Directions: Questions 1 and 2 are long free-response questions that should require about 15 minutes each to answer. Questions 3 and 4 are short free-response questions that should require about 5 minutes each to answer. Read each question carefully and write your response. Answers must be written out. Outline form is not acceptable. It is important that you read each question completely before you begin to write.
Question 1: Rates of Reactions (14 points)
A common reaction used to study reaction kinetics is the dissolution of sodium thiosulfate in hydrochloric acid. This process is monitored by cloudiness in the container for the following reaction:
2HCl(aq) + Na2S2O3(aq) → S(s) + SO2(g) + 2NaCl(aq) + H2O(l)
a) Write 3 – 4 sentences to describe the difference between the chemical reaction if it were set up so that test tubes containing a 0.1 M HCl solution and a 0.1 M Na2S2O3 solution were incubated on ice or in a 70°C water bath before the reaction began. Be sure to include observations and the effect on the rate constant. (6 points)
b) Suppose 1 M Na2S2O3 increased the rate of the reaction but 1 M HCl did not affect the reaction rate. Write 3 – 4 sentences to describe what these results tell about the rate law and the mechanism for the reaction. Explain your reasoning. (4 points)
c) Write 3 – 5 sentences to identify at least two other ways to increase the rate of a reaction and describe why they could not be used for this reaction. (4 points)
Question 2: Successful Collisions (12 points)
a) The graph below shows the energy profile for an exothermic reaction. Write 3 – 5 sentences to describe the reaction profile and the energy terms that are necessary for a reaction to be successful. (4 points)
b) The diagram below shows the proper orientation of two reactants in order for a successful collision to occur. Use this model to explain why some collisions are not successful by describing the other ways these two reactants can collide. Draw additional images to illustrate. (4 points)
c) Write 4 – 5 sentences to describe how the number of successful collisions can be increased. (4 points)
Question 3: Reaction Mechanisms (10 points)
a) What would you expect to be the rate law of the following reaction if it were a binary interaction that was dependent on both reactants? (2 points)
2NO(g) + Cl2(g) → 2NOCl(g)
b) Suppose, however, that the reaction followed a two-step mechanism:
Identify the rate law for this stepwise reaction. (3 points)
c) Identify the intermediate in the reaction mechanism. (2 points)
d) Write 2 – 3 sentences to describe why the reaction mechanism in (b) is possible, but also explain why it is only called a possible mechanism. (3 points)
Question 4: Catalysts (14 points)
a) A catalyst works to lower the activation energy of a chemical reaction. Write 3 – 4 sentences to describe at least four ways that a catalyst can accomplish this task. (4 points)
b) A catalytic converter is a catalyst that removes some of the pollutants, such as NO2 and CO gas, generated by an automobile. Write 3 – 4 sentences to describe the type of catalyst that is used for the catalytic converter and identify why a catalyst is needed for this reaction. (4 points)
c) A chemical reaction has an activation energy of 437 kJ/mol. A set of catalysts was tested to determine the activation energy. The results are listed in the table below. Draw the activation energy of the uncatalyzed reaction and the catalyzed reactions and identify the best catalyst. (6 points)
| Catalyst | Ea (kJ/mol) |
| Platinum | 143 |
| Palladium | 154 |
| Nickel | 177 |
