Changes Which Affect the Equilibrium (AQA A-Level Chemistry): Revision Notes

1.8.5 Changes Which Affect the Equilibrium

Factors Affecting $K_c$

The value of the equilibrium constant, $K_c$, is influenced only by temperature changes. Changes in concentration or the addition of a catalyst do not affect the value of $K_c$

Effect of Temperature

Temperature plays a crucial role in shifting the equilibrium and changing the value of $K_c$. The direction of the shift depends on whether the reaction is exothermic or endothermic.

For an Exothermic Reaction ( $\Delta H < 0$):

• Increasing the temperature shifts the equilibrium towards the reverse reaction, which is endothermic. This reduces the concentration of the products and increases the concentration of the reactants, causing the value of $K_c$ to decrease.

• Decreasing the temperature favours the forward reaction, increasing the concentration of the products and reducing the concentration of the reactants. This causes $K_c$ to increase. For an Endothermic Reaction ( $\Delta H > 0$ ):

• Increasing the temperature shifts the equilibrium to the forward reaction, raising the concentration of the products and increasing $K_c$

• Decreasing the temperature shifts the equilibrium to the reverse reaction, reducing the product concentration and lowering $K_c$

Factors That Do Not Affect $K_c$

Change in Concentration

Changes in the concentration of reactants or products do not affect $K_c$. When concentration changes occur, the system adjusts to maintain the equilibrium constant by shifting the position of equilibrium according to Le Chatelier's principle.

Addition of a Catalyst

A catalyst speeds up both the forwards and reverse reactions equally, allowing the system to reach equilibrium faster. However, it does not change the equilibrium position or the value of $K_c$.