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CHAPTER 2
ATOMS AND MOLECULES
 Safety Box

Be careful not to get any of the solutions on your hands or clothing. They may be caustic.

Disposal of solutions may pose environmental hazards. Carefully follow your instructor's directions.

Chemicals used as reagents, such as bromthymol blue or sodium iodide, may permanently stain clothing.
Use with caution.
OBJECTIVES
Be able to do the following:
1. Define these terms in writing:
atom
ionic bond
orbital
balanced
ion
element
nucleus
atomic number
isotope
acid
hydrogen ion
mass number
molecule
base
hydroxide ion
energy levels
compound
covalent bond
reactant
pH scale
electron
neutron
product
periodic table of
the elements
proton
2. Determine the number of protons, neutrons, and electrons for any atom, given the information on a
periodic chart.
3. Draw and label a diagram of any atom with an atomic number less than 20, given the information on a
periodic chart.
4. Diagram a conceivable molecule using all the atoms given, and show the shape of that molecule and the proper
number of bonds for each atom.
5. Describe in writing and/or by a diagram the characteristics of ionic and covalent bonds.
6. Determine if a given solution is an acid or a base by using any of the following indicators:
a. pHydrion paper
b. bromthymol blue
c. phenolphthalein
d. pH meter
7. Interpret the pH number of a solution.
8. Interpret a chemical equation and point out the reactants and the products.
Time
1½ to 2 hours
Remarks
Designed to assist the students in understanding Chapters 2 and 3 of the text.
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Equipment (for five laboratory groups)
13 pairs of scissors
13 pairs of forceps
Several rolls of pHydrion paper/charts or 50 pieces of pHydrion paper ½ inch long
5 dropper bottles of bromthymol blue
5 dropper bottles of phenolphthalein
5 beakers with solutions having varying pHs and a dropper for each beaker
5 test tube racks/18 tubes
5 dropper bottles of 10% sodium chloride solution
5 dropper bottles of 1% silver nitrate solution
5 dropper bottles of sodium iodide solution
5 dropper bottles of lead nitrate
5 dropper bottles of 0.1 N hydrochloric acid
A supply of sodium bicarbonate
Molecular stick models, if available
5 pH meters
Optional
Other indicator, if desired.
Potential Problems
The students may compare the color change of the bromthymol blue indicator to the pHydrion color chart. Caution
them against this. Students should realize that physical evidence that a reaction has occurred is not the same as a
chemical reaction. A precipitate can form from a settling mixture that has not had a chemical change.
ANSWERS
The Structure of Atoms
1. Table 2.A
Element
Carbon
Hydrogen
Nitrogen
Oxygen
Sodium
Magnesium
Phosphorus
Chlorine
Potassium
Calcium
Symbol
C
H
N
O
Na
Mg
P
Cl
K
Ca
Protons
6
1
7
8
11
12
15
17
19
20
6
Neutrons
6
0
7
8
12
12
16
18
20
20
Electrons
6
1
7
8
11
12
15
17
19
20
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2. Name of the atom you diagrammed in figure 2.4.
Nitrogen. 7 protons, 7 neutrons, 7 electrons. Electrons as shown in the diagram.
3. Name of the atom you diagrammed in figure 2.5.
Will vary with the atom.
Ionic Bonds
Eight ionic compounds that can be named in figure 2.6:
H2O
H2O2
HCl
NaCl CaO
Na2O CaCl2
NaOH
*Caution students that H2O is held together by polar covalent bonds, and therefore, is not a true ionic compound.
1.
Protons in a sodium atom (Na)
11
Electrons in a sodium atom (Na)
11
2.
Protons in a sodium ion (Na+)
11
Electrons in a sodium ion (Na+)
10
3.
Protons in a chlorine atom (Cl)
17
Electrons in a chlorine atom (Cl)
17
4.
Protons in a chloride ion (Cl–)
17
Electrons in a chloride ion (Cl–)
18
5.
Protons in a sodium chloride
molecule (NaCl)
28
Electrons in a sodium chloride
molecule (NaCl)
28
6.
Protons in a calcium atom (Ca)
20
Electrons in a calcium atom (Ca)
20
7.
Protons in a calcium ion (Ca++)
20
Electrons in a calcium ion (Ca++)
18
8.
Protons in an oxygen atom (O)
8
Electrons in an oxygen atom (O)
8
9.
Protons in an oxide ion (O–)
8
Electrons in an oxide ion (O–)
10
10.
Protons in a calcium oxide
molecule (CaO)
28
Electrons in a calcium oxide
molecule (CaO)
28
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Simple Molecules
1. Methane (one carbon atom and four hydrogen atoms): CH4
2. Ammonia (one nitrogen atom and three hydrogen atoms): NH3
3. Water (one oxygen atom and two hydrogen atoms): H2O
4. Ethane (two carbon atoms and six hydrogen atoms): C2H6
5. Ethyl alcohol: C2H5OH
More Complex Molecules
6. Ethene (two carbon atoms and four hydrogen atoms): C2H4
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7. Aldehyde (one carbon atom, two hydrogen atoms, and one oxygen atom): CH2O
8. Acetic acid: CH3COOH
9. Amino acid: (add —COOH and —NH2 to the carbon skeleton)
10. Sugar: C6H12O6
Note: These are two of several correct answers.
Acids, Bases, and pH
1. Which of the following pH values indicates the greater concentration of acid:
pH 1 or pH 4? pH 1
pH 5 or pH 3? pH 3
2. Which of the following pH values indicates the greater concentration of base:
pH 8 or pH 10? pH 10
pH 7 or pH 11? pH 11
3. What does a pH of 7 indicate?
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neutral
Many kinds of materials change color as the pH of a solution changes. We can make use of this property to
determine the pH of unknown solutions.
Table 2.2 Unknowns*
Solution
PH paper
Color After Adding
Bromthymol Blue
Yellow
Color After Adding
Phenolphthalein
Clear
pH Meter
Readings
3.1
A
3
B
9
Blue
Faint pink
8.9
C
7
Blue-green
Clear
7.2
D
5
Yellow
Clear
5.0
E
11
Blue
Pink
11.1
*(These are example results. Results will vary depending on solutions provided.)
1. Bromthymol blue changes color when mixed with an acid. What color does it become?
yellow
2. What color is it when in a base?
blue
3. What color would you expect it to be if it were in a neutral solution?
green
4. Phenolphthalein changes to pink in the presence of bases.
5. What is the advantage of using pHydrion paper rather than bromthymol blue or phenolphthalein?
It gives a more accurate reading of the pH.
6. Which reagent could be considered to be an acid indicator?
bromthymol blue
Reactions
1. Complete the equation by writing out the name of the products.
a. What physical changes do you observe as evidence that a chemical reaction has taken place?
A white precipitate forms.
b. Complete the equation by writing out the name of the products.
Sodium chloride + Silver nitrate ------> Sodium nitrate + Silver chloride
2. Mix a couple of drops of sodium iodide solution with lead nitrate solution.
a. What is the physical evidence that a reaction has taken place?
A yellow precipitate forms.
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b. Fill in the name of the reactants and the products below the chemical formulas.
NaI + Pb(NO3)2 ------> PbI2 + 2 NaNO3
_______ + _______ -------> _______ + ________
c. Is this equation balanced? If not, be sure to balance it.
3. a. What is the physical evidence that a reaction has taken place?
yellow precipitate
b. Fill in the names of the reactants and the products below the chemical formulas.
____NaI + Pb(NO3)2 ------> PbI2 + ____ NaNO3
Sodium iodide + lead nitrate ------> lead iodide + sodium nitrate
Is this equation balanced? If not, be sure to balance it.
2 NaI + Pb(NO3)2 ------> PbI2 + 2 NaNO3
4. Place a small amount of sodium bicarbonate in a test tube and add a couple of drops of hydrochloric acid.
a. What physical evidence indicates that a reaction took place?
Bubbles occur due to production of a gas.
b. Complete the equation by writing in the names and the chemical symbols for the end products that are
not listed.
HCl + NaHCO3 ------> CO2 +
+
------> CO2 + H2O + NaCl
water + sodium chloride
c. Is the equation balanced? Yes.
END-OF-EXERCISE QUESTIONS
1. List several differences between ionic and covalent bonds.
Ionic bonds occur when ions of opposite charge are mutually attracted. Acids and bases are examples of
ionic compounds. Covalent bonds are strong chemical bonds that occur when atoms share electrons.
Methane and sugar are examples of covalent compounds.
2. What information can be obtained from the periodic table of elements?
From the periodic table, an element's symbol, atomic number, number of protons, atomic mass, number
of neutrons, and number of electrons can be obtained.
3. What do you need to know to diagram an atom?
number of protons, number of neutrons, and number and location of electrons
4. What does a single straight line (—CH3) extending from an atomic symbol represent?
one bond, two shared electrons
5. How does one know how many covalent bonds a particular atom will form?
The number of electrons in an atom's outermost energy level determines the number of covalent bonds it
will form.
6. Which groups of elements on the periodic table tend to form ions?
elements in columns IA, IIA, UIA, and UIIA
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7. What is the difference between —OH and OH–?
The line on —OH represents a covalent bond; OH– is a hydroxyl ion, the line represents an
extra electron.
8. Define pH.
pH is a measure of hydrogen ion concentration.
9. Explain how one might use bromythymol blue and phenolphthalein to test the pH of a swimming pool.
If the pool water is acidic, it will turn bromthymol blue to yellow. Phenolphthalein will turn pink if the
pool water is a base.
10. List three kinds of physical changes that indicate that a chemical reaction has taken place.
Change in color, change in temperature, and change in state, such as formation of a gas or solid when
mixing two liquids at room temperature are three physical changes.
SOURCES OF MATERIALS
Standard materials are available from most supply houses.
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