Effect of Change in Temperature

The enthalpy change quoted with an equilibrium equation always refers to the forward reaction. The backward reaction has the same ΔH with the opposite sign.By Le Chatelier's principle:• Increasing the temperature shifts the equilibrium in the endothermic direction, which absorbs heat energy and opposes the rise.
• Decreasing the temperature shifts the equilibrium in the exothermic direction, which releases heat energy.Example 1.N₂O₄(g) ⇌ 2NO₂(g)    ΔH = +58 kJ mol⁻¹The forward reaction is endothermic. Heating the mixture shifts the equilibrium to the right, so more brown NO₂ forms and the colour darkens. Cooling it in ice turns it paler.Example 2.N₂(g) + 3H₂(g) ⇌ 2NH₃(g)    ΔH = −92 kJ mol⁻¹The forward reaction is exothermic, so a lower temperature gives a higher equilibrium yield of ammonia. However, a lower temperature also makes the reaction slower, so a compromise temperature is used in industry.Temperature is different from concentration and pressure in one important way: a change in temperature changes the value of K_c. For an exothermic forward reaction, K_c decreases as the temperature rises. For an endothermic forward reaction, K_c increases as the temperature rises.