Edexcel IAL Chemistry 13.6 Temperature and equilibrium constants

Treat temperature as the only listed condition that changes K: favouring the endothermic direction increases its relative equilibrium terms and changes the constant accordingly.

Exam points

Predict whether K rises or falls after heating from the endothermic reaction direction.
Infer whether the forward reaction is exothermic or endothermic from the temperature trend in K.
Use the K expression to connect changed equilibrium partial pressures with the new value.

[Maximum number: 1]

The equation for the reaction between iron and steam is shown.

3 \mathrm{Fe}(\mathrm{~s})+4 \mathrm{H}_{2} \mathrm{O}(\mathrm{~g}) \rightleftharpoons \mathrm{Fe}_{3} \mathrm{O}_{4}(\mathrm{~s})+4 \mathrm{H}_{2}(\mathrm{~g}) \quad \Delta H=-151.2 \mathrm{~kJ} \mathrm{~mol}^{-1}

What is the effect on the equilibrium constant for this reaction if small pieces of iron are replaced by iron powder and if the temperature is increased?
□ A
□ B
□ C
□ D

Table for Question 13.6—The effect of changing the temperature on the equilibrium constant (Kc and Kp) for both exothermic and endothermic reactions question 1 — Edexcel A-Level Chemistry A2

The only correct answer is D ( $K_{\mathrm{c}}$ unchanged; $K_{\mathrm{c}}$ decreases)

A is incorrect because $K_{\mathrm{c}}$ is not affected by the state of division of the solid and $K_{c}$ decreases for an exothermic reaction

B is incorrect because $K_{c}$ decreases for an exothermic reaction

C is incorrect because $K_{\mathrm{c}}$ is not affected by the state of division of the solid

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13.6—The effect of changing the temperature on the equilibrium constant (Kc and Kp) for both exothermic and endothermic reactions question 1