Increasing temperature favours the endothermic reaction direction

The position of equilibrium depends on temperature according to Le Chatelier's principle:

• For an exothermic reaction, increasing temperature shifts the equilibrium towards the reactants (left).
• For an endothermic reaction, increasing temperature shifts the equilibrium towards the products (right).

This adjustment happens so that the system can counteract the change in temperature.

Example:

N_2(g) + 3H_2(g) ⇌ 2NH_3(g), ΔH = $-$92.2 kJ mol^-1 (exothermic reaction)

If temperature is raised, the equilibrium position will shift to the left to absorb heat.

Temperature changes affect Kc

The equilibrium constant (K_c_) is influenced solely by temperature. Changes in concentration or pressure have no impact on K_c.

When the position of equilibrium is altered due to a temperature shift:

• A decrease in the amount of product at equilibrium causes K_c_ to decrease.
• An increase in the amount of product at equilibrium causes K_c_ to increase. This ensures that the value of K_c_ aligns with the new position of equilibrium.

Example:

2SO_2(g) + O_2(g) ⇌ 2SO_3(g), ΔH = $-$197 kJ mol^-1

Raising the temperature causes the equilibrium to shift left towards the reactants, absorbing heat and resulting in less SO₃ at the new equilibrium position.

With a decrease in SO₃:

• The proportion of products reduces.
• Consequently, K_c_ (the ratio of products to reactants) decreases. The impact of temperature changes on exothermic and endothermic reactions is summarised in the table below:

Concentration and pressure changes do not affect Kc

While shifts in concentration or pressure can move the equilibrium position, altering the quantities present to counteract the change, K_c_ remains unchanged at a specific temperature.

For example:

N_2(g) + 3H_2(g) ⇌ 2NH_3(g)

• Adding more N_2_ leads to an increased formation of NH_3_ at the new equilibrium. However, the equilibrium ratio of [NH_3_]^2^ to [N_2_][H_2_]^3^ still remains equal to keep value of K_c_ constant.
• Thus, changes in concentration and pressure do not directly influence K_c_ - its value is only determined by temperature.

Catalysts do not change Kc or equilibrium position

Catalysts increase the rate of both the forward and reverse reactions, reducing the time needed to reach equilibrium following a change.

However, catalysts neither alter the equilibrium position nor the value of K_c_.

The table below summarises the effects of changing reaction conditions on the position of equilibrium and the value of K_c_:

Kp changes with temperature

The equilibrium constant K_p changes with temperature in the same way as the equilibrium constant K_c.

• Increasing temperature causes K_p to increase for endothermic reactions and decrease for exothermic reactions.
• Decreasing temperature has the opposite effects on K_p. Changes in pressure and adding a catalyst do not affect the value of K_p, only shifting the equilibrium position. The value of K_p itself depends only on temperature, just like K_c.

The effects of temperature changes on exothermic and endothermic reactions can be summarised in the following table: