Changes in Temperature & Pressure (AQA A-Level Chemistry): Revision Notes

5.3.3 Changes in Temperature & Pressure

Temperature Effects on Equilibrium Position

In a reversible reaction, temperature changes influence the equilibrium position according to the reaction's enthalpy change ($\Delta H$). Le Chatelier's Principle suggests that the system will adjust to counteract the temperature change:

Exothermic Reactions

(e.g., forwards reaction releases heat)

Increasing temperature shifts equilibrium towards the reactants (left), favouring the endothermic reverse reaction and decreasing the equilibrium constant $K_p$.

Endothermic Reactions

(e.g., forwards reaction absorbs heat)

Increasing temperature shifts equilibrium towards the products (right), favouring the forwards reaction and increasing $K_p$.

Pressure Effects on Equilibrium Position

Pressure changes only affect equilibria involving gases and are significant when there's a different number of moles of gas on each side of the equation:

• Increase in pressure: Equilibrium shifts towards the side with fewer gas molecules to counterbalance the pressure increase.
• Decrease in pressure: Equilibrium shifts towards the side with more gas molecules. Note: Pressure changes do not affect $K_p$; only temperature can alter $K_p$.

Catalyst Effects on Equilibrium

Catalysts speed up the rate at which equilibrium is reached but do not affect the equilibrium position or the value of $K_p$. Catalysts lower the activation energy for both forwards and reverse reactions, allowing equilibrium to be achieved more rapidly without altering the composition of the equilibrium mixture.