AP Chemistry 5.6: Reaction Energy Profiles

Practice AP Chemistry questions on relating activation energy and temperature to reaction rate, using particle-energy reasoning from a reaction energy profile.

Exam points

Explain how chemical systems reach equilibrium through competing forward and reverse reaction rates.
Apply equilibrium expressions and reaction quotients to predict composition changes in reversible reactions.

[Maximum number: 1]

The half-life $\left(t_{1 / 2}\right)$ of the catalyzed isomerization of cis-2-butene gas to produce trans-2-butene gas, represented above, was measured under various conditions, as shown in the table below.

Table for Question 5.6.A—Represent the activation energy and overall energy change in an elementary reaction using a reaction energy… question 1 — AP Chemistry

The half-life of the reaction in trial 4 is less than the half-life in trial 1. Explain why, in terms of activation energy.

The temperature is higher in trial 4, meaning that the $K E_{\text {avg }}$ of the
molecules is greater. Consequently, in this trial a greater fraction of collisions have sufficient energy to overcome the activation energy barrier, thus the rate is greater.

1 point is earned for a correct answer with justification.

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5.6.A—Represent the activation energy and overall energy change in an elementary reaction using a reaction energy… question 1