Quiz Four Practice, Chemistry 111, Lycoming College, Feb. 16, 2005, Dr. Mahler

Fair Game for Friday’s Quiz: All previous material (Ch. 11, 12, 13). New Material – Ch. 13 section 9, Chapter 14 Sections 1, 2 and 3. Relevant Homework: Ch. 13 all assigned, but especially 61, 65, 67, 68 and 85c (all mechanisms); Ch. 14 problems 22, 25, 29, 32, 35, 38 and 41 (keys on web: http://www.lycoming.edu/chem/spring2005/111/keys.htm)

Practice problems:

1) Possible essay / short answer type questions:

• Discuss mechanisms, molecularity and assumptions used to solve mechanisms.
• Why do we not include species that are pure liquids or solids in the equilibrium expression?
• Explain how the equilibrium constant, K, changes if a reaction is a) reversed, b) multiplied by a constant, or c) how to find K for a reaction that is the sum of several reactions, each with their own K values.

2) A reaction was believed to occur by the following mechanism:

Step 1)             A₂ à A + A and its reverse, A + A à A₂

Step 2)             A + A + B à A₂B

Step 3)             A₂B + C à A₂C + B

a) Write the overall reaction. b) Write the rate law for each step and indicate its molecularity. c) Identify each species as a reactant, product, intermediate, or catalyst.

3) The following mechanism has been proposed for the reaction of chlorine and chloroform, CHCl₃:

Step 1)             Cl₂ à Cl + Cl and its reverse, Cl + Cl à Cl₂                 (both fast)

Step 2)             CHCl₃ + Cl à CCl₃ + HCl                                                      (slow)

Step 3)             CCl₃ + Cl à CCl₄                                                       (fast)

Write the rate law based on this mechanism, clearly stating any assumptions / reasoning.

4) Write the expression for K_c for each of the following reactions:

a) NH₄HS (s) ó NH₃ (g) + H₂S (g)

b) ZnO (s) + CO (g) ó Zn (s) + CO₂ (g)

c) SbCl₅ (g) ó SbCl₃ (g) + Cl₂ (g)

d) H₂ (g) + I₂ (g)  ó 2 HI (g) 

5) Calculate the following (all at equilibrium):

a) The concentration of H₂ (g) in reaction 4d), above, if at 500 K, K_c = 160, [HI] = 2.21 mmol/L, and [I₂] = 1.46 mmol/L.

b) At 500 K, K_c = 3.5 x 10^-4, [SbCl₅] = 0.15 M, [SbCl₃] = 0.20 M. What is [Cl₂]? See 4c).