Nitrogen and hydrogen react together in a closed container to form ammonia according to the equation: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). At a certain temperature the equilibrium constant, Kc, has a value of 6.2 × 10⁻² mol⁻² dm⁶. The temperature is then lowered and the value of Kc increases significantly. Explain what the change in Kc tells us about the position of equilibrium, and what this indicates about whether the forward reaction is exothermic or endothermic.

A larger Kc (6.2×10⁻²) indicates that at equilibrium the concentration of NH₃ is relatively high compared with N₂ and H₂. When the temperature is lowered the Kc increases further, meaning the product concentration becomes even greater.

This shows that the equilibrium position has shifted to the right – more ammonia is formed.

According to Le Chatelier’s principle, lowering the temperature favours the exothermic direction because the system releases heat. The fact that the equilibrium shifts to the right when the temperature is reduced therefore tells us that the forward reaction (N₂ + 3H₂ → 2NH₃) is exothermic.

Kc changes only with temperature; it is unaffected by changes in concentration or pressure.