H 2O(l) from its elements?
3
213. Given the reaction (see image):
Once equilibrium is reached, which statement is accurate?
1. The concentration of Ag+(aq) is greater than the concentration of Cl(aq).
3. The rates of the forward and reverse reactions are equal.
2. The AgCl(s) will be completely consumed.
4. The entropy of the forward reaction will continue to decrease.
3 Equilibrium is defined as the condition in which the rates of the forward and reverse reactions are equal.
2
214. In most aqueous reactions as temperature increases, the effectiveness of collisions between reacting particles
1. decreases
3. remains the same
2. increases
2 Temperature is a measure of the average kinetic energy of the reacting particles. As the average kinetic energy increases (due to an increase in
temperature), the number of effective collisions between the reacting particles also increases.
2
215. Based on the nature of the reactants in each of the equations below, which reaction at 25°C will occur at the fastest rate?
1. C(s) + O2(g) --> CO2(g)
3. CH3OH(l) + CH3COOH(l) --> CH3COOCH3(aq) + H2O(l)
2. NaOH(aq) + HCl(aq) --> NaCl(aq) + H2O(l)
4. CaCO3(s) --> CaO(s) + CO2(g)
2 Systems containing dissolved ions will react very rapidly since no bonds need to be broken. In this reaction, the ions Na+(aq), OH-(aq), H+(aq), and
CL-(aq) undergo a double replacement at a very rapid rate.
3
216. Given the reaction at equilibrium (see image):
The concentration of A(g) can be increased by
1. lowering the temperature
3. increasing the concentration of AB(g)
2. adding a catalyst
4. increasing the concentration of B(g)
3 According to LeChâtelier's principle, an increase in concentration of the product (AB) will shift the reaction so that some of the product is removed.
In other words, the reaction will shift to the left. As a result, the concentration of A (and of B) will increase.
Wrong Choices Explained:
(1) Lowering the temperature will favor the exothermic reaction. The reaction will shift toward the right, leading to a decrease in the concentration of
A.
(2) Adding a catalyst will not shift the reaction in either direction.
(4) Increasing the concentration of B will shift the reaction toward the right, leading to a decrease in the concentration of A.
3
217. At STP, which 4.0-gram zinc sample will react fastest with dilute hydrochloric acid?
1. lump
3. powdered
2. bar
4. sheet metal
3 One of the factors that affect the rate of a reaction is the surface area of the reactants. Since powdered zinc has the greatest surface area, it will react
most rapidly with the hydrochloric acid.
4
218. Which information about a chemical reaction is provided by a potential energy diagram?
1. the oxidation states of the reactants and products
3. the change in solubility of the reacting substances
2. the average kinetic energy of the reactants and products
4. the energy released or absorbed during the reaction
4 Refer to the potential energy diagram shown (see image).
4
219. A catalyst works by
1. increasing the potential energy of the reactants
3. decreasing the potential energy of the products
2. increasing the energy released during a reaction
4. decreasing the activation energy required for a reaction
4 A catalyst increases the rate of a reaction by providing an alternative pathway for the reaction to occur. (This is somewhat like certain roads in
Europe that pass through mountains rather than go around them.) The alternative pathway has a lower activation energy than the original pathway.
As a result, molecules with lower kinetic energies are capable of reacting, and the speed of the reaction is increased.
1
220. Even though the process is endothermic, snow can sublime. Which tendency in nature accounts for this phase change?
1. a tendency toward greater entropy
3. a tendency toward less entropy
2. a tendency toward greater energy
4. a tendency toward less energy
1 Spontaneous processes depend on two factors: energy and entropy. Processes that are exothermic and lead to greater entropy will always be
spontaneous. Nevertheless, it is possible for an endothermic reaction (such as sublimation) to occur spontaneously if the entropy increase and/or the
temperature is sufficiently high.
2
221. Given the reaction at equilibrium:
Which change will result in a decrease in the amount of NO(g) formed?
1. decreasing the pressure
3. increasing the concentration of O2(g)
2. decreasing the concentration of N2(g)
4. increasing the temperature
2 When the concentration of N2(g) is decreased, the system will shift to the left in order to restore some of the removed N2(g). As a result, the
concentration of NO(g) will decrease.
Wrong Choices Explained:
(1) Since the equation has 2 moles of reactants and products, a change in pressure will leave the system unaffected.
(3) Increasing the O2(g) concentration will shift the reaction toward the right, leading to an increase in NO(g).
(4) The forward reaction is endothermic. An increase in temperature favors the endothermic reaction, leading to an increase in NO(g).
3
222. Given the balanced equation representing a reaction:
2HCl(aq) + Na2S2O3(aq) --> S(s) + H2SO3(aq) + 2NaCl(aq)
Decreasing the concentration of Na2S2O3(aq) decreases the rate of reaction because the
1. activation energy decreases
3. frequency of effective collisions decreases
2. activation energy increases
4. frequency of effective collisions increases
3 The rate of a chemical reaction depends on the frequency of effective collisions among the reacting particles. Any factor that decreases the collision
frequency will decrease the rate of the reaction. Such factors include decreasing the concentration of the reactants and lowering the temperature.
Wrong Choices Explained:
(1), (2) Activation energy does not depend on the concentration of the reactants. It depends on the nature of the reactants.
2
223. Given the equation representing a reaction at equilibrium:
Which change favors the reverse reaction?
1. decreasing the concentration of HI(g)
3. increasing the concentration of I2(g)
2. decreasing the temperature
4. increasing the pressure
2 According to Le Chatelier's principle, a decrease in temperature favors the exothermic reaction in a system at equilibrium. In this case, the reverse
reaction is exothermic.
Wrong Choices Explained:
(1), (3) Decreasing the concentration of HI(g) favors the forward reaction, as does increasing the I2(g) concentration.
(4) Increasing the pressure has no effect since there are 2 moles of reactants and 2 moles of products in the equation.
4
224. A chemical reaction between iron atoms and oxygen molecules can only occur if
1. the particles are heated
3. there is a catalyst present
2. the atmospheric pressure decreases
4. there are effective collisions between the particles
4 A chemical reaction occurs when the reacting particles are able to undergo effective collisions. An effective collision is one in which the colliding
particles have sufficient kinetic energies and are oriented properly toward one another.
4
225. Given the equation representing a phase change at equilibrium: (see accompanying image)
Which statement describes this equilibrium?
1. The H2O(s) melts faster than the H2O(l) freezes.
3. The mass of H2O(s) must equal the mass of H2O(l).
2. The H2O(l) freezes faster than the H2O(s) melts.
4. The mass of H2O(l) and the mass of H2O(s) each remain constant.
4 When a system reaches a state of equilibrium, the rates of the forward and reverse reactions become equal. In this example, the rate at which the ice
melts is equal to the rate at which the water freezes. As a result, the masses of the water and the ice become constant, though not necessarily equal to
each other.
4
226. In a chemical reaction, the difference between the potential energy of the products and the potential energy of the reactants is equal to the
1. activation energy
3. heat of fusion
2. entropy of the system
4. heat of reaction
4 Refer to the accompanying potential energy diagram. An inspection of this diagram reveals that the difference in potential energy between the
reactants and products is equal to the heat of reaction (ΔH). In this case, the heat of reaction is negative.
1
227. Given the potential energy diagram and equation representing the reaction between substances A and D: (see accompanying diagram)
According to Table I, substance G could be
1. HI(g)
3. CO2(g)
2. H2O(g)
4. C2H6(g)
1 The diagram in this question indicates that substance G has a higher potential energy than substances A and G. This reaction is endothermic and has
a positive ΔH. Use Reference Table I. Of the choices given, only the product in choice (1), HI(g), has a positive ΔH.
4
228. Given the equation representing a system at equilibrium: (see accompanying image)
Which changes occur when the temperature of this system is decreased?
1. The concentration of H2(g) increases and the concentration of N2(g)
increases.
3. The concentration of H2(g) decreases and the concentration of NH3(g)
decreases.
2. The concentration of H2(g) decreases and the concentration of N2(g)
increases.
4. The concentration of H2(g) decreases and the concentration of NH3(g)
increases.
4 In the forward reaction, energy appears as a product, i.e., it is an exothermic reaction. If the temperature of the system is lowered, Le Chatelier's
principle predicts that the system will shift to the right in order to produce more heat energy. As a result, the concentration of both H 2(g) and N2(g)
will decrease and the concentration of NH3(g) will increase.
2
229. Which statement correctly describes a chemical reaction at equilibrium?
1. The concentrations of the products and reactants are equal.
3. The rate of the forward reaction is less than the rate of the reverse
reaction.
2. The concentrations of the products and reactants are constant.
4. The rate of the forward reaction is greater than the rate of the reverse
reaction.
1
230. The solid and liquid phases of water can exist in a state of equilibrium at 1 atmosphere of pressure and a temperature of
1. 0oC
3. 273oC
2. 100oC
4. 373oC
4
231. Increasing the temperature increases the rate of a reaction by
1. lowering the activation energy
3. lowering the frequency of effective collisions between reacting
molecules
2. increasing the activation energy
4. increasing the frequency of effective collisions between reacting
molecules
3
232. Given the equation (see image) representing a phase change at equilibrium:
Which statement is true?
1. The forward process proceeds faster than the reverse process.
3. The forward and reverse processes proceed at the same rate.
2. The reverse process proceeds faster than the forward process.
4. The forward and reverse processes both stop.
3 Equilibrium is defined as the state in which the rates of two opposing processes (that is, the forward and reverse processes) are equal.
4
233. A 5.0-gram sample of zinc and a 50.-milliliter sample of hydrochloric acid are used in a chemical reaction. Which combination of these samples
has the fastest reaction rate?
1. a zinc strip and 1.0 M HCl(aq)
3. zinc powder and 1.0 M HCl(aq)
2. a zinc strip and 3.0 M HCl(aq)
4. zinc powder and 3.0 M HCl(aq)
4 Two factors that affect reaction rate are concentration and surface area. As the concentration increases, the rate increases. Therefore, a 3.0 M HCl
solution will produce a faster reaction rate than will 1.0 M HCl. As the surface area increases, the reaction rate increases. Therefore, powdered zinc
will produce a faster reaction rate than a zinc strip.
2
234. For a given reaction, adding a catalyst increases the rate of the reaction by
1. providing an alternate reaction pathway that has a higher activation
energy
3. using the same reaction pathway and increasing the activation energy
2. providing an alternate reaction pathway that has a lower activation
energy
4. using the same reaction pathway and decreasing the activation energy
2 Activation energy is the minimum energy needed to produce a reaction. As the activation energy of a reaction is lowered, the rate of the reaction
will increase because more of the particles have the necessary minimum energy. A catalyst lowers the activation energy by providing an alternate
pathway for the reaction.
3
235. Given the equation representing a reaction at equilibrium:
Which change causes the equilibrium to shift to the right?
1. decreasing the concentration of H2(g)
3. increasing the concentration of N2(g)
2. decreasing the pressure
4. increasing the temperature
3 When the concentration of N2(g) is increased, the system will shift to the right in order to remove some of the added N2.
Wrong Choices Explained:
(1) When the concentration of H2(g) is decreased, the system will shift to the left in order to add back some of the removed H 2.
(2) When the pressure is decreased (by increasing the volume of the container), the system will shift to the left in order to produce a greater number
of molecules.
(4) The forward reaction is exothermic, and the reverse reaction is endothermic. When the temperature is increased, the endothermic reaction is
favored, which causes the system to shift to the left.
1
236. How is a chemical reaction affected by the addition of a catalyst?
1. The activation energy decreases.
3. The number of collisions between particles decreases.
2. The heat of reaction increases.
4. The potential energy of the reactants increases.
1 A catalyst increases the rate of a chemical reaction by providing an alternative pathway that has a lower activation energy than the uncatalyzed
reaction.
2
237. Systems in nature tend to undergo changes toward
1. lower energy and less disorder
3. higher energy and less disorder
2. lower energy and more disorder
4. higher energy and more disorder
2 In order to achieve stability, a system needs its energy as low as possible and as much disorder as possible. Therefore, processes tend to move
toward lower energy and more disorder.
1
238. Which 1-mole sample has the least entropy?
1 The entropy (disorder) of the particles of a substance depends on the phase of the substance. A solid has the least entropy, and a gas has the greatest
entropy.
2
239. At 20.oC, a 1.2-gram sample of Mg ribbon reacts rapidly with 10.0 milliliters of 1.0 M HCl(aq). Which change in conditions would have caused
the reaction to proceed more slowly?
1. increasing the initial temperature to 25oC
3. using 1.2 g of powdered Mg
2. decreasing the concentration of HCl(aq) to 0.1 M
4. using 2.4 g of Mg ribbon
2 Three factors that affect reaction rate are concentration, surface area, and temperature. As the concentration increases, the rate increases. Therefore,
a 0.1 M solution of HCl will produce a slower rate than a 1.0 M solution of HCl.
Wrong Choices Explained:
(1) If the temperature is increased, the rate will increase.
(3) If the surface area is increased, the rate will increase.
(4) Using more Mg ribbon will have no affect on the rate since the concentration of the magnesium is unchanged.
4
240. Changes in activation energy during a chemical reaction are represented by a
1. cooling curve
3. ionization energy diagram
2. heating curve
4. potential energy diagram
4 A potential energy diagram traces a chemical reaction from reactants to products. The potential energy diagram shown illustrates the quantities that
can be determined during the course of a reaction:
Wrong Choices Explained:
(1), (2) Cooling and heating curves trace the phases and the phase changes of single substances.
2
241. Given the equation representing a reaction:
Which statement describes this reaction at equilibrium?
1. The concentration of N2O4(g) must equal the concentration of NO2(g).
3. The rate of the forward reaction is greater than the rate of the reverse
reaction.
2. The concentration of N2O4(g) and the concentration of NO2(g) must be 4. The rate of the reverse reaction is greater than the rate of the forward
constant.
reaction.
2 At equilibrium, the rates of the forward and reverse reactions are equal. As a result, the concentrations of N 2O4(g) and NO2(g) do not change.
1
242. A catalyst increases the rate of a chemical reaction by
1. lowering the activation energy of the reaction
3. raising the temperature of the reactants
2. lowering the potential energy of the products
4. raising the concentration of the reactants
1 The activation energy is the minimum energy needed by the reactants to begin a chemical reaction. A catalyst provides an alternative path with a
lower activation energy for the reaction. As a result, more atoms can react in a given amount of time. In other words, the rate of the reaction
increases.
3
243. A solution that is at equilibrium must be
1. concentrated
3. saturated
2. dilute
4. unsaturated
3 When a solution is in equilibrium, the rate at which excess solute dissolves must equal the rate at which the dissolved solute leaves the solution.
Since these two rates are equal, dissolving additional solute at a given temperature and pressure is not possible. In other words, the solution is
saturated.
2
244. Given the reaction:
Which change would cause an immediate increase in the rate of the forward reaction?
1. increasing the concentration of NO(g)
3. decreasing the reaction temperature
2. increasing the concentration of N2(g)
4. decreasing the reaction pressure
2 Increasing the concentration of N2(g) would increase the number of effective collisions between N2(g) and O2(g), leading to an increase in the rate
of the forward reaction.
Wrong Choices Explained:
(1) Increasing the concentration of NO(g) would lead to an increase in the rate of the reverse reaction.
(3) Decreasing the temperature always decreases the rates of both the forward and the reverse reactions.
(4) Decreasing the reaction pressure effectively decreases the concentrations of (gaseous) reactants and products. As a result, the rates of the forward
and reverse reactions would decrease.
1
245. Which 10-milliliter sample of water has the greatest degree of disorder?
1. H2O(g) at 120°C
3. H2O(l) at 20°C
2. H2O(l) at 80°C
4. H2O(s) at 0°C
1 A substance in the gas phase always has a greater degree of disorder than the same substance in either the solid or liquid phases. If two samples of a
substance are at different temperatures, the sample at the higher temperature has the greater disorder.
3
246. Which statement must be true about a chemical system at equilibrium?
1. The forward and reverse reactions stop.
3. The rate of the forward reaction is equal to the rate of the reverse
reaction.
2. The concentration of reactants and products are equal.
4. The number of moles of reactants is equal to the number of moles of
product.
3 A chemical system is considered to be at equilibrium when the rates of its forward and reverse reactions are equal.
2
247. Adding a catalyst to a chemical reaction results in
1. a decrease in activation energy and a decrease in the reaction rate
3. an increase in activation energy and a decrease in the reaction rate
2. a decrease in activation energy and an increase in the reaction rate
4. an increase in activation energy and an increase in the reaction rate
2 A catalyst increases the rate of a reaction by providing an alternative pathway for the reaction to occur. (This is somewhat like certain roads in
Europe that pass through mountains rather than go around them.) The alternative pathway has a lower activation energy than the original pathway. As
a result, molecules with lower kinetic energies are capable of reacting and the speed of the reaction is increased.
2
248. Systems in nature tend to undergo changes toward
1. lower energy and lower entropy
3. higher energy and lower entropy
2. lower energy and higher entropy
4. higher energy and higher entropy
2 Changes in nature are directed by two factors: energy and entropy. Systems in nature will tend to undergo spontaneous changes if the energy of the
system is decreased and the entropy (disorder) of the system is increased.
1
249. Given the balanced equation shown in the diagram:
Which statement best describes this process?
1. It is endothermic and entropy increases.
3. It is exothermic and entropy increases.
2. It is endothermic and entropy decreases.
4. It is exothermic and entropy decreases.
1 Since 34.89 kJ appears on the left side of the equation, it indicates that heat has been absorbed by the system. This is an endothermic reaction. In
the dissolving process, the KNO3 crystal is separated into ions, which are more randomly arranged in the solution than in the crystal. Therefore, the
entropy of the system increases.
4
250. A 1.0-gram piece of zinc reacts with 5 milliliters of HCl(aq). Which of these conditions of concentration and temperature would produce the
greatest rate of reaction?
1. 1.0 M HCl(aq) at 20.oC
o
2. 1.0 M HCl(aq) at 40. C
3. 2.0 M HCl(aq) at 20.oC
4. 2.0 M HCl(aq) at 40.oC
4 Reaction rates depend on temperature and the concentration of reactants. A reaction that occurs at 40°C will be faster than the reaction at 20°C.
Furthermore, 2.0 M HCl is more concentrated than 1.0 M HCl. Using 2.0 M HCl will produce a faster reaction than using 1.0 M HCl.
5
251. Which expression represents the ΔH for a chemical reaction in terms of the potential energy, PE, of its products and reactants?
1. PE of products + PE of reactants
3. PE of products × PE of reactants
2. PEof products - PE of reactants
4. PE of products ÷ PE of reactants
2 The heat of a reaction, ΔH, is the difference between the potential energies of the products and reactants.
2
252. Which factors must be equal in a reversible chemical reaction at equilibrium?
1. the activation energies of the forward and reverse reactions
3. the concentrations of the reactants and products
2. the rates of the forward and reverse reactions
4. the potential energies of the reactants and products
2 When a reaction system reaches equilibrium, the rates of the forward and reverse reactions are equal.
3
253. A closed container holds 3.0 moles of CO2 gas at STP. What is the total number of moles of Ne(g) that can be placed in a container of the same
size at STP?
1. 1.0 mole
3. 3.0 moles
2. 1.5 moles
4. 0.0 moles
3
254. Which statement best describes a chemical reaction when it reaches equilibrium?
1. The concentrations of reactants and products are the same.
3. The forward and reverse reaction rates are the same.
2. The concentrations of the reactants decrease to zero.
4. The forward reaction rate decreases to zero.
1
255. Which reaction has the greatest increase in entropy?
1
256. In a potential energy diagram, the difference between the potential energy of the products and the potential energy of the reactants is equal to the
1. heat of reaction
3. activation energy of the forward reaction
2. entropy of the reaction
4. activation energy of the reverse reaction
2
257. As the concentration of reacting particles increases, the rate of reaction generally
1. decreases
3. remains the same
2. increases
2
258. As the temperature of a gas increases at constant pressure, the volume of the gas
1. decreases
2. increases
4
3. remains the same
259. Based on Reference Table H, which sample has the highest vapor pressure?
1. water at 20oC
3. ethanol at 50oC
2. water at 80oC
4. ethanol at 65oC
1
260. Which sample has the greatest entropy?
1. NH3(g)
3. NH3(s)
2. NH3(l)
4. NH3(aq)
3
261. The activation energy required for a chemical reaction can be decreased by
1. increasing the surface area of the reactant
3. adding a catalyst to the reaction
2. increasing the temperature of the reactant
4. adding more reactant
2
262. Given the potential energy diagram of a chemical reaction: . . . (see image)
Which arrow represents the potential energy of the reactants?
1. A
3. C
2. B
4. D
2
263. Given the reaction at equilibrium: . . . (see image)
Which change will shift the equilibrium to the right?
1. increasing the temperature
3. decreasing the amount of SO2(g)
2. increasing the pressure
4. decreasing the amount of O2(g)
3
264. What occurs when the temperature is increased in a system at equilibrium at constant pressure?
1. The rate of the forward reaction increases, and the rate of the reverse
reaction decreases.
3. The rate of the endothermic reaction increases.
2. The rate of the forward reaction decreases, and the rate of the reverse
reaction increases.
4. The rate of the exothermic reaction decreases.
1
265. Ammonia is produced commercially by the Haber reaction: . . . (see image)
The formation of ammonia is favored by
1. an increase in pressure
3. removal of N2(g)
2. a decrease in pressure
4. removal of H2(g)
1
266. Given the equilibrium reaction at STP: . . . (see image)
Which statement correctly describes this system?
1. The forward and reverse reaction rates are equal.
3. The concentrations of N2O4 and NO2 are equal.
2. The forward and reverse reaction rates are both increasing.
4. The concentrations of N2O4 and NO2 are both increasing.
2
267. Which process is accompanied by a decrease in entropy?
1. boiling of water
3. subliming of iodine
2. condensing of water vapor
4. melting of ice
1
268. A catalyst is added to a system at equilibrium. If the temperature remains constant, the activation energy of the forward reaction
1. decreases
3. remains the same
2. increases
2
269. As carbon dioxide sublimes, its entropy
1. decreases
3. remains the same
2. increases
3
270. Given the reaction at 25oC:
Zn(s) + 2 HCl(aq) --> ZnCl2(aq) + H2(g)
The rate of this reaction can be increased by using 5.0 grams of powdered zinc instead of a 5.0-gram strip of zinc because the powdered zinc has
1. lower kinetic energy
3. more surface area
2. lower concentration
4. more zinc atoms
3 Zinc powder has a much larger surface area than a strip of zinc. As a result, many more atoms of zinc are exposed to the HCl when the zinc is in the
powdered form.
Wrong Choice Explained:
(4) The powdered zinc has the same number of atoms as the zinc strip since both have a mass of 5.0 grams. However, the powder has more exposed
atoms than the strip.
3
271. Which statement about a system at equilibrium is true?
1. The forward reaction rate is less than the reverse reaction rate.
3. The forward reaction rate is equal to the reverse reaction rate.
2. The forward reaction rate is greater than the reverse reaction rate.
4. The forward reaction rate stops and the reverse reaction rate continues.
3 In chemistry, equilibrium is defined as the state that exists when the forward and reverse reaction rates are equal.
3
272. A compound has a molar mass of 90. grams per mole and the empirical formula CH2O. What is the molecular formula of this compound?
1. CH2O
3. C3H6O3
2. C2H4O2
4. C4H8O4
3 Use the Periodic Table of the Elements. The molar mass of the empirical formula is 30. grams per mole (12 + 2 + 16). Divide this mass into the
molar mass of the compound:
90. g/mol / 30. g/mol = 3
The formula of the compound must be 3 times the empirical formula: C3H6O3.
1
273. Matter that is composed of two or more different elements chemically combined in a fixed proportion is classified as
1. a compound
3. a mixture
2. an isotope
4. a solution
1 A compound is defined as the chemical combination of two or more elements in a fixed proportion.
1
274. Given the balanced equation representing a reaction:
2CO(g) + O2(g) → 2CO2(g)
What is the mole ratio of CO(g) to CO2(g) in this reaction?
1. 1:1
3. 2:1
2. 1:2
4. 3:2
1 The coefficients of a balanced chemical reaction determine the mole ratios of the substances taking part in the reaction. Since the respective
coefficients of CO(g) and CO2(g) are 2 and 2, the mole ratios of these substances are 2:2 or, more simply, 1:1.
4
275. Given the balanced equation representing a reaction:
H+(aq) + OH-(aq) → H2O(l ) + 55.8 kJ
In this reaction there is conservation of
1. mass, only
3. mass and energy, only
2. mass and charge, only
4. mass, charge, and energy
4 Every balanced equation must conserve mass, charge, and energy.
1
276. Which list of formulas represents compounds, only?
1. CO2, H2O, NH3
3. H2, Ne, NaCl
2. H2, N2, O2
4. MgO, NaCl, O2
1 A compound consists of a chemical combination of two or more elements in which the composition of each element is fixed. Of the choices given,
only choice (1), CO2, H2O, and NH3, meets these criteria.
Wrong Choices Explained:
(2) H2, N2, and O2 are all elements.
(3) H2 and Ne are elements.
(4) O2 is an element.
2
277. What is the chemical formula for iron(III) oxide?
1. FeO
3. Fe3O
2. Fe2O3
4. Fe3O2
2 Use the Periodic Table of the Elements. The charge of the oxide ion is 2–; the charge of the iron(III) ion is 3+. In an ionic compound such as
iron(III) oxide, the sum of the charges must equal 0. This occurs in only choice (2), Fe2O3.
4
278. Which equation shows conservation of mass and charge?
1. NH4Br --> NH3 + Br2
3+
2+
2. 2Mg + Fe --> Mg + 3Fe
3. H2SO4 + LiOH --> Li2SO4 + H2O
4. Cu + 2Ag+ --> Cu2+ + 2Ag
4 The equation in choice (4) is balanced for mass since 1 mole of Cu and 2 moles of Ag are present on both sides of the equation. Similarly, this
equation is balanced for charge since the total charge on each side of the equation equals +2.
Wrong Choices Explained:
(1), (3) These equations are balanced for charge but not for mass.
(2) This equation is not balanced for either charge or mass.
2
279. What is the percent composition by mass of hydrogen in NH4HCO3 (gram-formula mass = 79 grams/mole)?
1. 5.1%
3. 10.%
2. 6.3%
4. 50.%
2 First let us construct a table to calculate the molar mass of NH4HCO3:
Now use the percent composition formula on Reference Table T:
% composition by mass = (mass of part)/(mass of whole) X 100%
% composition of hydrogen by mass = 5.0 g/mol / 79.0 g/mol X 100% = 6.3%
1
280. Which equation represents a decomposition reaction?
1 In a decomposition reaction, one reactant forms two or more simpler products. Of the choices given, only the reaction shown in choice (1) meets
this criterion.
Wrong Choices Explained:
(2) This is a single replacement reaction.
(3) This is a synthesis reaction.
(4) This is a double replacement reaction.
2
281. A compound has the empirical formula CH2O and a gram-formula mass of 60. grams per mole. What is the molecular formula of this
compound?
1. CH2O
3. C3H8O
2. C2H4O2
4. C4H8O4
2 The empirical formula CH2O has a molar mass of 30. grams per mole (12 + 2 + 16 grams per mole). Since the molar mass of the compound is twice
that of its empirical formula, the molecular formula must be twice that of the empirical formula, C2H4O2.
4
282. Which formula represents strontium phosphate?
1. SrPO4
3. Sr2(PO4)3
2. Sr3PO8
4. Sr3(PO4)2
4 Use the Periodic Table of the Elements and Reference Table E. Strontium ions have a charge of 2+ (Sr2+), and phosphate ions have a charge of 3−
(PO43−). In order to be neutral, there must be 3 strontium ions for every 2 phosphate ions, Sr 3(PO4)2.
4
283. Which Lewis electron-dot diagram represents calcium oxide?
4 Lewis electron-dot diagrams for simple positive ions (such as Ca2+) are drawn without their valence electrons. Lewis electron-dot diagrams for
simple negative ions (such as O2−) are drawn with all 8 valence electrons and enclosed in brackets. Of the choices given, the Lewis electron-dot
diagram shown in choice (4) meets both criteria.
2
284. Which type of matter is composed of two or more elements that are chemically combined in a fixed proportion?
1. solution
3. homogeneous mixture
2. compound
4. heterogeneous mixture
2 A compound is defined as a combination of two or more elements present in a fixed proportion.
2
285. A balanced equation representing a chemical reaction can be written using
1. chemical formulas and mass numbers
3. first ionization energies and mass numbers
2. chemical formulas and coefficients
4. first ionization energies and coefficients
2 A balanced chemical equation must contain the chemical formulas of those substances that participate in the reaction, as well as the numerical
coefficients to demonstrate the conservation of mass and charge.
1
286. Every water molecule has two hydrogen atoms bonded to one oxygen atom. This fact supports the concept that elements in a compound are
1. chemically combined in a fixed proportion
3. physically mixed in a fixed proportion
2. chemically combined in proportions that vary
4. physically mixed in proportions that vary
1 A compound is defined as a substance in which the elements present are chemically combined in a fixed proportion. This definition allows a
chemical formula to be written for each compound.
2
287. The percent composition by mass of nitrogen in NH4OH (gram-formula mass = 35 grams/mole) is equal to
1. 1
3. 3
2. 2
4. 4
2 The mass of nitrogen in NH4OH is 14. Use the percent composition formula given on Reference Table T: (see Image)
(By the way, NH4OH is a compound that does not exist!)
2
288. Given the balanced equation representing a reaction:
2NaCl(l) → 2Na(l) + Cl2(g)
A 1170.-gram sample of NaCl(l) completely reacts, producing 460. grams of Na(l). What is the total mass of Cl2(g) produced?
1. 355 g
3. 1420. g
2. 710. g
4. 1630. g
2 This is simply a problem about the conservation of mass: The total mass of the reactant (1170. grams) must equal the total masses of the products.
Since the Na accounts for 460. grams, the mass of the Cl2 must be the difference between 1170. grams and 460. grams, or 710. grams.
1
289. Given the accompanying formula representing a hydrocarbon:
The molecular formula and the empirical formula for this hydrocarbon are
1. C5H10 and CH2
3. C4H8 and CH2
2. C5H10 and CH3
4. C4H8 and CH3
1 In order to determine the molecular formula, count the total number of carbon and hydrogen atoms. The molecular formula is C 5H10. In order to
determine the empirical formula, reduce the molecular formula to smallest whole numbers by dividing by 5. The empirical formula is CH2.
2
290. Which polyatomic ion contains the greatest number of oxygen atoms?
1. acetate
3. hydroxide
2. carbonate
4. peroxide
2 See Reference Table E. Choice (2), the carbonate ion, CO32–, contains 3 oxygen atoms, the greatest number of those given in the question.
Wrong Choices Explained:
(1) The acetate ion, C2H3O2–, contains 2 oxygen atoms.
(3) The hydroxide ion, OH–, contains 1 oxygen atom.
(4) The peroxide ion, O22–, contains 2 oxygen atoms.
4
291. Which formula represents an ionic compound?
1. H2
3. CH3OH
2. CH4
4. NH4Cl
4 The compound NH4Cl is composed of NH4+ and Cl– ions.
Wrong Choices Explained:
(1) H2 is the diatomic molecule of the element hydrogen.
(2), (3) CH4 and CH3OH are molecular (covalent) compounds.
2
292. An ion of which element has a larger radius than an atom of the same element?
1. aluminum
3. magnesium
2. chlorine
4. sodium
2 When an atom forms a negative ion, the radius of the ion is larger than the radius of its parent atom. Of the choices given, only choice (2), chlorine,
forms a negative ion.
3
293. What is the total number of different elements present in NH4NO3?
1. 7
3. 3
2. 9
4. 4
3 There are three elements in the compound: N, H, and O.
Wrong Choice Explained:
(2) You counted the total number of atoms in the compound.
1
294. Given the balanced equation representing a reaction:
Cl2(g) → Cl(g) + Cl(g)
What occurs during this change?
1. Energy is absorbed and a bond is broken.
3. Energy is released and a bond is broken.
2. Energy is absorbed and a bond is formed.
4. Energy is released and a bond is formed.
1 When a chlorine molecule (Cl2) dissociates, the single bond is broken and a pair of chlorine atoms are formed: (see image)
Anytime a bond is broken, energy is absorbed.
3
295. What is the net charge on an ion that has 9 protons, 11 neutrons, and 10 electrons?
1. 1+
3. 1-
2. 2+
4. 2-
3 Each proton contributes a charge of 1+, and each electron contributes a charge of 1–. Neutrons have no effect on the charge. The charge on the ion
is 1– : (9+) + (10–).
3
296. At standard pressure, a certain compound has a low boiling point and is insoluble in water. At STP, this compound most likely exists as
1. ionic crystals
3. nonpolar molecules
2. metallic crystals
4. polar molecules
3 Nonpolar molecules have low boiling points because the intermolecular forces among these molecules are weak. Moreover, nonpolar compounds
are insoluble in polar liquids such as water because the intermolecular forces of both substances are too dissimilar.
1
297. Given the formula of a substance:
What is the total number of shared electrons in a molecule of this substance?
1. 22
3. 9
2. 11
4. 6
1 Each line in the formula represents a pair of shared electrons. Since there are 11 lines in the formula, a total of 22 electrons must be shared.
3
298. Atoms of which element have the greatest tendency to gain electrons?
1. bromine
3. fluorine
2. chlorine
4. iodine
3 The tendency of an atom to gain electrons can be determined by inspecting the electronegativity value of the atom: the higher the electronegativity,
the greater the tendency to gain electrons. See Reference Table S. Of the choices given, choice (3), fluorine, has the highest electronegativity value.
1
299. An atom of which element has the greatest attraction for the electrons in a bond with a hydrogen atom?
1. chlorine
3. silicon
2. phosphorus
4. sulfur
1 The attraction for electrons in a chemical bond between two atoms is determined by the difference in the electronegativities of the two atoms. See
Reference Table S. Of the four choices given, choice (1), chlorine, has the highest electronegativity and, therefore, the greatest attraction for electrons
in a bond with a hydrogen atom.
2
300. Which property could be used to identify a compound in the laboratory?
1. mass
3. temperature
2. melting point
4. volume
2 Any property that can be used to identify a substance in a laboratory must be unique to that substance under a given set of conditions. Such
properties include melting point, boiling point, density, and solubility. In contrast, mass, temperature, and volume can be common to any number of
substances at a given time and cannot be used to identify the particular substance.
3
301. Which statement describes what occurs as two atoms of bromine combine to become a molecule of bromine?
1. Energy is absorbed as a bond is formed.
3. Energy is released as a bond is formed.
2. Energy is absorbed as a bond is broken.
4. Energy is released as a bond is broken.
3 When two atoms such as bromine form a molecule, a chemical bond forms between the atoms. As a result, the potential energy of the molecule is
lower than the potential energy of the two individual atoms and energy is released.
4
302. Given a formula for oxygen(see image):
What is the total number of electrons shared between the atoms represented in this formula?
1. 1
3. 8
2. 2
4. 4
4 In a covalent bond between two atoms, each pair of shared electrons is represented by a horizontal line. The double bond between the two oxygen
atoms in this question represents the four electrons shared by the two atoms.
3
303. Which formulas represent two polar molecules?
1. CO2 and HCl
3. H2O and HCl
2. CO2 and CH4
4. H2O and CH4
3 Polar molecules contain at least one polar bond and are asymmetrical. Of the choices given, choice (3), H2O and HCl, meet both criteria.
Wrong Choices Explained:
(1) CO2 contains 2 polar bonds. However, its linear shape makes it symmetrical and therefore nonpolar.
(2), (4) CH4 contains 4 polar bonds. However, its tetrahedral shape makes it symmetrical and therefore nonpolar.
1
304. What is the total number of valence electrons in a sulfide ion in the ground state?
1. 8
3. 16
2. 2
4. 18
1 A sulfur (S) atom has 6 valence electrons since it is located in Group 16 of the Periodic Table of the Elements. An atom of sulfur gains 2 electrons
when it becomes a sulfide ion. Therefore, a sulfide ion has 8 valence electrons.
1
305. Which type of substance can conduct electricity in the liquid phase but not in the solid phase?
1. ionic compound
3. metallic element
2. molecular compound
4. nonmetallic element
1 In order to conduct electricity, mobile charges must be present. In the solid phase, the ions in ionic compounds are rigidly held in place and the
substance cannot conduct. When melted, the ions are released and the substance can conduct electricity since the ions have become mobile.
Wrong Choices Explained:
(2), (4) Molecular compounds and nonmetallic elements do not possess mobile charges. They do not conduct electricity in either the solid or the
liquid phases.
(3) Metallic elements possess mobile electrons in their structures. They conduct electricity in both the solid and the liquid phases.
1
306. Why is a molecule of CO2 nonpolar even though the bonds between the carbon atom and the oxygen atoms are polar?
1. The shape of the CO2 molecule is symmetrical.
3. The CO2 molecule has a deficiency of electrons.
2. The shape of the CO2 molecule is asymmetrical.
4. The CO2 molecule has an excess of electrons.
1 Molecules with polar bonds are nonpolar if their bonds are arranged symmetrically. The two C=O bonds in CO 2 are symmetrically arranged to form
a linear molecule.
1
307. Which formula represents a molecular compound?
1. HI
3. KCl
2. KI
4. LiCl
1 A binary compound is molecular when the electronegativity difference between its elements is less than 1.7. Use Reference Table S. Of the choices
given, only choice (1), HI, has an electronegativity difference less than 1.7.
2
308. Given the accompanying formula representing a molecule:
The molecule is
1. symmetrical and polar
3. asymmetrical and polar
2. symmetrical and nonpolar
4. asymmetrical and nonpolar
2 The formula represents the compound ethyne, a linear molecule. Since the molecule is symmetrical, the polarities of its individual bonds cancel
each other. So the molecule is nonpolar.
4
309. Which compound has both ionic and covalent bonds?
1. CO2
3. NaI
2. CH3OH
4. Na2CO3
4 See Reference Table E. Generally, compounds that contain polyatomic ions have both ionic and covalent bonding. The covalent bonding exists
within the polyatomic ion. Only choice (4), Na2CO3, is an ionic compound that joins Na+ to the polyatomic ion CO32–.
2
310. Which Group 15 element exists as diatomic molecules at STP?
1. phosphorus
3. bismuth
2. nitrogen
4. arsenic
2 Seven common elements exist as diatomic molecules: H2, O2, N2, F2, Cl2, Br2, and I2. Only N2 is located in Group 15.
4
311. What is the total number of electrons shared in a double covalent bond?
1. 1
3. 3
2. 2
4. 4
4 In a double covalent bond, two pairs or 4 electrons are shared between two atoms.
3
312. Given the balanced equation representing a reaction in the accompanying diagram:
Br2 + energy → Br + Br
Which statement describes the energy change and bonds in this reaction?
1. Energy is released as bonds are broken.
3. Energy is absorbed as bonds are broken.
2. Energy is released as bonds are formed.
4. Energy is absorbed as bonds are formed.
3 Note that the energy term appears as a reactant. This indicates that energy is absorbed by each Br 2 molecule. Note also that the product consists of 2
Br atoms. This indicates that the single bond in each Br2 molecule has been broken.
4
313. The bond between which two atoms is most polar?
1. Br and Cl
3. I and Cl
2. Br and F
4. I and F
4 The polarity of the bond between two atoms can be estimated by determining the difference in the electronegativities of the two atoms. Use
Reference Table S. Of the choices given, choice (4), I and F, has the greatest difference in electronegativity. Therefore, the bond between I and F is
the most polar.
3
314. In the formula X2(SO4)3, the X represents a metal. This metal could be located on the Periodic Table in
1. Group 1
3. Group 13
2. Group 2
4. Group 14
3 Use Reference Table E. The sulfate ion has a charge of 2–(SO42–). Since 3 sulfate ions are present in the compound, the total charge from sulfate
must be 6–. The 2 ions of element X must therefore have a total charge of 6+. So, each ion of X must carry a charge of 3+ (X3+). Refer to the Periodic
Table of the Elements. Of the choices given, only Group 13 contains metallic elements whose ions carry a charge of 3+.
4
315. Which element forms an ionic compound when it reacts with lithium?
1. K
3. Kr
2. Fe
4. Br
4 Lithium (Li, atomic number 3) is a Group 1 element that forms a positive ion. In order to form an ionic compound, it must combine with a negative
ion. Use the Periodic Table of the Elements. Of the choices given, only choice (4), Br (atomic number 35), forms a negative ion.
2
316. What was concluded about the structure of the atom as the result of the gold foil experiment?
1. A positively charged nucleus is surrounded by positively charged
particles.
3. A negatively charged nucleus is surrounded by positively charged
particles.
2. A positively charged nucleus is surrounded by mostly empty space.
4. A negatively charged nucleus is surrounded by mostly empty space.
2 An English physicist, Ernest Rutherford, beamed alpha particles at thin gold foils. When he examined the scattering patterns of the alpha particles,
he concluded that most of the volume of the atom was empty space and that most of the mass of the atom was concentrated in a dense, positively
charged nucleus.
1
317. An atom is electrically neutral because the
1. number of protons equals the number of electrons
3. ratio of the number of neutrons to the number of electrons is 1:1
2. number of protons equals the number of neutrons
4. ratio of the number of neutrons to the number of protons is 2:1
1 Protons and electrons have equal but opposite charges, and neutrons have no charge. An atom is neutral because the number of protons equals the
number of electrons.
2
318. How do the energy and the most probable location of an electron in the third shell of an atom compare to the energy and the most probable
location of an electron in the first shell of the same atom?
1. In the third shell, an electron has more energy and is closer to the
nucleus.
3. In the third shell, an electron has less energy and is closer to the
nucleus.
2. In the third shell, an electron has more energy and is farther from the
nucleus.
4. In the third shell, an electron has less energy and is farther from the
nucleus.
2 As a shell number increases, the energy of an electron occupying the shell increases and the probability is that the electron will be located farther
from the nucleus.
3
319. Which two particles make up most of the mass of a hydrogen-2 atom?
1. electron and neutron
3. proton and neutron
2. electron and proton
4. proton and positron
3 The nucleus contains most of the mass of an atom. The nucleus of a hydrogen-2 atom contains 1 proton and 1 neutron.
4
320. In the wave-mechanical model of the atom, orbitals are regions of the most probable locations of
1. protons
3. neutrons
2. positrons
4. electrons
4 According to the wave-mechanical model of the atom, locating the exact position of an electron around the nucleus is not possible. One can
calculate only the probability that an electron will most likely be found within a certain region in space. These regions are called orbitals.
3
321. Which phrase describes an atom?
1. a positively charged electron cloud surrounding a positively charged
nucleus
3. a negatively charged electron cloud surrounding a positively charged
nucleus
2. a positively charged electron cloud surrounding a negatively charged
nucleus
4. a negatively charged electron cloud surrounding a negatively charged
nucleus
3 An atom is a neutral particle that contains equal numbers of protons and electrons. The protons are located within the positively charged nucleus,
and the negatively charged electrons are located in a cloud that surrounds the nucleus.
1
322. Which total mass is the smallest?
1. the mass of 2 electrons
3. the mass of 1 electron plus the mass of 1 proton
2. the mass of 2 neutrons
4. the mass of 1 neutron plus the mass of 1 electron
1 Protons and neutrons each have an approximate mass of 1 atomic mass unit. The mass of an electron is approximately 0.0005 atomic mass unit.
Therefore, of the choices given, the mass of 2 electrons is the smallest.
2
323. What is the total number of protons in an atom with the electron configuration 2-8-18-32-18-1?
1. 69
3. 118
2. 79
4. 197
2 An atom is a neutral particle containing an equal number of protons and electrons. The sum of the number of electrons in the configuration 2-8-1832-18-1 adds to 79; this is also the number of protons present.
4
324. Which particle has the least mass?
4 Use Reference Table O. Of the choices given, choice (4), 0 -1He, which is an electron, is the least massive particle.
Wrong Choices Explained:
(1) The symbol 4 2He represents an alpha particle, which is approximately 8,000 times more massive than an electron.
(2) The symbol 1 1H represents a proton, which is approximately 2,000 times more massive than an electron.
(3) The symbol 1 0n represents a neutron, which is approximately 2,000 times more massive than an electron.
3
325. What information is necessary to determine the atomic mass of the element chlorine?
1. the atomic mass of each artificially produced isotope of chlorine, only
3. the atomic mass and the relative abundance of each naturally occurring
isotope of chlorine
2. the relative abundance of each naturally occurring isotope of chlorine,
only
4. the atomic mass and the relative abundance of each naturally occurring
and artificially produced isotope of chlorine
3 By definition, the atomic mass of an element is the weighted average of the element's naturally occurring isotopes. In order to calculate such an
average, the mass and relative abundance of each isotope must be known.
1
326. In an atom of argon-40, the number of protons
1. equals the number of electrons
3. is less than the number of electrons
2. equals the number of neutrons
4. is greater than the number of electrons
1 The mass number of an isotope is the sum of the number of protons and neutrons contained within its nucleus.
1
327. An electron in a sodium atom moves from the third shell to the fourth shell. This change is a result of the atom
1. absorbing energy
3. gaining an electron
2. releasing energy
4. losing an electron
1 By definition, an atom is a neutral particle in which the numbers of protons and electrons are equal.
2
328. Which electron configuration represents an excited state for a potassium atom?
1. 2-8-7-1
3. 2-8-8-1
2. 2-8-7-2
4. 2-8-8-2
2 Use the Periodic Table of the Elements. Each element is accompanied by its ground-state electron configuration. Since an atom is a neutral particle,
the number of electrons in an electron configuration is also the atomic number of the element. An atom in an excited state does not match its groundstate configuration because one or more electrons have been promoted to higher energy levels. In choice (2), 2-8-7-2, an electron in the third energy
level of the atom has been promoted to the fourth energy level.
2
329. Given the bright-line spectra of three elements and the spectrum of a mixture formed from at least two of these elements:
Which elements are present in this mixture?
1. E and D, only
3. D and G, only
2. E and G, only
4. D, E, and G
1 To determine the composition of the mixture, its spectrum must be compared with the spectra of elements D, E, and G. All of the spectral lines of
elements D and E appear in the spectrum of the mixture, but only one spectral line of element G is present in the mixture. Therefore, the mixture
contains only elements D and E.
2
330. What is the total number of valence electrons in a calcium atom in the ground state?
1. 8
3. 18
2. 2
4. 20
2 Valence electrons are those found in the outermost shell of an atom of ion. Refer to the Periodic Table of the Elements. The ground-state electron
configuration of calcium (Ca, atomic number 20) is 2–8–8–2. There are 2 electrons in the outermost shell of this atom in the ground state.
3
331. Which subatomic particles are located in the nucleus of an He-4 atom?
1. electrons and neutrons
3. neutrons and protons
2. electrons and protons
4. neutrons, protons, and electrons
3 The nuclei of all atoms (with the exception of H–1) contain protons and neutrons.
1
332. In the late 1800s, experiments using cathode ray tubes led to the discovery of the
1. electron
3. positron
2. neutron
4. proton
1 The physicist J. J. Thomson used cathode rays to identify the properties of the negative particles known as electrons.
4
333. The atomic mass of titanium is 47.88 atomic mass units. This atomic mass represents the
1. total mass of all the protons and neutrons in an atom of Ti
3. weighted average mass of the most abundant isotope of Ti
2. total mass of all the protons, neutrons, and electrons in an atom of Ti
4. weighted average mass of all the naturally occurring isotopes of Ti
4 The average atomic mass of an element is calculated by determining the weighted average of all of the element's naturally occurring isotopes. The
most abundant, naturally occurring isotopes have the greatest effect on the value of the average atomic mass.
2
334. The isotopes K-37 and K-42 have the same
1. decay mode
3. mass number for their atoms
2. bright-line spectrum
4. total number of neutrons in their atoms
2 The isotopes K-37 and K-42 are isotopes of the same element, potassium. Each element has a unique bright-line spectrum. Therefore, K-37 and K42 will have the same bright-line spectrum.
Wrong Choices Explained:
(1) Use Reference Table N. The decay mode of K-37 is β+; the decay mode of K-42 is β-.
(3) The mass number of K-37 is 37, and the mass number of K-42 is 42.
(4) An atom of K-37 has 18 neutrons in its nucleus, while an atom of K-42 has 23 neutrons in its nucleus.
2
335. The accompanying table shows the number of subatomic particles in atom X and in atom Z.
Atom X and atom Z are isotopes of the element
1. aluminum
3. magnesium
2. carbon
4. nitrogen
2 An element is defined by the number of protons contained in each atom of the element, which is its atomic number. Refer to the Periodic Table of
the Elements. The element carbon (C) has an atomic number of 6.
2
336. The greatest composition by mass in an atom of 178O is due to the total mass of its
1. electrons
3. positrons
2. neutrons
4. protons
2 An atom of 17O contains 8 protons, 9 neutrons, and 8 electrons. The masses of these subatomic particles in decreasing order are mneutron > mproton >>
melectron. Since the most massive particle (the neutron) is present in the greatest quantity (9), it follows that neutrons contribute most to the mass
composition of 17O.