ACS Bezique Events Practice Test

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How does a catalyst affect activation energy and the temperature dependence of the rate constant, according to the Arrhenius equation?

The catalyst lowers the activation energy, but decreases k. The catalyst changes only the pre-exponential factor A. The catalyst lowers the activation energy, increasing k. The catalyst has no effect on Ea or k.

Correct Answer:

The catalyst lowers the activation energy, increasing k.

Explanation:

In the Arrhenius view, the rate constant is k = A exp(-Ea/RT). A catalyst provides an alternative pathway with a lower activation energy Ea. Because the exponent becomes less negative when Ea is smaller, the exponential factor increases, so the rate constant k is larger at any given temperature. Regarding temperature dependence, the slope of a plot of ln k versus 1/T is -Ea/R. Lowering Ea reduces the magnitude of this slope, meaning k becomes less sensitive to temperature changes. So the catalyst not only increases k by lowering Ea, but also makes the temperature dependence weaker. While some catalysts can affect the pre-exponential factor A, the typical and most direct description is that Ea is lowered, leading to a higher k and diminished temperature sensitivity.

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