Leaders: / Lindsey and Elizabeth
Course: / Chem 178
Date: / 4-13-15

Exam 3

Supplemental InstructionIowa State University

1. A catalyst increases the rate of a reaction by doing which of the following?

a. Increasing reactant concentrations

b. Increasing temperature

c. Decreasing temperature

d. Increasing activation energy of a reaction

e. Decreasing activation energy of a reaction

2. Given the following mechanism for a reaction:

Cl2(g)  2Cl(g)

2NO(g) + 2Cl(g)  N2(g) + 2 ClO(g)

2ClO(g)  Cl2(g) + O2(g)

Which of the following is a catalyst in the reaction?

a. Cl2

b. N2O5

c. N2

d. ClO

e. O2

3. What are the units of k for the rate law Rate=k[A][B]2 when the concentration unit is mol L-1?

a. s-1

b. s

c. L mol-1 s-1

d. L2 mol-2 s-1

e. L2 – s2 mol-2

4. The rate law for the reaction between NO(g) and H2(g) is rate=k[H2][NO]2.. Which statement is true based on the rate law?

a. NO(g) is consumed twice as fast as H2(g)

b. The rate-determining step involves a three-body collision

c. If the concentration of NO is doubled the rate doubles

d. The reaction is third order

e. The chemical reaction is H2(g) + 2NO(g)  2HO(g) + N2(g)

5. For which of the following changes is S negative?

a. H2O(l)  H2O(s)

b. CO(g) + 0.5O2(g)  CO2(g)

c. 2SO3(g)  2SO2(g) + O2(g)

d. CO2(s)  CO2(g)

e. 2AgCl(s)  2Ag(s) + Cl2(g)

6. Given the energy profile diagram, which of the following phrases could directly describe the kind of reaction shown:

a. (1) and (3)

b. (2) and (4)

c. (1), (3), and (5)

d. (2), (4), and (5)

e. (1), (2), (3), (4), and (5)

7. Given that a reaction is exothermic and has an activation energy of 50 kJ/mol, which of the following statements are correct? (more than one answer!)

1. The reverse reaction has an activation energy equal to 50 kJ/mol

2. The reverse reaction has an activation energy less than 50 kJ/mol

3. The reverse reaction has an activation energy greater than 50 kJ/mol

4. The reaction rate increases with temperature

5. The reaction rate decreases with temperature

8. What can we say about catalysts given the following information?

CO(g) + 3H2(g)  CH4(g) + H2O(g)using Ni catalyst

CO(g) + 3H2(g)  CH3OH(g)using ZnO/Cr2O3 catalyst

a. Catalysts are nonspecific in their activity

b. Catalysts are highly specific in their activity

c. Ni is a better catalyst than ZnO/Cr2O3

d. ZnO/Cr2O3 is a better catalyst than Ni

e. Most metals can act as a catalyst

9. The following mechanism is proposed:

H2 (g) + ICl (g) => HI (g) + HCl (g)

HI (g) + ICl (g) => I2 (g) + HCl (g)

a. Write the balanced equation for the overall reaction.

b. Identify all the intermediates in the reaction

c. If the first step is slow and the second step is fast, which rate law do you expect to be observed for the overall reaction?

10. What is the difference between average rate and instantaneous rate?

11. For the following reaction (use for questions 7-9): SO2 + O3 => SO3 + O2

Experiment / [SO2], M / [O3], M / Initial Rate, M/s
1 / 0.25 / 0.40 / 0.118
2 / 0.25 / 0.20 / 0.118
3 / 0.75 / 0.20 / 1.062
  1. What is the rate law expression?
  1. What is the rate constant?
  1. What are the units of the rate constant?
  1. What is the order of the rate law?

12.

NH4NO3(s) + H2O(l) NH4+(aq) + NO3-(aq)

Compound Hfo(kJ/mol) S(J/mol-K)

NH4NO3(s) -365.56 151.08

NH4+(aq) -132.51 113.4

NO3-(aq) -205.0 146.4

  1. Calculate ΔS for this reaction.
  2. Is this reaction spontaneous at 20 ˚C?

13. What is the solubility of AgCl in mol/L in a 0.2 M MgCl2solution?

Ksp= 1.8x10-10for AgCl

14. If x equals the solubility of Ag2CO3, then how is the value of Kspfor Ag2CO3related to the value of x?

15. Ag+is carefully and slowly added to a solution containing Cl–, Br–, and I–. Which compound would precipitate out of solution first?

a. AgCl (Ksp=1.8x10-10)

b. AgBr (Ksp=5x10-13)

c. AgI (Ksp=8.3x10-17)

d. All simultaneously precipitate

16. The solubility of BaCO3in water is increased by which of the following?

a. Addition of NaOH

b. Addition of HNO3

c. Increasing pH of solution

d. Both a & c

17. The heat of vaporization, ΔHvap of carbon tetrachloride, CCl4, at 25ᵒC is 43.0 kJ/mol. If 1 mol of liquid CCl4 has an entropy of 214 J/K, what is the ΔS of the system?

18. The half-life of strontium-90 is 25 years. How much strontium-90 will remain after 100 years if the initial amount if 4.0g?

19. N2O5decomposes into the solvent CCl4as follows:

2 N2O54 NO2+ O2

The specific rate constant for the first-order decomposition at 45oC is 6.32x10-4s-1.

a. What is the concentration of N2O5remaining after 2 hours if the initial concentration of N2O5was 0.5M?

b. How much time is required for 90% of the N2O5to decompose?