Download ANSWERS Problem Set 5a – Chemical Reactions

ANSWERS Problem Set 5a – Chemical Reactions
1) Define a chemical reaction. Use the terms “reactant and product” and write at
least three sentences in your definition. Don’t give examples… just a definition.
A chemical reaction is a change of bonds in a substance or between
substances. The bonds can be broken, rearranged or new bonds can be
formed. The chemical make up of the products will be different from the
chemical make up of the reactants.
2) Determine if the following are chemical reactions or physical changes.
a. You crumple this problem set up. Physical change
b. You tear this problem set up. Physical change
c. You light this paper on fire and it turns into ash. Chemical reaction
d. You place a piece of calcium in a beaker of water. chemical reaction
e. You place a sponge in a beaker of water. Physical change
f. You strike lead with a hammer and the metal bends. Physical change
g. The addition of a solution of lead (II) nitrate to a solution of sodium
phosphate to produce a solution of sodium nitrate and solid particles of
lead (II) phosphate. chemical reaction
h. You breathe in O2 gas and exhale CO2 gas… chemical reaction
3) Give three types of evidence that indicate a chemical reaction has taken place.
Answers will vary – heat, light, change in temp, unexpected color, sound, odor,
gases, precipitates temp, physical state, texture, bubbles, smell,
4) The mass of the reactants must equal the mass of the products in any chemical
change. Why? The LAW of conservation of mass
5) Why does your car stop running when you run out of gas? There are no more
molecules of gas to burn which will then produce the energy when they are
burned to move the pistons and in turn move the car.
6) Glow sticks are actually two sticks in one… What does snapping the tube do and
how does this lead to a chemical reaction? Any chemical reaction can only take
place when the reactants are allowed to touch and mix. The glass tube was
separating the two reactants and not allowing this mixing to take place.
7) When balancing a chemical reaction the subscripts in the formulas can NEVER be
changed. Why? Subscripts define which compounds are being used in the
reaction. The coefficients are used to define the quantity of each compound,
not the chemical make-up.
8) The reaction below is balanced, but there are only ten molecules of reactants
while there are twelve molecules of products. How can this be balanced?
C6H12 + 9 O2  6 CO2 + 6 H2O
Chemical reactions are not balanced by molecules or compounds, but only by
atomic values. The number of atoms that are reacted must add up to the number of
atoms that are produced. There are 6 carbon atoms, 12 hydrogen atoms and 18
oxygen atoms on both sides of this reaction.
9) What is the difference between a word equation and a chemical equation?
Word equations give the names of the substances, while chemical equations give the
formulas and the quantities (and ideally the phases, too).
10) Which numbers in the following balanced reaction of aluminum and sulfuric acid
are coefficients and which are subscripts?
Subscripts are underlined, Coefficients are boxed
3 H2SO4+ 2 Al  Al2(SO4)3 + 3 H2
11) Write the word equation for the following balanced reactions:
a. 2 Mg (s) + O2 (g)  2 MgO (s)
solid magnesium reacts with oxygen gas to produce solid magnesium oxide in a
synthesis reaction.
b. HCl (aq) + NaOH (aq)  H2O (l) + NaCl (aq)
Aqueous hydrochloric acid reacts with aqueous sodium hydroxide to produce water
and aqueous sodium chloride in a neutralization reaction. (double replacement)
c. 2 NH4NO3 (s) 2 N2 (g) + O2 (g) + 4 H2O(g)
solid ammonium nitrate decomposes to form nitrogen gas, oxygen gas and steam
d. NaOH (aq)+ AgNO3 (aq)  AqOH (s) + NaNO3 (aq)
An aqueous solution of sodium hydroxide reacts with an aqueous solution of silver
nitrate to produce solid silver hydroxide and aqueous sodium nitrate in a double
replacement reaction.
12) Balance the following reactions –
a. 2 C2H6 (g) + 7 O2 (g)  4CO2 (g) + 6 H2O (g)
b. 2 NaCl (aq) + Pb(NO3)2 (aq)  2 NaNO3 (aq) + PbCl2 (s)
c. Ca (s) + 2 H2O (l)  Ca(OH)2 + H2 (g)
d. Ca(HCO3)2 (s)  CaO (s) + 2 CO2 (g) + H2O (g)
e. 6 Cu (s) + S6 (g)  6 CuS (s)
13) When nitric acid is added to a solid piece of copper, ...
a. Write the word equation for the statement above.
Nitric acid reacts with solid copper to produce gaseous nitrogen dioxide,
hydrogen gas, oxygen gas, and a solution of copper (II) nitrate.
b. Write the unbalanced formula equation.
HNO3 (aq) + Cu (s)  NO2( g) + H2O (l) + Cu(NO3)2 (aq)
c. Count the atoms for each element as a reactant (make a table)
Reactant
hydrogen
nitrogen
oxygen
Copper
amount
1
1
3
1
d. Count the atoms for each element as a product (make a table)
Product
hydrogen
nitrogen
oxygen
Copper
amount
2
3
9
1
e. Balance the reaction
4 HNO3 (aq) + Cu (s)  2 NO2( g) + 2 H2O(l) + Cu(NO3)2 (aq)
14) Write a balanced chemical reaction for the following word equations.
a. Potassium chlorate decomposes to form potassium chloride and oxygen
gas: 2 KClO3 (s)  2 KCl (s) + 3 O2(g)
b. Aqueous solutions of copper (II) nitrate and sodium hydroxide react to
form solid copper (II) hydroxide and a solution of sodium nitrate.
Cu(NO3)2 (aq) + 2 NaOH (aq)  Cu(OH)2 (s) + 2 NaNO3(aq)
c. Diphosphorous tetrabromide gas reacts with fluorine gas to produce
diphosphorous tetrafluoride gas and bromine liquid.
P2Br4 (g) + 2 F2 (g)  P2F4 (g) + 2 Br2(l)
d. Octane (C8H12) and oxygen react to produce carbon dioxide and water.
C8H12 (l) + 11 O2 (g)  8 CO2 (g) + 6 H2O (g)
15) Why is being able to name compounds correctly so important in chemistry?
Names and formulas are specific for each compound. Any errors in names or
formulas do not represent the chemistry of the reaction properly.
16) List the diatomic elements – give their names and molecular formulas.
H2 (g) – hydrogen, O2 (g) oxygen, N2 (g) nitrogen, F2( g) fluorine, Cl2 (g) chlorine,
Br2 (l) bromine, I2 (s) iodine
(H O N Cl Br I F)
Multiple Choice Section (again, record your answers on this sheet)
17) _B_ This law states the mass of reactants has to be equal to the mass of the
products. B) The Law of Conservation of Mass
18) _D_ Which of the following is a chemical change?
d. Letting the milk sit out overnight and waking up to find sour milk
19) _C_ Which of the following is NOT a chemical change?
c. Ice in a beaker over the Bunsen burner turn into water
20) _B_ What are the coefficients that will balance this reaction?
___ HCl (aq) + ___ Mg(OH)2 (aq)  ___ H2O (l) + ___MgCl2 (aq)
b. 2, 1, 2, 1
21) _D__ What is the “x” in the following equation?
C5H12 + _x_ O2  5 CO2 + 6H2O
a. 4
b. 5.5
c. 16
d. 8
e. 10
22) _D_ According to the diagram on the right, which equation would
correctly represent the reaction?
a)
b)
c)
d)
e)
8X + 8Y 8Z
8X + 2Y 8Z
8 X + 16 Y  24 Z
X + 2Y 2Z
2X + Y 3Z
Y
ZZ
X
Section 10-2: Classifying Chemical Reactions
23) Balance these reactions and identify their type
a. synthesis _3 H2 (g) + ___ N2 (g)  _2_ NH3 (g)
b. decomposition _2_ H2O (g)  _2_ H2 (g) + ___ O2 (g)
c. synthesis _2_ H2 (g) + ___ O2 (g) _2_ H2O (aq)
d. single replacement_2_ Na(s) + _2_ HCl(aq)  _ H2 (g) + _2_ NaCl (aq)
e. single replacement__ Na(s) + __ AgNO3(aq)  _ Ag (s) + _ NaNO3 (aq)
f. decomposition ___ H2CO3 (aq)  ___ H2O (l) + ___ CO2 (g)
g.
double replacement_
NaOH(aq) + __ HCl(aq)  _ H2O(l) + _ NaCl (aq)
h. double replacement Na2CO3(aq) + CaSO4 (aq)  Na2SO4 (aq) + CaCO3 (s)
i. single replacement _4_ H2O (l) + _3_ Fe(s)  _ Fe3O4 (s) + _4_ H2 (g)
j. synthesis_4_ Al(s) + _3_ O2 (g)  _2_ Al2O3 (g)
k. combustion ___ CH4 (g) + _2_ O2 (g)  ___ CO2 (g) + _2_ H2O (g)
24) What product is always needed for a combustion reaction to take place? Oxygen Gas
(O2) What are the two products that are always produced in the complete combustion
reaction of an organic fuel? Carbon Dioxide (CO2) and Water (H2O)
25) What three elements are produced in the decomposition of sodium sulfate? Sodium,
Sulfur and Oxygen
26) What four elements are needed to run the synthesis reaction that forms the ionic
compound ammonium sulfate? Nitrogen, hydrogen, sulfur and oxygen
27) What does the activity series show? The activity series lists several metal
elements in order of willingness to react. The elements at the top are very
reactive, those at the bottom are not. What is the rule of thumb that you should
remember when looking at the activity series? The atom must be more active
than the ion for the reaction to occur.
28) Use your activity series to determine which reactants will react:
a. Na (s) + HCl (aq)
or
H2 (g) + NaCl (aq)
b. Mg (s) + NaCl (aq)
or
Na (s) + MgCl2 (aq)
c. Au (s) + FeCl3 (aq)
or
Fe (s) + AuCl (aq)
Multiple Choice section
29) Which product is missing from this single replacement reaction?
Cu (s) + H2SO4 (aq)  CuSO4 (aq) + ???
a. H (g)
b. H2 (g)
c. CuO (s)
d. O2 (g)
e. CO2 (g)
30) Lead(IV) oxide changes to lead(II) oxide when heated. Which set of masses
could occur in this experiment?
2 PbO2 (s)  2 PbO (s) + O2 (g)
a) test tube
test tube + PbO2
test tube + PbO
19.0 g
18.7 g
18.0 g
b) test tube
test tube + PbO2
test tube + PbO
19.0 g
19.3 g
18.0 g
c) test tube
test tube + PbO2
test tube + PbO
19.0 g
22.0 g
21.8 g
d) test tube
test tube + PbO2
test tube + PbO
19.0 g
22.0 g
22.3 g
Single Replacement Reactions
31) Will these single replacement reactions will run based upon the activity series?
a. Ag (s) + Fe(NO3)3aq) no way – Ag atoms are less active than Fe ions
b. Fe (s) + AgNO3 (aq) yessir! Fe atoms are more active than Ag ions
c. Al (s) + Cu(NO3)2 (aq) yup! Al atoms are more active than Cu ions
32) Given the following data, write an activity series or the elements A, B and C.
Atoms of A react with ions of B
A must be > B
Atoms of A react with ions of C
A must be > B
Atoms of B do not react with ions of C
B must be < C
Atoms of B do not react with ions of A
B must be < A
Atoms of C react with ions of B
C must be > B
Atoms of C do not react with ions of A
C must be < A
So… A > C > B
33) Draw a picture showing the following reaction:
12 molecules of hydrogen gas as a reactant
3 molecules of nitrogen gas as a reactant
The maximum molecules of ammonia (NH3) gas that can be produced
Light grey is for hydrogen and dark grey is for nitrogen.
+
3H2 (g) + N2 (g)  2NH3 (g)
There are 3 molecules of hydrogen that do not react – they are in excess.
34) Define “Reactant in Excess” and “Limiting Reactant.” Why are these two terms
important in industrial production of compounds?
The reactant is excess is the substance that remains after the limiting reactant runs
out. The limiting reactant is the reactant that runs out before any other in a
chemical reaction – it limits the amount of product made. The limiting reactants in
most industrial reactions are typically the most expensive compounds.
35) Explain why “Theoretical Yield” is usually more than the “Actual Yield” for most
reactions.
The theoretical yield is typically more than actual yield because of human errors,
laboratory conditions are not always ideal, the concentrations of products slows
reactions down by getting in the way… others.
36) Determine the Percentage Yield if a reaction has the theoretical yield of 3.45
grams of product and the actual yield of 3.25 grams of product.
3.25 g / 3.45 g = 94.2 % yield
37) Determine the actual yield of a reaction that has a 77 % yield and a theoretical
yield of 2.35 grams.
Actual Yield / 2.35 g = 77%
Actual = (.77)(2.35g) 1.81 g
38) Determine the theoretical yield of a reaction if it has a 85% yield and an actual
yield of 3.22 grams.
3.22 g / Theoretical Yield = 85% Theoretical = 3.22 / 0.85 3.79 g
Use this reaction for the next four questions:
Cu (s) + 4 HNO3 (aq)  Cu(NO3)2 (aq) + 2 NO2 (g) + 2 H2O (l)
39) 3.0 moles of copper react completely in a reaction.
a. How many moles of nitric acid are needed?
3.0 mol Cu x (4 mol HNO3 / 1 mol Cu) = 12.0 mol HNO3
b. How many moles of copper (II) nitrate are produced?
3.0 mol Cu x (1 mol Cu(NO3)2 / 1 mol Cu) = 3.0 mol Cu(NO3)2
c. How many moles of water are produced?
3.0 mol Cu x (2 mol H2O / 1 mol Cu) = 6.0 mol H2O
d. How many moles of nitrogen dioxide are produced?
3.0 mol Cu x (2 mol NO2 / 1 mol Cu) = 6.0 mol NO2
e. How many liters of NO2 are produced if the lab was performed at STP?
6.0 mol NO2 x 22.4 L/ 1mol = 134.4 L
40) If 1.2 moles of copper react with 4.0 moles of HNO3, what is the limiting
reactant?
HAVE
NEED
1.2 mol Cu x (4mol HNO3 / 1 mol Cu) = 4.8 mol HNO3
4.0 mol HNO3 x (1 mol Cu / 4 mol HNO3) = 1 mol Cu
You don’t have enough HNO3 so HNO3 is the limiting reactant.
41) If 1.2 moles of copper are placed into 4.9 moles of HNO3, how much of the
reactant in excess remains at the end of the reaction?
HAVE
NEED
1.2 mol Cu x (4mol HNO3 / 1 mol Cu) = 4.8 mol HNO3
4.9 mol HNO3 x (1 mol Cu / 4 mol HNO3) = 1.22 mol Cu
Cu limits the reaction and there is 0.10 mol of HNO3 left over. (4.9 – 4.8)
42) How many grams of water can 12.5 grams of anhydrous chromium (III) sulfate
salt absorb if the formula of the hydrated salt is Cr2(SO4)3.18H2O?
Cr2(SO4)3.18H2O (s)  Cr2(SO4)3 (s) + 18H2O (g)
12.5 g Cr2(SO4)3 x (1 mol /392 g ) = 0.0319 mol Cr2(SO4)3
0.0319 mol Cr2(SO4)3 x (18 mol H2O/1mole Cr(SO4)3 ) = .574 mol H2O
0.574 mol H2O x ( 18 g/1 mol) = 10.33 g H2O
35) What mass of magnesium oxide should be produced from 3.55 grams of
magnesium and an excess of oxygen gas?
2Mg (s) + O2 (g)  2MgO (s)
3.55 g Mg x (1 mol /24.3 g Mg) = 0.146 mol Mg
0.146 mol Mg x (2 mol MgO/ 2 mol Mg) = 0.146 mol MgO
0.146 mol MgO (40.3 g MgO/ 1 mol MgO) = 5.88 g MgO
36) How many grams of copper will be produced from reacting 3.44 grams of iron
with 3.55 grams of Cu(NO3)2 that have been dissolved in water? (you need to
determine the limiting reactant)
3 Cu(NO3)2 (aq) + 2 Fe (s)  2 Fe(NO3)3 (aq) + 3 Cu (s)
3.44 g Fe x (1 mol Fe/ 55.85 g Fe) = 0.0615 mol Fe (HAVE)
3.55 g Cu(NO3)2 ( 1 mol/187.55 g) = 0.0189 mol Cu(NO3)2 (HAVE)
0.0615 mol Fe x (3 mol Cu(NO3)2 / 2 mol Fe) = 0.09225 mol Cu(NO3)2 NEEDED
0.0189 mol Cu(NO3)2 x (2 mol Fe/3 mol Cu(NO3)2 ) = 0.0126 mol Fe NEEDED
You have enough Fe, you don’t have enough Cu(NO3)2.
0.0189 mol Cu(NO3)2 x (3 mol Cu/3 mol Cu(NO3)2 ) = 0.0189 mol Cu produced
0.0189 mol Cu x (63.5 g Cu/ 1 mol Cu) =1.20 g Cu can be made.
37) What mass of CO2 gas will be produced from the combustion of 50.00 grams of
octane, C8H18?
2 C8H18 (g) + 25 O2 (g)  16 CO2 (g) + 18 H2O (g)
50.00 g C8H18 x (1 mol/114 g) = 0.439 mol C8H18
0.439 mol C8H18 x ( 16 mol CO2 / 2 mol C8H18 ) =3.51 mol CO2
3.51 mol CO2 (44.0 g/1 mol) = 154.5 g CO2