Introduction to Chemical Principles, 11e (Stoker)
Chapter 16 Reaction Rates and Chemical Equilibrium
16.1 Multiple Choice
1) Most reactions are carried out in liquid solution or in the gaseous phase because in such
situations ________.
A) activation energies are higher
B) reactant activation energies are lower
C) kinetic energies of reactants are lower
D) reactant collisions occur more frequently
2) For a collision between molecules to result in a reaction, the molecules must possess a
favorable orientation relative to each other ________.
A) and exchange electrons
B) and “stick together” for at least 10 seconds
C) and a certain minimum energy
D) and be in the gaseous state
3) Which of the following statements about “activation energy” is correct?
A) Activation energy is low for reactions which take place quickly.
B) Activation energy is the maximum energy a reacting molecule may possess.
C) Activation energy is the energy given off when reactants collide.
D) Activation energy is high for reactions which take place rapidly.
4) The minimum combined kinetic energy reactant particles must possess in order for their
collision to result in a reaction is called the ________.
A) dissociation energy
B) activation energy
C) collision energy
D) orientation energy
5) Whether a reaction is exothermic or endothermic is determined by ________.
A) the activation energy
B) the physical state of the reaction system
C) an energy balance between bond breaking and bond forming resulting in a net loss or gain of
energy
D) whether a catalyst is present
6) Which reaction below is endothermic?
A) NH3 + hr NH4Br + heat
B) CH4 + N2 + heat hn + NH3
C) 2NO2 N2 + 2O2 + heat
D) PCl3 + Cl2 PCl5 + heat
7) Which of the following changes would most likely decrease the rate of a reaction?
A) increasing the activation energy for the reaction
B) decreasing the activation energy for the reaction
C) increasing the reaction temperature
D) increasing the concentration of a reactant
8) A catalyst works by ________.
A) increasing the rate of a reaction by decreasing the heat of reaction
B) increasing the rate of a reaction by lowering the activation energy of the forward reaction only
C) increasing the rate of a reaction by increasing the activation energy of the reverse reaction
only
D) increasing the rate of a reaction by providing an alternative pathway with a lower activation
energy
3
9) Increasing the temperature at which a chemical reaction occurs ________.
A) will lower the activation energy, thus increasing the reaction rate
B) will raise the activation energy, thus decreasing the reaction rate
C) causes more reactant collisions to take place in a given time, thus increasing the reaction rate
D) will cause fewer reactant collisions to take place
10) Catalysts are correctly characterized by each of the following statements except one. The
exception is ________.
A) catalysts can be either solids, liquids or gases
B) catalysts lower the activation energy for a reaction
C) catalysts do not “actively” participate in a reaction
D) catalysts are not “consumed” in a reaction
11) Reaction A releases 24 kJ/mole and has an activation energy of 98 kJ/mole. Which statement
is correct concerning Reaction A?
A) The overall reaction is endothermic with a high activation energy.
B) The overall reaction is endothermic with a low activation energy.
C) The overall reaction is exothermic with a high activation energy.
D) The overall reaction is exothermic with a low activation energy.
12) Reaction conditions for a hypothetical reaction, X + Y W + Z, are given below.
Predict which reaction should occur at a faster rate. (M = molarity)
Reaction
Activation energy
Temperature
1
low
low
2
low
high
3
high
high
4
high
low
A) 1
B) 2
C) 3
D) 4
4
13) Which response includes all the factors that would increase the rate of a chemical reaction,
and no others?
I. Lowering the temperature of the chemical reaction.
II. Significantly increasing the concentration of one of the reactants.
III. Adding a catalyst to an uncatalyzed reaction.
IV. Decrease the reaction surface area.
V. Increasing the pressure on a gaseous reaction system.
A) I and IV
B) II, III and V
C) I, IV and V
D) III only
14) Increasing the temperature of a chemical reaction increases the rate of reaction because
________.
A) the activation energy decreases
B) the activation energy increases
C) the collision frequency of reactant molecules increases
D) both the collision frequency and collision energies of reactant molecules increases
15) Reaction conditions for a hypothetical reaction, A + B C, are given below. Predict
which reaction should occur at the slowest rate.
Reaction
Activation energy
Temperature
1
102 kJ/mol
14 °C
2
48 kJ/mol
22 °C
3
102 kJ/mol
22 °C
4
48 kJ/mol
34 °C
A) 1
B) 2
C) 3
D) 4
16) The equilibrium constant for the following hypothetical reaction is two (Keq = 2). The initial
concentrations present are [A] = 4, [B] = 8, and [AB] = 16 at 25 °C
AB + Heat A (g) + B (g)
After a stress has been absorbed by the reaction, the new equilibrium concentrations are: [A] = 2,
[B] = 1, and [AB] = 64. The stress absorbed by the equilibrium system ________.
A) was a change in the reaction temperature evident by the change in the value of the Keq
B) was a decrease in the concentration of [A]
C) was a decrease in the concentration of [B]
D) was an increase in the concentration of [AB]
17) Which of the following conditions characterizes a system in a state of chemical equilibrium?
A) Concentrations of reactants and products are equal.
B) Rate of forward reaction has dropped to zero.
C) Reactants are being consumed at the same rate they are being produced.
D) Reactant molecules no longer react with each other.
18) Chemical equilibrium is reached in a system when ________.
A) product and reactant concentrations remain constant
B) reactant concentrations steadily decrease
C) product molecules begin reacting with each other
D) complete conversion of reactants to products has occurred
19) A mixture of 1.40 moles of A and 2.30 moles of B was allowed to reach equilibrium. At
equilibrium, it is found that 0.90 mole of A is present. How many moles of C are present at
equilibrium?
3 A (g) + 2 B (g) 4 C (g)
A) 1.6 moles
B) 0.67 mole
C) 1.1 moles
D) 1.3 moles
20) A chemical equilibrium expression depends on the ________ of a reaction.
A) rate
B) stoichiometry
C) stoichiometry and mechanism
D) mechanism
21) 0.56 mol of NO and 0.38 mol of Br2 are placed in a container and allowed to react until
equilibrium is established. At equilibrium there is 0.47 mol of NOBr present
2 NO (g) + Br2 (g) 2 NOBr (g)
What is the composition of the equilibrium mixture in terms of moles of each substance present?
A) 0.09 mol NO 0.15 mol Br2
B) 0.33 mol NO 0.09 mol Br2
C) 0.56 mol NO 0.38 mol Br2
D) 0.47 mol NO 0.47 mol Br2
22) In writing an equilibrium constant expression which of the following is incorrect?
A) Product concentrations are always placed in the numerator of the expression.
B) Concentrations are always expressed as molarities.
C) Concentrations of pure solids and pure liquids, when placed in the equilibrium expression, are
never raised to any power.
D) Reactant concentrations are always placed in the denominator of the expression.
23) Given the following reaction, the equilibrium expression will be:
4 CuO (s) + CH4 (g) CO2 (g) + 4 Cu (s) + 2 H2O (g)
A) [CH4]/[CO2][H2O]2
B) [CuO]4/[Cu]4
C) [Cu]4/[CuO]4
D) [CO2][H2O]2 /CH4]
24) Which of the following is the correct equilibrium expression for the reaction
CS2 (g) + 4 H2 (g) CH4 (g) + 2 H2S (g)
A)
B)
C)
D)
25) Which of the following is the correct equilibrium expression for the reaction
2 Ag (s) + Cl2 (g) 2 AgCl (s)
A)
B)
C)
D)
26) Identify the reaction that is described by the following equilibrium expression:
K = [H2]2 [O2]/[H2O]2
A) 2 H2 (g) + O2 (g) 2H2O (g)
B) H2O (g) H2 (g) + ½ O2 (g)
C) H2O (g) 2H (g) + O (g)
D) 2H2O (g) 2 H2(g) + O2 (g)
27) If, at equilibrium, most of the reactants remain unreacted, the equilibrium constant would be
expected to have ________.
A) a very large numerical value
B) a very small numerical value
C) a numerical value slightly greater than 1.0
D) a numerical value slightly less than 1.0
28) When the position of an equilibrium is described as being “far to the left” it means that
________.
A) very few reactant molecules are present in the equilibrium mixture
B) very few product molecules are present in the equilibrium mixture
C) significant amounts of both products and reactants are present in the equilibrium mixture
D) the rate of the reverse reaction is greater than that of the forward reaction
29) The grams of products present after a chemical reaction reaches equilibrium ________.
A) will always be greater than the grams of reactants present
B) will always be less than the grams of reactants present
C) must equal the grams of reactants present
D) may be less than, equal to, or greater than the grams of reactants present, depending upon the
chemical reaction under study
30) Which statement is false? An equilibrium constant for a particular reaction:
A) changes when the temperature increases.
B) changes when a catalyst is added.
C) does not change when an additional quantity of a reactant is added.
D) does not change when a product is removed.
31) Which one of the following will change the value of an equilibrium constant?
A) changing temperature
B) adding other substances that do not react with any of the species involved in the equilibrium
C) varying the initial concentrations of reactants
D) varying the initial concentrations of products
32) What is the equilibrium constant value if at equilibrium the concentrations are [NH3]= 0.40
M, [H2]= 0.12 M and [N2] = 0.040 M at a certain temperature?
2 NH3 (g) N2 (g) + 3 H2 (g)
A) 4.8 x 10−2
B) 4.3 x 10−4
C) 6.8 x 10−9
D) 7.2 x 1015
33) Calculate the equilibrium constant for the reaction below if a 3.25 L tank was found to
contain 0.343 mol O2, 0.0212 mol SO3 and 0.00419 mol SO2 at equilibrium.
2 SO3 (g) 2 SO2 (g) + O2 (g)
A) 6.78 x 10-2
B) 1.34 x 10-2
C) 4.12 x 10-3
D) 4.35 x 10-2
34) Given the following decomposition reaction: PCl5 (g) PCl3 (g) + Cl2 (g), when 0.84
moles of PCl5 is placed in a 1.0 L container it was found that when the reaction reaches
equilibrium, 0.72 moles of PCl5 still remains. What is the value of the equilibrium constant for
this reaction?
A) 0.62
B) 0.72
C) 0.020
D) 0.12
35) For a reaction which has an equilibrium constant of 3.8 x 10−16 at 25 °C, the position at
equilibrium is best described as ________.
A) mostly products
B) mostly reactants
C) same amount of products and reactants
D) significant amounts of both products and reactants
36) All of the following factors may shift the position of an equilibrium except one. The
exception is ________.
A) reduction of reaction volume
B) increase reaction temperature
C) double the pressure
D) addition of a catalyst
37) What effect does a catalyst have on an equilibrium?
A) it increases the rate at which an equilibrium is reached without changing the composition of
the reaction.
B) it slows the reverse reaction
C) it increases the rate of the forward reaction
D) it shifts the reaction to the right
38) If at equilibrium, reactant concentrations are slightly smaller than product concentrations, the
equilibrium constant would be ________.
A) a number >> 1
B) a number << 1
C) a number slightly lower than one
D) a number slightly higher than one
39) The expression for the equilibrium constant, Keq, for the reaction below is:
4 NH3 (g) + 5 O2 (g) 4 NO (g) + 6 H2O (g)
A)
B)
C)
D)
40) The expression for the equilibrium constant, Keq, for the reaction below is:
2 NaIO3 (s) 2 NaI (s) + 3 O2 (g)
A)
B)
C)
D) Keq = [O2]3
41) At a given temperature, K = 46.0 for the reaction
4 HCl (g) + O2 (g) 2 H2O (g) + 2 Cl2 (g)
At equilibrium, [HCl] = 0.150, [O2] = 0.395, and [H2O] = 0.625. What is the concentration of
[Cl2] at equilibrium?
A) 1.26 M
B) 0.00653 M
C) 0.153 M
D) 0.438 M
42) If the K eq = 2.2 x 1012 then which of the following statements is true?
A) mostly products are present
B) mostly reactants are present
C) some of both reactants and products are present
D) none of the above
43) If the K eq = 5.6 x 10-10 then which of the following statements is true?
A) mostly products are present
B) mostly reactants are present
C) some of both reactants and products are present
D) none of the above
44) Sulfur combines with hydrogen to form hydrogen sulfide, a toxic gas that is a product of
decay of organic material.
S (g) + H2 (g) H2S (g) K eq = 2.8 x 10-21
Which of the following statements is true concerning the equilibrium system?
A) Increasing the volume of the sealed reaction container would shift the equilibrium to the right.
B) Decreasing the concentration of H2 would shift the equilibrium to the right.
C) Decreasing the concentration of H2S would shift the equilibrium to the left.
D) Very little hydrogen sulfide gas is present in the equilibrium.
45) Hydrogen gas reacts with iron(III) oxide and produces iron metal which can be used to
produce steel according to the reaction below. Choose the response that is incorrect concerning
the equilibrium system.
Fe2O3 (s) + 3 H2 (g) + heat 2 Fe (s) + 3 H2O (g)
A) Lowering the reaction temperature will increase the concentration of hydrogen gas.
B) Increasing the pressure on the reaction chamber favors the formation of products.
C) Continually removing water from the reaction chamber increases the yield of iron.
D) Decreasing the volume of hydrogen gas will reduce the yield of iron.
46) CO2 and H2 are allowed to react until an equilibrium is established as follows:
CO2 (g) + H2 (g) H2O (g) + CO (g)
What will be the effect on the equilibrium after adding H2O to the equilibrium mixture?
A) The equilibrium will shift to the left.
B) H2 concentration will decrease and CO2 concentration will increase.
C) CO and CO2 concentrations will increase.
D) H2 concentration will decrease and H2O concentration will increase.
47) According to Le Chatelier’s principle, which of the following changes will shift to the left the
position of the equilibrium to the left of the following reaction?
N2 (g) + 3 H2 (g) 2 NH3 (g) + Heat
A) Increase the concentration of N2.
B) Decrease the pressure on the system.
C) Decrease the temperature.
D) Increase the concentration of H2.
48) In which of the following equilibrium systems will the equilibrium shift to the left when the
pressure of the system is increased?
A) N2 (g) + 3 H2 (g) 2 NH3 (g)
B) 4 NH3 (g) + 5 O2 (g) 4 NO (g) + 6 H2O (g)
C) 2 SO2 (g) + O2 (g) 2 SO3 (g)
D) H2 (g) + Cl2 (g) 2 HCl (g)
49) In which of the following reactions that are at equilibrium will a shift to the left occur when
the pressure is decreased?
A) 2 HCl (g) H2 (g) + Cl2 (g)
B) 2 SO3 (g) 2 SO2 (g) + O2 (g)
C) N2 (g) + 3 H2 (g) 2 NH3 (g)
D) N2O4 (g) 2 NO2 (g)
50) The following reaction is exothermic. Which of the following will drive the reaction to the
right (towards products)?
CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g)
A) a decrease in temperature
B) an increase in temperature
C) the removal of CH4
D) the addition of CO2
51) For a chemical reaction at equilibrium, which of the following would always decrease the
concentrations of the products?
A) Increase the temperature.
B) Decrease the temperature.
C) Decrease the pressure.
D) Decrease the concentration of a gaseous or aqueous reactant.
52) The following reaction is endothermic: CaCO3 (s) CaO (s) + CO2 (g). Which of
the following will cause the reaction to shift towards making more carbon dioxide gas?
A) increasing the temperature of the reaction
B) decreasing the temperature of the reaction
C) increasing the pressure of the system
D) increasing the pressure and decreasing the temperature of the system
53) Consider the following chemical system at equilibrium.
Heat + 6 H2O (g) + 2 N2 (g) 4 NH3 (g) + 3O2 (g)
Which of the following stresses would shift the equilibrium to the left?
A) increasing the concentration of O2
B) increasing the reaction temperature
C) increasing the concentration of H2O
D) decreasing the concentration of NH3
54) Consider the following system at equilibrium:
N2 (g) + 3 H2 (g) 2 NH3 (g) + 92. 94 kJ
Which of the following changes will shift the equilibrium to the right?
1. Increasing the temperature 2. Decreasing the temperature
3. Increasing the volume 4. Decreasing the volume
5. Removing some NH3 6. Adding some NH3
7. Removing some N2 8. Adding some N2.
A) 1, 4, 6, 7
B) 2, 3, 5, 8
C) 1, 6, 8
D) 2, 4, 5, 8
55) Coal burning plants release sulfur dioxide, a toxic gas, into the atmosphere. Nitrogen
monoxide is released into the atmosphere via industrial processes and the combustion engine.
Sulfur dioxide can also be produced in the atmosphere by the reaction of sulfur trioxide and
nitrogen monoxide according to the following equilibrium. reaction
SO3 (g) + NO (g) + Heat SO2 (g) + NO2 (g)
Which of the following stresses will not shift the equilibrium to the right?
A) decrease the reaction chamber volume (pressure increase)
B) temperature increase
C) [NO2] decrease
D) [NO] increase
56) Which of the following conditions would force the reaction to go to completion?
2 N2 (g) + 6 H2O (g) + heat 4 NH3 (g) + 3 O2 (g)
A) continual removal of N2 gas
B) decrease the pressure on the reaction vessel
C) continual addition of NH3 gas to the reaction mixture
D) increase reaction temperature
16.2 Short Answer
1) a) Draw an energy diagram for the following reaction, whose activation energy is 75 kJ/mol
and whose overall energy for the reaction is exothermic. Be sure to label all parts of the graph,
including potential energy of reactant/products, Ea and enthalpy of reaction. b) How does a
catalyst speed up a reaction? Explain and illustrate it on your graph.
H2 (g) + I2 (g) —> 2 HI (g) + heat
2) A 0.20 mole sample of C and a 0.10 mole sample of B are placed in a reaction chamber. At
equilibrium, 0.080 mole of A is present. What is the composition of the equilibrium mixture in
terms of moles of each substance present.
4 C (g) + B (g) 2 A (g)
3) The value of the equilibrium constant (Keq) for the reaction
N2O4 (g) 2 NO2
is ________ if the concentrations of each species at equilibrium are [N2O4] = 0.300 and [NO2]
= 0.0054. The value of (K) indicates that the ________ reaction is favored.
4) Write the expression for the equilibrium constant for the following reaction.
CuO (s) + H2 (g) Cu (s) + H2O (g)
5) Write the expression for the equilibrium constant for the following reaction.
2 Na2O (s) 4 Na (s) + O2 (g)
6) The equilibrium constant is 8.5 x 10-3 for the reaction shown below. If 0.055 mol of IBr is
placed in a 3.00 L container, what is the concentration of each substance remaining after
equilibrium is established?
2 IBr (g) I2 (g) + Br2 (g)