CHAPTER 6: ANSWERS TO SELECTED PROBLEMS
SAMPLE PROBLEMS (“Try it yourself”)
6.1
a) This is a chemical change. When you digest food, you break down the chemical
compounds into simpler molecules.
b) This is a physical change. The chemical formula of alcohol does not change when you
dissolve it in water.
6.2
The first reaction is correct. Sodium is a metal, and metal atoms do not form chemical
bonds to one another. Writing Na3 implies that three sodium atoms link together to form a
molecule, which is not the case.
6.3
This expression contains a total of six copper atoms and three oxygen atoms.
6.4
This chemical equation is not balanced. There are four oxygen atoms on the left side of
the arrow, but only three oxygen atoms on the right side. (The iron atoms are balanced, with two
on each side, but every element must be balanced in a chemical equation.)
6.5
2 Al + 6 HCl → 2 AlCl3 + 3 H2
6.6
Mg(s) + 2 HCl (aq) → MgCl2(aq) + H2(g)
6.7
In amu: 53.96 amu of Al reacts with 212.7 amu of Cl2.
In grams: 53.96 g of Al reacts with 212.7 g of Cl2.
6.8
Your body uses 1.12 g of O2. (The calculator answer is 1.12183607 g.) The mass ratio
here is 342.296 g of sucrose to 384 g of oxygen.
6.9
When 146.416 g of Cd(OH)2 reacts with 185.412 g of Ni(OH)2, the reaction absorbs 252
kcal of heat, and it produces 112.4 g of Cd and 219.428 g of Ni(OH)3. Since this reaction
absorbs heat, ΔH is a positive number.
6.10
You must burn 4.44 g of H2. (The calculator answer is 4.4440292 g.)
6.11
The peanut butter contains 200 Calories. (The calculator answer is 204 Calories.)
6.12
C4H10O + 6 O2 → 4 CO2 + 5 H2O
6.13 The second reaction is a precipitation reaction, because the reactants are all in solution
but one of the products is a solid. In the first reaction, there is a solid reactant, so the first
reaction is not a precipitation.
6.14
Fe2+(aq) + 2 OH–(aq) → Fe(OH)2(s)
6.15
The first reaction is faster, because it has a lower activation energy.
6.16
Product
energy
Energy
Reactant
energy
Start of reaction
8 kcal
(activation
energy)
7 kcal
(heat of
reaction)
End of reaction
6.17 The reaction will go forward, so it can get rid of the excess HCO3– ions. It will make
C3H5O3– and H2CO3, and it will consume HC3H5O3 and HCO3–.
6.18 The reaction will go forward, so it can replace the H2CO3 that you removed. It will make
C3H5O3– and H2CO3, and it will consume HC3H5O3 and HCO3–.
END OF SECTION PROBLEMS
Section 6.1
6.1
a) This is a physical change. The water changes from a liquid to a solid, but it remains
H2O.
b) This is a chemical change. The electrical current separates the hydrogen atoms from
the oxygen atoms.
c) Dissolving a solid in water is generally described as a physical change. However,
when the solid dissociates, it is reasonable to call the process a chemical change.
6.2
The steam weighs 5.00 grams. This is an example of the law of mass conservation.
6.3
When gasoline burns, the products are invisible gases. These gases do not remain in the
container, so we cannot see them, but they still obey the law of mass conservation. The mass of
the gases equals the mass of the original gasoline plus the oxygen that reacted with it.
6.4
Lead and gold are different elements. No chemical reaction can change an atom of one
element into an atom of another element.
Section 6.2
6.5
a) This equation is not balanced, because it has two oxygen atoms on the left side of the
arrow and only one oxygen atom on the right side.
b) This equation is not balanced, because it has four oxygen atoms on the right side of the
arrow and only two oxygen atoms on the left side.
c) This equation is not balanced, because it has four silver atoms on the right side of the
arrow and only two silver atoms on the left side.
d) This equation is balanced.
6.6
a)
b)
c)
d)
e)
Mg + Cl2 → MgCl2 (The original equation was balanced.)
2 Al + 3 S → Al2S3
P4 + 6 Cl2 → 4 PCl3
2 MgI2 + O2 → 2 MgO + 2 I2
Na2S + 2 AgF → 2 NaF + Ag2S
6.7
When you write “2 N”, you mean that the two nitrogen atoms are not bonded to one
another. Writing “N2” means that the atoms are linked by a chemical bond.
6.8
The answer is incorrect, because the student has changed a chemical formula. When you
balance a chemical equation, you cannot change any of the subscript numbers in the formulas,
because these tell you how the atoms are bonded to one another. Aluminum and chlorine can
only combine to form AlCl3, never AlCl2.
6.9
2 Na(s) + 2 H2O(l) → 2 NaOH(aq) + H2(g)
6.10 One molecule of gaseous sulfur trioxide reacts with two formula units of aqueous
potassium hydroxide, forming one formula unit of aqueous potassium sulfate and one molecule
of liquid water.
Section 6.3
6.11
16.042 g of CH4 reacts with oxygen to form 36.032 g of H2O.
6.12 a) 3.51 g of HCl. (The calculator answer is 3.50539716 g.) The mass relationship is
78.004 g of Al(OH)3 to 109.374 g of HCl.
b) 1.73 g of H2O. (The calculator answer is 1.73221886 g.) The mass relationship is
78.004 g of Al(OH)3 to 54.048 g of H2O.
6.13 a) 147 g of fruit sugar. (The calculator answer is 146.6495181 g.) The mass relationship
is 92.136 g of C2H6O to 180.156 g of C6H12O6.
b) 71.6 g of carbon dioxide. (The calculator answer is 71.64951811 g.) The mass
relationship is 92.136 g of C2H6O to 88.02 g of CO2.
Section 6.4
6.14
a) The surroundings become cooler.
c) ΔH is a positive number.
b) This reaction is endothermic.
6.15
When 171.316 g of Ba(OH)2 reacts, 5.5 kcal of heat is absorbed.
6.16 43.2 kcal of heat is given off. (The calculator answer is 43.19557535 kcal.) The
relationship is 156.216 g of C6H6 to 1562 kcal of heat.
6.17
You must use 12.5 g of benzene. (The calculator answer is 12.50128041 g.)
6.18 a) Your body gets 0.0905 kcal of energy (heat). (The calculator answer is 0.09047714
kcal.) The relationship is 180.156 g of C6H12O6 to 16.3 kcal of heat.
b) Your body obtains roughly 44 times as much energy when it burns glucose as it does
when it ferments glucose. (4 kcal ÷ 0.0905 kcal = 44.2)
6.19
The hamburger contains roughly 270 Calories (rounded to the nearest 10 Calories).
6.20
The manufacturer must remove 5 g of sugar.
Section 6.5
6.21
a) C7H8 + 9 O2 → 7 CO2 + 4 H2O
b) 2 C4H10 + 13 O2 → 8 CO2 + 10 H2O
c) C4H10O + 6 O2 → 4 CO2 + 5 H2O
6.22
Reactions a and d are combustion reactions.
6.23
Reaction b is the only precipitation reaction.
6.24
Ag+(aq) + Br–(aq) → AgBr(s)
Section 6.6
6.25
a) The rate increases, because the concentration of O2 has increased.
b) The rate decreases. All reactions slow down when they are cooled.
c) The rate increases. The bubbles provide a larger surface area where the gaseous
oxygen and the wine are in contact.
6.26
The powdered limestone dissolves faster, because it has a larger surface area.
6.27 Stirring breaks the fat into small droplets, increasing the surface area where the fat is in
contact with the NaOH solution.
6.28
The second reaction (using FeCl2) has the larger activation energy.
6.29
ENERGY
no catalyst (red curve)
with catalyst (green curve)
21
kcal
12
kcal
energy of
products
energy of
reactants
Section 6.7
6.30 An equilibrium mixture is a stable mixture of products and reactants that is formed by a
reversible reaction.
6.31
Only statement c is correct.
6.32
Statement a is correct.
6.33
a) The reaction goes backward to remove the excess glucose.
b) The reaction goes forward to remove the excess sucrose.
c) The reaction goes forward to make more fructose.
6.34 The concentration of fructose will decrease. The reaction goes backward, to remove
some of the glucose we added. The backward direction consumes both glucose and fructose, so
the number of molecules of fructose in our mixture decreases.
CUMULATIVE PROBLEMS (Odd-numbered problems only)
6.35 In a chemical change, the chemical formulas of the starting materials and products are
different. In a physical change, the starting materials and the products have the same chemical
formulas.
6.37
a) physical
b) chemical
c) chemical
6.39
a) physical
b) chemical
c) physical
d) physical
6.41 Rusting is a chemical reaction in which iron combines with oxygen from the air to form a
compound. The compound weighs more than the original iron because it also contains some
oxygen.
6.43 When your body burns food to obtain energy, one of the products is water. The
difference between the mass of water you take in and the mass of water you put out is the water
that your body makes as it burns food.
6.45
a) not balanced
b) balanced
c) balanced
d) not balanced
6.47 This is not a reasonable answer, because the student has changed the chemical formula of
the product.
6.49
a) S + 2 Cl2 → SCl4
b) 2 Ag + S → Ag2S
c) 2 K + Cl2 → 2 KCl
6.51
a) CaO + 2 HCl → CaCl2 + H2O
b) 4 Fe + 3 O2 → 2 Fe2O3
c) CH4 + 2 O2 → CO2 + 2 H2O
6.53
a) 2 C4H10 + 13 O2 → 8 CO2 + 10 H2O
b) 2 AlCl3 + 3 H2O → Al2O3 + 6 HCl
c) 2 AgNO3 + MgI2 → 2 AgI + Mg(NO3)2
6.55
a) 2 Al(OH)3 + 3 H2SO4 → Al2(SO4)3 + 6 H2O
b) 4 C5H11NO2 + 27 O2 → 20 CO2 + 22 H2O + 2 N2
6.57
PCl3(s) + 3 H2O(l) → H3PO3(aq) + 3 HCl(aq)
6.59
a) The molecule contains eight sulfur atoms, so its formula is S8.
b) S8 + 8 O2 → 8 SO2
6.61
223.4 g of Fe reacts with oxygen to form 319.4 g of Fe2O3.
6.63 a) The fish makes 0.227 g of ammonia (the calculator answer is 0.22690822 g). The
mass relationship is 150.14 g of C2H5NO2 to 34.068 g of NH3.
b) The fish makes 26.4 g of water (the calculator answer is 26.44123518 g). The mass
relationship is 34.068 g of NH3 to 36.032 g of H2O.
6.65 a) You will need 34.1 g of oxygen (the calculator answer is 34.11676698 g). The mass
relationship is 1277.964 g of C39H74O6 to 3488 g of O2.
b) You will make 33.6 g of CO2 and 13.0 g of H2O (the calculator answers are
33.57665005 g and 13.04011694 g, respectively). The mass relationship is 1277.964 g of
C39H74O6 to 3432.78 g of CO2 to 1333.184 g of H2O.
c) You use a total of 46.6 g of reactants (12.5 g of trilaurin and 34.1 g of oxygen), and
you make a total of 46.6 g of products (33.6 g of carbon dioxide and 13.0 g of water). The
reactants and products have equal masses, obeying the law of mass conservation.
6.67 When you burn 10.0 g of alanine, you form 3.37 g of urea. When you burn 10.0 g of
asparagine, you form 4.55 g of urea. Therefore, the asparagine forms a larger mass of urea when
it burns. (The mass relationships are 178.192 g of alanine to 60.062 g of urea, and 132.124 g of
asparagine to 60.062 g of urea.)
6.69 a) 0.0454 g of oxygen will react (the calculator answer is 0.04542254 g). The mass
relationship is 352.248 g of C6H8O6 to 32 g of O2.
b) 34.9 mL of oxygen (the calculator answer is 34.94041785 mL).
6.71
a) 0.729 g of HCl (the calculator answer is 0.72850435 g).
b) 0.0200 moles of HCl (the calculator answer is 0.01998202 moles).
c) 2000 mL of 0.01 M HCl.
6.73
The products are CO2 and H2O (carbon dioxide and water).
6.75
a) C7H16 + 11 O2 → 7 CO2 + 8 H2O
b) 2 C5H12O + 15 O2 → 10 CO2 + 12 H2O
6.77 You will form 16.9 g of CO2 (the calculator answer is 16.90351821 g). You must start by
writing the balanced equation for the combustion reaction. The equation is 2 C2H2 + 5 O2 → 4
CO2 + 2 H2O. Using this, you should find that the mass ratio is 52.072 g of C2H2 to 176.04 g of
CO2.
6.79
Reactions b, c and e are precipitation reactions.
6.81
a) Ca2+(aq) + 2 F–(aq) → CaF2(s)
b) 2 Ag+(aq) + S2–(aq) → Ag2S(s)
c) Fe3+(aq) + 3 OH–(aq) → Fe(OH)3(s)
6.83
a) This is an endothermic reaction.
b) The surroundings become cooler.
c) ΔH is a positive number.
6.85
When 104.058 g of NaHSO3 reacts, 10.4 kcal of heat is absorbed.
6.87 a) The surroundings become hotter. ΔH is a negative number, which tells us that the
reaction produces heat.
b) Heat is a product in this reaction.
c) 2 Mg(s) + O2(g) → 2 MgO + 287 kcal
6.89 The reaction produces 2.6 kcal of heat (the calculator answer is 2.57998624 kcal). The
relationship is 180.156 g of C6H12O6 to 16.6 kcal of heat.
6.91 The reaction produces 153 g of ethanol (the calculator answer is 152.6349398 g). The
relationship is 92.136 g of C2H6O to 16.6 kcal of heat.
6.93 The heat of reaction is 529 kcal (the calculator answer is 529.128 kcal). The heat of
reaction is the amount of energy you get when you burn 44.094 g of propane. We can write the
complete reaction as:
C3H8 + 5 O2 → 3 CO2 + 4 H2O + 529 kcal
6.95 a) The mammal produces 3.50 kcal of heat (the calculator answer is 3.50184071 kcal).
The relationship is 178.192 g of C3H7NO2 to 624 kcal of heat.
b) The nutritional value of alanine is 3.50 Calories, which is somewhat lower than the
Calorie value for proteins. (The nutritional value of protein is an average value for all of the
amino acids in typical proteins.)
6.97 a) The normal combustion reaction produces 4.35 kcal of heat (the calculator answer is
4.35485319 kcal). The relationship in normal combustion is 356.384 g of C3H7NO2 to 1552 kcal
of heat. When your body burns alanine, you produce 3.50 kcal of heat (from Problem 6.96).
b) Your body loses roughly 20% of the heat of combustion.
6.99
The cottage cheese contains roughly 230 Calories (rounded to the nearest 10 Calories).
6.101 The Twinkie® contains roughly 5 g of fat. (Subtracting the Calories from protein and
carbohydrate gives us 48 Calories from fat, which works out to 5.333333333 g according to the
calculator.)
6.103 Your body obtains 7.10 kcal when it burns 1.00 g of ethanol, so the Calorie value is 7.10
Calories per gram. This is substantially more energy than you obtain from a gram of
carbohydrate.
6.105 a) You need 400 calories.
b) Your body must burn 0.1 g of carbohydrate. (Remember that a Calorie is actually a
kilocalorie, so you need only 0.4 Calories to melt the ice.)
6.107 You should eat no more than 67 grams of fat. (30% of 2000 Calories is 600 Calories.)
6.109 a) The rate will increase.
c) The rate will increase.
b) The rate will decrease.
d) The rate will increase.
6.111 Catalysts lower the activation energy of the reaction, and they position the reactants in
the correct orientation. Lowering the activation energy has a greater impact on the reaction rate.
6.113 Bacteria cannot regulate their temperature, so they cool down when you put the food into
the refrigerator. All reactions slow down at colder temperatures, including reactions in living
organisms, so the reactions that produce the unpleasant products occur more slowly in
refrigerated food.
6.115 a) The HCl-catalyzed reaction is slower, so it must have the larger activation energy.
b)
ENERGY
red curve: catalyzed by HCl
(high activation energy)
green curve: catalyzed by trypsin
(low activation energy)
energy of
reactants
energy of
products
6.117 NaOH is a more effective catalyst, because the reaction has a lower activation energy
when you use NaOH. The lower the activation energy, the faster the reaction.
6.119 The energy of the reactants is 20 kcal and the top of curve is 60 kcal, so the activation
energy is 40 kcal (60 – 20 = 40). The reactant energy is 20 kcal and the product energy is 0 kcal,
so the heat of reaction is –20 kcal. (The heat of reaction is negative because the energy of the
products is lower than the energy of the reactants.) This is an exothermic reaction.
6.121 a) This reaction is reversible. The ethanol is continually breaking down into C2H4 and
H2O, so we also have some C2H4 and H2O in our mixture.
b) Some of the ethanol will break down into C2H4 and H2O.
6.123 a) The reaction goes forward, consuming some of the CO that we added.
b) The reaction goes backward, consuming some of the CO2 that we added.
c) The reaction goes forward, making H2 to replace some of the H2 we removed.
6.125 a) Adding CO will drive the reaction forward, making some H2 and CO2, so adding CO
will increase the amount of H2 in the equilibrium mixture.
b) Removing CO will drive the reaction backward, consuming some of the H2 in our
original mixture, so removing CO will not increase the amount of H2 in the equilibrium mixture.
c) Adding CO2 will drive the reaction backward, consuming some of the H2 in our
original mixture, so adding CO2 will not increase the amount of H2 in the equilibrium mixture.
d) Removing CO2 will drive the reaction forward, making some H2 and CO2, so adding
CO2 will increase the amount of H2 in the equilibrium mixture.