Exam 1A, Spring 2001

1. When aqueous hydrogen azide reacts with aqueous hydrogen peroxide the products are water and nitrogen gas.

a. At some temperature, when the initial concentration of hydrogen peroxide is 0.025 M, after 5 s the concentration has decreased to 0.019 M. What is the initial rate of loss of hydrogen peroxide? Give the units.

b. For the same experiment as part a), what is the initial rate of gain of nitrogen gas?

2. The following data was measured for the reaction: 2 NO₂(g) → 2 NO(g) + O₂(g) at 573 K.

t (s) [NO₂] (M) t (s) [NO₂] (M)

0 1.00 0 0.50

0.25 0.88 1.25 0.37

0.55 0.77 2.40 0.30

0.95 0.66 3.10 0.27

1.40 0.57 3.70 0.25

1.85 0.50 4.25 0.23

2.10 0.47 5.05 0.21

2.75 0.40 5.65 0.20

Find the rate law for the reaction, determine the value of the rate constant, and give the units of the rate constant.

3. For the reaction C₂H₅Br(aq) + OH^–(aq) → C₂H₅OH(aq) + Br^–(aq) the rate constant at 25 ^oC was found to be 8.8×10^–5 M^–1s^–1 while at 35 ^oC the rate constant was measured to be 2.8×10^–4 M^–1s^–1. Estimate the activation energy in units of kJ/mol and determine the total order of the reaction.

4. Write the mass action expression for K_c for the following reactions:

a. CO₂(g) →← CO(g) + O₂(g)

b. CH₄(g) + O₂(g) →← CH₃OH(l)

c. CaCO₃(s) →← CaO(s) + CO₂(g)

5. Find the equilibrium constant K_c for the reaction: CdS(s) + 2 H₃O⁺(aq) →← Cd²⁺(aq) + H₂S(aq) + 2 H₂O(l), given the following data (T = 25 ^oC):

CdS(s) + H₂O(l) →← HS^–(aq) + Cd²⁺(aq) + OH^–(aq) K_c = 8×10^–28

H₂S(aq) + H₂O(l) →← H₃O⁺(aq) + HS^–(aq) K_c = 1.0×10^–7

H₃O⁺(aq) + OH^–(aq) →← 2 H₂O(l) K_c = 1.0×10¹⁴

6. Consider the reaction nitrogen gas with oxygen gas to form nitrogen monoxide gas, which is an endothermic reaction. Which way does the reaction shift if:

a. Nitrogen is added to the reaction mixture?

b. The total pressure is increased by decreasing the volume of the reaction vessel?

c. The temperature is increased?

7. The following equilibrium data was obtained for the reaction H₂(g) + I₂(g) →← 2 HI(g):

Experiment[H₂]_e (M) [I₂]_e (M) [HI]_e (M)

10.0032583 0.0012949 0.015869

20.0046981 0.0007014 0.013997

30.007106 0.007106 0.005468

Find the average K_c and K_p for this reaction.

Answers