Chemistry 1312, Exam 3

Spring, 2000

I. Multiple Choice. (90 points) Select the best response for each of the following and record your answers on the Scantron. Do not erase; erasures may result in a misgrading of your response.

  1. All of the following statements about catalysts are true except
    a.the catalyzed reaction has a greater reaction rate
    b.the products of the reaction remain the same
    c.the activation energy for the reaction is lowered
    d.the enthalpy of the reaction is decreased
    e.the mechanism of the reaction is altered
  2. The experimental data for a chemical reaction is: D H°rxn = +27.7kJ and the Ea for the forward reaction is 130 kJ. What is the Ea for the reverse reaction?
    a.130 kJ
    b.157.7 kJ
    c.72.3 kJ
    d.27.7 kJ
    e.none of the above
  3. Kinetic data collected from a chemical reaction is plotted. Which set of plotted data produces a linear relationship for a 1st order reaction with respect to the reactant designated as "A"?
    a.[A]t vs. time
    b.1/[A]t vs. time
    c.ln 1/[A]t vs. time
    d.ln [A]t vs. time
    e.ln [A]t vs. 1/T
  4. Which set of plotted kinetic data will provide a positive slope equal to the value of the rate constant for reactant "A" in a chemical reaction?
    a.ln [A]t vs. 1/T
    b.ln [A]t vs. time
    c.[A]t vs. time
    d.ln k vs. 1/T
    e.1/[A]t vs. time
  5. The half-life for a first order reaction is 20 seconds. What fraction of the original sample will be left after 100 seconds?
    a.none
    b.1/4
    c.1/16
    d.1/32
    e.1/64
  6. To determine the activation energy for a chemical reaction
    a.the rate constant for the reaction must be determined at two different temperatures
    b.the natural logarithm of the concentration of a reactant is plotted vs. time
    c.the positive slope of the natural logarithm of the rate constant vs. temperature must be determined
    d.the y-intercept of the natural logarithm of the rate constant vs. the reciprocal temperature must be determined
    e.the enthalpy change and the entropy change of the reaction must be determined
  7. All of the following factors affect reaction rates except
    a.color of reactants
    b.concentration of reactants
    c.temperature of the reaction system
    d.macro particle size of reactants
    e.presence of a catalyst
  8. The following reaction is exothermic: CO(g) + Cl2(g) ® COCl2(g). What can be inferred from the reaction?
    a.the reaction is spontaneous only at high temperatures
    b.the reaction is spontaneous only at low temperatures
    c.the reaction is spontaneous at all temperatures
    d.the reaction is nonspontaneous at all temperatures
    e.the activation energy of the reaction system is low
  9. The initial reaction rate for a chemical reaction increases by a factor of 8 when the concentration of one of the reactants increases by a factor of 2. What is the order of the reaction with respect to that one reactant?
    a.1st
    b.2nd
    c.3rd
    d.4th
    e.need additional data
  10. The rate law for a chemical reaction is: R = k[A][B]2. What is/are the units of the rate constant if the reaction rate is measured in mol/L•min?
    a.mol/L•min
    b.min-1
    c.L4/mol4•min
    d.L2/mol2•min
    e.L3/mol3•min
  11. All of the following chemical reactions are entropy driven except
    a.N2(g) + 3 H2(g) ® 2 NH3(g)
    b.CH3OH(l) ® CH3OH(g)
    c.2 Fe2O3(s) + C(gr)     ® 4 Fe(s) + 3 CO2(g)
    d.CaCO3(s)     ¾ D® CaO(s) + CO2(g)
  12. Which of the following expressions must be true condition for a spontaneous process to occur?
    a.    DSsys > 0
    b.    DSsys + DSsurr < 0
    c.DSsurr + DSsys > 0
    d.DStotal < 0
    e.DSsurr + DSsys = 0
  13. A chemical reaction is known to be spontaneous at high temperatures but nonspontaneous at low temperatures. Therefore, the reaction
    a.must be endothermic with a decrease in entropy
    b.must be endothermic with an increase in entropy
    c.must be exothermic with a decrease in entropy
    d.must be exothermic with an increase in entropy
    e.must be exothermic with no change in entropy
  14. For the reaction, PbO(s) + C(gr) ® Pb(s) + CO(g), DH°rxn = +108 kJ/mol and DS°sys = +190 J/mol•K. At what temperature is the system at equilibrium?
    a.568 K
    b.295 K
    c.1.76 K
    d.298 K
    e.none of the above
  15. Reactions for which DG° < 0 are
    a.spontaneous for all conditions
    b.spontaneous under standard conditions
    c.not spontaneous under standard conditions
    d.not spontaneous under any conditions
    e.at equilibrium under standard conditions
  16. The second law of thermodynamics means that
    a.the entropy of a system always increases
    b.the entropy of the surroundings always increases
    c.spontaneous changes increase the entropy of the universe
    d.spontaneous changes occur with an increase in the free energy
    e.the entropy of a perfectly crystalline substance is zero at 0 K

II. Exercises. (50 points) Complete the following exercises in the space provided. Your organized and chemically logical (neat) work must be shown; correct answers only account for 10% of the credit indicated.

1. (15 points) Dinitrogen pentoxide, N2O5, decomposes at 60°C:
2 N2O5(g) ® 4 NO2(g) + O2(g)
The decomposition reaction follows first order kinetics with a rate constant of 5.2 x 10-3 s-1.

a. (5 pts) What is the half-life for the decomposition of N2O5(g) at 60°C?

133 seconds

b. (10 pts) If the pressure of N2O5(g) in a reaction vessel is initially 37.0 torr and then heated to 60°C, what will be the pressure of the N2O5(g) in the reaction vessel after 10 minutes?

1.63 torr

2. (15 points) Given the following rate data for the reaction, NO2(g) + CO(g) ® NO(g) + CO2(g)

[NO]o [CO]o Initial rate (mol/L•s)
0.25 0.40 3.1 x 10-5
0.25 0.80 3.1 x 10-5
0.50 0.80 1.2 x 10-4
0.35 0.35 ???

a. (5 pts) The order of the reaction with respect to NO is _____2nd________


b. (5 pts) The order of the reaction with respect to CO is ______0__________.

c. (5 pts) What is the predicted reaction rate for the 4th reaction?

6.1 x 10-5 mol/L s

3. (20 points) Consider the reaction, 2 SO2(g) + O2(g) ® 2 SO3(g)

 

SO2(g)

O2(g)

SO3(g)

DH°f (kJ/mol)

–296.13

0.0

–395.72

S°(J/mol•K)

248.12

205.03

256.72

a. (5 pts) What is the standard entropy change of the system?

-187.83 J/K

b. (5 pts) What is the standard enthalpy change of the system?

-199.18 kJ

c. (10 pts) Is the reaction spontaneous at standard conditions? Work must be shown.

–143.21 kJ
Reaction is spontaneous at standard conditions because the standard free energy value is negative.

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