Chemistry 1312
Exam 3, Spring, 2001

I. Multiple Choice (80 points) Select the best response for each of the following and record on the scantron. Do not erase; erasures may result in a misgrading of your response.

1. For the reaction, S₂Cl₂ + CCl₄ ® CS₂ + 3 Cl₂, the reaction is first order in S₂Cl₂ and second order in CCl₄. Therefore,
a. the reaction rate increases by a factor of four when the [S₂Cl₂] doubles and the [CCl₄] doubles
b. the reaction rate increases by a factor of two when only the [CCl₄] doubles
c. the reaction rate increases by a factor of four when only the [S₂Cl₂] doubles
d. the reaction rate increases by a factor of eight when the [S₂Cl₂] doubles and the [CCl₄] doubles
e. additional data is necessary

2. An entropy increase is expected for all of the following changes except one. Which one?
a. C(diamond) ® C(graphite)
b. 2 H₂O₂(l) ® 2 H₂O(l) + O₂(g)
c. CO(g) + 2 H₂(g) ® CH₃OH(l)
d. Ba(OH)₂(s) ® BaO(s) + H₂O(g)
e. 2 NOCl(g) ® 2 NO(g) + Cl₂(g)

3. Heat is absorbed for the spontaneous reaction, N₂F₄(g) ® 2 NF₂(g). Which of the following statements is true about the reaction?
a. DG°_rxn is positive
b. DS°_sys is positive
c. DS°_total is negative
d. DS°_surr is positive
e. DH°_rxn is negative

4. For CF₄(g), DG°_f = –635.1 kJ/mol, DH°_f = –679.9 kJ/mol, and S° = +262.3 J/mol•K. All of the following statements are true regarding CF₄(g) except
a. the formation of CF₄(g) from its elements occurs rapidly
b. CF₄(g) is thermodynamically stable relative to its elements at 25°C
c. the formation of CF₄(g) from its elements is an exothermic process at 25°C
d. the formation of CF₄(g) from its elements is spontaneous at 25°C
e. the total entropy change for the formation of CF₄(g) from its elements is greater than zero at 25°C

5. One of the following statements is false regarding a reaction system. Which one?
a. If DS°_sys + DS°_surr < 0, the reaction is spontaneous
b. If +DS°_sys > –DS°_surr, the reaction is spontaneous
c. If DG° < 0 the reaction is spontaneous
d. If +DS°_surr > –DS°_sys, the reaction is spontaneous
e. If DS°_total > 0, the reaction is spontaneous

6. Indicate the conditions in which a reaction is spontaneous but only at high temperatures.
a. DH°_rxn < 0, DS°_sys > 0
b. DH°_rxn > 0, DS°_sys > 0
c. DH°_rxn < 0, DS°_sys < 0
d. DH°_rxn > 0, DS°_sys < 0
e. none of the above

7. For the reaction, SO₂(g) + 2 H₂(g) ® S(s) + 2 H₂O(g), DG° = –156.7 kJ. Which of the following statement is true?
a. The reaction occurs spontaneously but is driven primarily by entropy change at standard conditions.
b. The reaction occurs spontaneously but is driven primarily by enthalpy change at standard conditions.
c. The reaction does not occur at standard conditions.
d. The reaction occurs spontaneously and is driven both by entropy and enthalpy changes at standard conditions.
e. The reaction occurs spontaneously but only at high temperatures.

8. The rate constant for a second order reaction can be obtained from the slope of a straight line from which of the following data plots?
a. ln [A]_t vs. time
b. ln k vs. ¹/_temperature
c. ln [A]_t vs. ¹/_time
d. ¹/_[A]t vs. time
e. ¹/_[A]t vs. temperature

9. For the 1^st order decomposition for an organophosphorus compound, the half-life is 16 hours at room temperature. How many grams of a 30 g sample will remain after 20 hours?
a. >15 g, but < 30 g
b. > 7.5 g, but < 15 g
c. > 3.7 g, but < 7.5 g
d. > 1.9 g. but < 3.7 g
e. > 0.9 g, but < 1.9 g

10. For the reaction, BF₃(g) + NH₃(g) ® F₃BNH₃(s), the following rate data were obtained.
[BF₃] [NH₃]_o – D[BF₃]/Dt, (mol/L•s)
0.25 0.250 0.213
1.00 0.250 0.852
1.00 0.010 0.341
The rate law for the reaction is
a. rate = k[BF₃][NH₃]^2..5
b. rate = k[BF₃]⁴[NH₃]^2..5
c. rate = k[BF₃][NH₃]
d. rate = k[BF₃]²[NH₃]
e. rate = k[BF₃]

11. Which statement about chemical thermodynamics is false?
a. The entropy of a perfect crystalline substance at 0 K equals zero.
b. The total entropy change for a spontaneous process is less than zero.
c. Energy is conserved in all chemical and physical changes.
d. The free energy change for all spontaneous processes is less than zero.
e. All exothermic reactions accompanied by an entropy increase of the system are spontaneous at all temperatures.

12. All of the following are characteristics of a catalyst except one. Which one?
a. A catalyst lowers the activation energy for the reaction.
b. A catalyst reroutes a chemical reaction over a lower energy barrier.
c. A catalyst may be consumed in a reaction, but then regenerated upon product formation.
d. A catalyst increases the kinetic energy of the reactant molecules.
e. A catalyst may be homogeneous or heterogeneous with regard to the reactants and products.

13. Identify the correct statement.
a. If the DS°_sys of a reaction increases, the DS°_surr must decrease.
b. For system at equilibrium, DS°_sys = DS°_surr = 0
c. If a reaction is spontaneous, DS°_sys + DS°_surr must increase
d. All spontaneous reactions occur quickly.
e. All spontaneous reactions are exothermic.

14. Reaction rates can be affected by each of the following factors except
a. changes in the density of reactants
b. changes in temperature of the system
c. changes in the surface area of reactants
d. addition or removal of a catalyst
e. changes in the concentration of reactants

15. The activation energy, E_a, for a reaction is 100 kJ and the enthalpy of reaction, DH_rxn, is +55 kJ. What is E_a for the reverse reaction?
a. 155 kJ
b. 145 kJ
c. 45 kJ
d. 55 kJ
e. none of the above, need more information

16. A set of kinetic data was plotted as ln k vs. ¹/_T. A linear plot with a negative slope was the result. What can be inferred from the plot?
a. the slope of the plot must have been equal to 2, the order of the reaction with respect to the reactant A
b. the slope of the plot must have been equal to 1, the order of the reaction with respect to the reactant A
c. the reaction was second order with respect to the reactant A
d. the slope of the plot was equal to ^–Ea/_R for the reaction
e. the reaction was first order with respect to the reactant A

II. Exercises (57 points). The organization of all exercises must be shown clearly and legibly in order to receive any partial credit. Answers without a logical problem setup will receive only 10% credit.

1. (4 points) What is the value (and units) for the rate constant in Multiple Choice Question No. 10?

rate = k[BF₃][NH₃]

k = 3.41 L/mol•s

2. (18 points) The rate constant for the 1^st order decomposition of a certain pesticide (labeled RXX) in water at 12°C is 1.45 y^-1
a. (3 pts) Write the rate law for the reaction.

rate = k[RXX]

b. (5 pts) What is the half-life (in years) for the decomposition of RXX at 12°C?

t_1/2 = 0.478 year

c. (10 pts) How much time (in years) is necessary for a 6.70 µg quantity of RXX to decrease to 1.0 µg?

t = 1.31 year

3. (35 points) Consider the following data for the reaction, 2 POCl₃(g) ® 2 PCl₃(g) + O₂(g)

                    DH°_f (kJ/mol) DG°_f (kJ/mol)         S° (J/mol•K)
POCl₃(g) –542.2              –502.5              325
PCl₃(g)       –288.07              –269.6              311.7
O₂(g)               0                   0                     205.0
a. (5 pts) What is the entropy change of the system at 25°C?

DS°_sys = +178.4 J/K

b. (10 pts) What is the entropy change of the surroundings at 25°C?

DH° = +508.26 kJ

DS°_surr = –1.71 kJ/K

c. (5 pts) What is the free energy change for the reaction at 25°C?

DG° = +465.8 kJ

d. (10 pts) Identify the temperature conditions at which the reaction is spontaneous. Show your work!

T = 2855 K; at T > 2855 K, the reaction is spontaneous

e. (5 pts) What is the equilibrium constant for the reaction at 25°C? R = 8.315 J/mol•K

K = 2.3 x 10^-82

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