Advanced Placement Chemistry 1998 Free Response Questions

6) Answer the following questions regarding the kinetics of chemical reactions.

(a) The diagram below at right shows the energy pathway for the reaction

O₃ + NO --> NO₂ + O₂.

Clearly label the following directly on the diagram.

(i) The activation energy (E_a) for the forward reaction

(ii) The enthalpy change (delta H) for the reaction

(b) The reaction 2 N₂O₅ --> 4 NO₂ + O₂ is first order with respect to N₂O₅.

(i) Using the axes at right, complete the graph that represents the change in [N₂O₅] over time as the reaction proceeds.

(ii) Describe how the graph in (i) could be used to find the reaction rate at a given time, t.

(iii) Considering the rate law and the graph in (i), describe how the value of the rate constant, k, could be determined.

(iv) If more N₂O₅ were added to the reaction mixture at constant temperature, what would be the effect on the rate constant, k? Explain.

(c) Data for the chemical reaction 2A --> B + C were collected by measuring the concentration of A at 10-minute intervals for 80 minutes. The following graphs were generated from analysis of data.

Use the information in the graphs above to answer the following.

(i) Write the rate-law expression for the reaction. Justify your answer.

(ii) Describe how to determine the value of the rate constant for the reaction.

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