6.8.4. Effect of Temperature Change

Whenever an equilibrium is disturbed by a change in concentration, pressure or volume, the composition of the equilibrium mixture changes because the reaction quotient, Qc no longer equals the equilibrium constant, Kc. However, when a change in temperature occurs, the value of equilibrium constant, Kc is changed.

When we change the temperature of a system at equilibrium, the equilibrium constant will also change. This means that if the temperature increases or decreases, it affects the balance between the reactants and products in such a way that the ratio of their concentrations at equilibrium will no longer reflect the previous equilibrium constant.

Think of a seesaw. If one side (temperature) is raised, the balance (equilibrium) shifts. In a chemical reaction, increasing temperature can favor the side with more energy (often the products in endothermic reactions) and decrease the concentration of the other side.

In general, the temperature dependence of the equilibrium constant depends on the sign of ∆H for the reaction.

• The equilibrium constant for an exothermic reaction (negative ∆H) decreases as the temperature increases. • The equilibrium constant for an endothermic reaction (positive ∆H) increases as the temperature increases.

Exothermic reactions release heat, so raising the temperature causes the reaction to shift back towards the reactants, thus decreasing the equilibrium constant. Conversely, endothermic reactions absorb heat, and raising the temperature favors the products, increasing the equilibrium constant.

Think of cooking: when cooking a steak (exothermic), if you lower the heat, it won't cook well (shifts back toward raw). While making a cake (endothermic), turning up the heat helps bake it properly (shifts to finished cake).

Production of ammonia according to the reaction, N2(g) + 3H2(g) → 2NH3(g); ∆H= −92.38 kJ mol–1 is an exothermic process. According to Le Chatelier’s principle, raising the temperature shifts the equilibrium to left and decreases the equilibrium concentration of ammonia.

In the synthesis of ammonia, because the reaction releases heat (it's exothermic), increasing the temperature will shift the reaction towards the reactants (N2 and H2) and away from the product (NH3). This is not desirable in industrial processes where ammonia production is sought, which is why lower temperatures are generally used, despite the slower reaction rates.

Imagine making ice cream: if the ice (a colder environment) melts (increasing temperature), the ice cream won't freeze properly. Similarly, in industry, they prefer lower temperatures to keep the reaction favoring ammonia production.

Effect of temperature on equilibrium can be demonstrated by taking NO2 gas (brown in colour) which dimerises into N2O4 gas (colourless).

2NO2(g) ⇌ N2O4(g); ∆H = −57.2 kJ mol–1. At low temperatures in beaker 1, the forward reaction of formation of N2O4 is preferred, as the reaction is exothermic, and thus, intensity of brown colour due to NO2 decreases.

When NO2 gas is cooled, it favors the formation of the colorless N2O4 gas, showing that lower temperatures promote the formation of products in exothermic reactions. This effect can be visually observed by the change in color from brown to colorless.

It's like wearing a sweater in winter: when it's cold (low temperature), you bundle up. Conversely, when it’s hot outside, you shed your coats. In chemistry, cooling a reaction shifts it to favor creating products, similar to how cooler weather favors bundling up.

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