Chemistry 1312, Exam 3
Spring, 1999

I. Multiple Choice (66 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. Which of the following data plots is used to determine the activation energy for a chemical reaction from its slope?
    a. ln [A]t vs. time
    b. ln k vs. 1/T
    c. k vs. Ea
    d. [A]t vs. time
    e. 1/[A]t vs. time
  2. Which of the following changes is not entropy driven?
    a. CaO(s) + 3 C(s)     => CaC2(s) + CO(g)
    b. H2O(s)     => H2O(l)
    c. 2 SO2(g) + O2(g)     => 2 SO3(g)
    d. 2 H2O(g)     => 2 H2(g) + O2(g)
    e. 2 NaNO3(s) —    D-> 2 NaNO2(s) + O2(g)
  3. One of the following statements is false in interpreting thermodynamic parameters. Which one?
    a.     DG for a chemical reaction in progress is never positive.
    b. If     DG°rxn is positive, the chemical reaction is nonspontaneous at standard conditions.
    c. The magnitude of     DG is a measure of the useful work that can be obtained from an existing chemical reaction.
    d. Ultimately, only entropy changes of the system and surroundings determine the spontaneity of a reaction.
    e    . The magnitude of DG is a measure of the equilibrium constant for the reaction.
  4. The rate law for the thermal decomposition of N2O(g) Æ products is a first order reaction. Which set of plotted data will yield a straight line?
    a. 1/[N2O]t vs. time
    b    . ln [N2O]t vs. time
    c. ln k vs. time
    d. ln [N2O]t vs.1/time
    e. [N2O]t vs. time
  5. Hydrogen peroxide decomposes according to the following mechanism:
    H2O2 + Br2     => 2 Br- + 2 H+ + O2
    H2O2 + 2 Br- + 2 H+     => 2 H2O + Br2
    Which of the following statements is not true for the reaction?
    a. The entropy of the reaction system increases
    b. Br- is the catalyst for the reaction
    c. The products of the reaction are H2O and O2
    d. Each step has an activation energy
    e. The concentration of Br2 remains constant in the reaction
  6. For the reaction, CH3OH(l) + CO(g) => CH3COOH(l), DH = –37.8 kJ
    This reaction will be
    a. nonspontaneous at all temperatures
    b. spontaneous at all temperatures
    c. spontaneous only at high temperatures
    d    . spontaneous only at low temperatures
    e. not enough information to determine
  7. For the reaction, 4 Fe(s) + 3 O2(g) => 2 Fe2O3(s), DG° = –1484.4 kJ
    Which of the following statements is true?
    a    . The reaction occurs spontaneously and is enthalpy driven
    b. The reaction does not occur spontaneously at low temperatures
    c. The reaction occurs spontaneously but is neither enthalpy or entropy driven
    d. The reaction does not occur spontaneously at 25°C
    e. The reaction occurs spontaneously and is entropy driven
  8. For the reaction, (CH3)3COH(l) => (CH3)2C=CH2(g) + H2O(l), the DH°rxn = + 51.9 kJ/mol and Ea = 274.1 kJ/mol. What is the activation energy for the reverse reaction?
    a. 51.9 kJ/mol
    b. 222.2 kJ/mol
    c. 274.1 kJ/mol
    d. 326 kJ/mol
    e. none of the above
  9. For a spontaneous process, all of the following are true statements except
    a    . DS for the system is always positive for a spontaneous process
    b.     DSsurr is always negative for an endothermic process
    c. +    DSsurr must be greater than -DS of the system for the process to be spontaneous
    d.     DStot for any spontaneous process must be greater than zero
    e. If     DHrxn is positive, then DSsys must be positive for a spontaneous process
  10. Chemical thermodynamics provides information about each of the following except:
    a. whether changes can occur spontaneously
    b. how rapidly a chemical reaction can occur
    c. the equilibrium constant for a reaction
    d. whether energy changes (at constant pressure) or entropy changes "drive" a reaction
    e. the amount of useful work that can be extracted from a reaction system
  11. All of the following parameters are necessary for an "effective" collision (i.e, one that potentially produces products) except
    a. the collision energy must be equal to or greater than the activation energy
    b. the molecule(s) must physically collide
    c. the molecular collisions must occur with the "proper" geometry
    d. the molecule(s) must have molecular motion
    e    . a catalyst must be present in the system
  12. A spontaneous reaction occurs at all temperatures when
    a. there is an increase in the free energy of the reaction system
    b. the reaction is endothermic accompanied by an increase in disorder of the reaction system
    c. the reaction is endothermic accompanied by an decrease in disorder of the reaction system
    d    . the reaction is exothermic accompanied by an increase in disorder of the reaction system
    e. the reaction is exothermic accompanied by an decrease in disorder of the reaction system

II. Exercises. (72 points) All work must be shown clearly and legibly in order to receive credit. Answers, without a logical presentation, will receive only 10% credit.

1. (33 points) Consider the following data for the reaction:
C2H4(g) + 3 O2(g) => 2 CO2(g) + 2 H2O(l)

C2H4 (g)

O2(g)

CO2(g)

H2O(l)
DG°f (kJ/mol)

+68.15

0

–394.36

–237.13
DH°f (kJ/mol)

+52.26

0

–393.51

–285.83

    a. (7 pts) Determine the DG° for the reaction. What does its value mean?

    DG° = -1331.13 kJ; Reaction is spontaneous at standard conditions

    b. (7 pts) Determine the     DH° for the reaction. What does its value mean?

    DH° = -1410.06 kJ; Reaction is exothermic.

    c. (7 pts) Determine the     DS° for the reaction. What does its value mean?

    DS°system   =  -265 J/K; A decrease in disorder of the reaction system.

    d. (7 pts) Determine the     DS° for the surroundings at 25°C. What does its value mean?

    DS°surr   =  +4.73 kJ/K; An increase in the disorder of the surroundings.

    e. (5 pts) Does the reaction violate the 2nd Law of Thermodynamics? Explain.

    DS°tot  =   + 4.73 kJ/K  +  (-0.265 kJ/K) = +4.47 kJ/K  >  0
    Since     DS°tot > 0, the reaction is spontaneous at standard conditions according to the second law of thermodynamics.

    2. (19 points) The rate constant for the 1st order dissociation of ethane, C2H6, into methyl radicals is 5.5 x 10-4s-1 at 700°C.
    C2H6(g)     => 2 •CH3(g)
    a. (5 pts) What is the half-life for the reaction at 700°C?

    t1/2 = 1260 seconds = 21 minutes

    b. (7 pts) If a 100 mg sample of ethane is confined to a 200 mL vessel and heated to 700°C, what will be its concentration after 25 hours?

    5.3 x 10-24 mol/L

    c. (7 pts) How much time (hours) will elapse for a sample of ethane to undergo a 30% dissociation?

    648 seconds, 10.8 minutes, or 0.18 hours

    3. (20 points) Initial reaction rate data were collected for the reaction:
    2 NO(g) + 2 H2(g)     => N2(g) + 2 H2O(g)

Exp’t No.

[NO]o

[H2]o

Initial Rate (mol/L•s)

1

0.420

0.122

0.136

2

0.210

0.122

0.0339

3

0.210

0.244

0.0678

4

0.150

0.350

??

a. (10 pts) The rate law is (must show work)

rate = k[NO]2[H2]

b. (5 pts) The value (and units) of the reaction rate constant is

k = 6.32 L2/mol2 s

c. (5 pts) What is the predicted reaction rate for Experiment No. 4?

rate = 4.98 x 10-2 mol/L s

Return to the Top of the Page

Return to Chemistry 1312 Exams Page