Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 13.7 Equilibrium explained using K-Study Notes - New Syllabus

13.7 Temperature Effect Explained via Equilibrium Constant

The effect of temperature on the position of equilibrium can be fully explained by the change in the value of the equilibrium constant (\( \mathrm{K_c} \) or \( \mathrm{K_p} \)). Rather than just applying Le Chatelier’s Principle, this provides a quantitative explanation of how equilibrium composition changes.

Key Idea

A change in temperature changes the value of \( \mathrm{K} \), and this change forces the equilibrium position to shift to a new composition consistent with the new \( \mathrm{K} \).

Link Between \( \mathrm{K} \) and Equilibrium Position

• \( \mathrm{K} \) determines the ratio of products to reactants at equilibrium.
• If \( \mathrm{K} \) increases → equilibrium shifts towards products.
• If \( \mathrm{K} \) decreases → equilibrium shifts towards reactants.

(i) Exothermic Reactions (\( \mathrm{\Delta H < 0} \))

• Increasing temperature → \( \mathrm{K} \) decreases.
• The ratio of products to reactants becomes smaller.
• Therefore, equilibrium shifts to the left.

Explanation

• The new lower value of \( \mathrm{K} \) requires fewer products relative to reactants.
• The system adjusts composition to match this new ratio.

(ii) Endothermic Reactions (\( \mathrm{\Delta H > 0} \))

• Increasing temperature → \( \mathrm{K} \) increases.
• The ratio of products to reactants becomes larger.
• Therefore, equilibrium shifts to the right.

Explanation

• The new higher value of \( \mathrm{K} \) requires more products relative to reactants.
• The system adjusts to achieve this new equilibrium ratio.

Important Insight

• Temperature is the only factor that changes \( \mathrm{K} \).
• Changes in concentration or pressure do not change \( \mathrm{K} \), so they do not alter the fundamental equilibrium ratio.

Key Features

• Equilibrium position is determined by the value of \( \mathrm{K} \).
• Temperature changes \( \mathrm{K} \), causing a shift in equilibrium.
• Exothermic: ↑T → ↓K → shift left.
• Endothermic: ↑T → ↑K → shift right.