Chapter 15, Problem 134CP

You are studying the kinetics of the reaction ${\text{H}}_2\text{(g)}+{\text{F}}_{\text{2}}\text{(g)}\to 2\text{HF(g)}$ and you wish to determine a mechanism for the reaction. You run the reaction twice by keeping one reactant at a much higher pressure than the other reactant (this lower-pressure reactant begins at $\text{1}\text{.000 atm}$ ). Unfortunately, you neglect to record whichreactant was at the higher pressure, and you forget itlater. Your data for the first experiment are as follows:

When you run the second experiment (in which the higher-pressure reactant is run at a much higher pressure), you determine the values of the apparent rate constants to be the same. It also turns out that you find data taken from another person in the lab. This individual found that the reaction proceeds $40.0$ times faster at $\text{55°C}$ than at $\text{35°C}$ . You also know, from the energy-level diagram, that there are three steps to the mechanism, and the first step has the highest activation energy. You look up the bond energies of the species involved and they are (in $\text{kJ/mol}$ ): $\text{H—H }\left(\text{432}\right)$ , $\text{F—F }\left(\text{154}\right)$ , and $\text{H—F }\left(\text{565}\right)$ .

1. Sketch an energy-level diagram (qualitative) that isconsistent with the one described.
2. Develop a reasonable mechanism for the reaction.Support your answer and explain the significance ofeach piece of information.
3. Which reactant was limiting in the experiments?