Dynamic equilibrium and reversible reactions, Le Chatelier's principle and the effects of concentration, pressure and temperature, the role of a catalyst, the equilibrium constant Kc, and applications to industrial processes such as the Haber process.

What this dot point is asking

CCEA wants you to describe dynamic equilibrium in reversible reactions, apply Le Chatelier's principle to predict the effect of changing concentration, pressure and temperature, explain the role of a catalyst, write and use the equilibrium constant $K_c$, and apply these ideas to industrial processes such as the Haber process.

Dynamic equilibrium

Le Chatelier's principle

A catalyst speeds up both the forward and backward reactions equally, so it lets equilibrium be reached faster but does not change its position.

Working through each change

For the exothermic reaction $\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3$, raising the ammonia concentration shifts the equilibrium back towards the reactants to remove the added ammonia, while removing ammonia (as in the real Haber plant) keeps pulling the equilibrium forward. Raising the pressure shifts the position towards the right because there are four gas moles on the left and only two on the right, so the system reduces the pressure by favouring the side with fewer molecules. Raising the temperature shifts an exothermic equilibrium backwards, because the system absorbs the added heat by favouring the endothermic (reverse) direction, which lowers the yield. Each prediction follows the same logic: identify the change, then identify the direction that opposes it.

The equilibrium constant

For the reaction $a\text{A} + b\text{B} \rightleftharpoons c\text{C} + d\text{D}$, the equilibrium constant is:

$K_c$ is constant at a fixed temperature. Only temperature changes its value; changing concentration or pressure shifts the position of equilibrium but leaves $K_c$ unchanged. A large $K_c$ (much greater than 1) means the products are favoured at equilibrium, while a small $K_c$ (much less than 1) means the reactants are favoured. The units of $K_c$ depend on the equation: they are found by writing $\text{mol dm}^{-3}$ for each concentration term and cancelling. For a reaction with equal numbers of moles on each side, such as $\text{H}_2 + \text{I}_2 \rightleftharpoons 2\text{HI}$, the units cancel completely and $K_c$ has no units.

The Haber process

The overall yield of the Haber process is improved not by pushing the equilibrium harder but by removing ammonia as it forms (by cooling and liquefying it) and recycling the unreacted nitrogen and hydrogen. Removing a product continually shifts the equilibrium forward, so the gases pass through the reactor many times and the cumulative conversion is high even though a single pass converts only about $15\%$.

Examples in context

Example 1. The Contact process for sulfuric acid. The key equilibrium $2\text{SO}_2(g) + \text{O}_2(g) \rightleftharpoons 2\text{SO}_3(g)$ is exothermic and runs with fewer gas moles on the product side. The same compromise reasoning as the Haber process applies: a moderate temperature of about $450\ \text{C}$ balances yield against rate, a vanadium(V) oxide catalyst speeds the reaction, and only a slightly raised pressure is needed because the yield is already over $95\%$ at modest pressure, so the cost of high-pressure plant is not justified. This is a standard CCEA application that contrasts with the Haber process on the pressure decision.

Example 2. Carbon dioxide in the blood. The equilibrium $\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^-$ keeps blood pH stable. When exercise produces extra carbon dioxide, Le Chatelier's principle predicts the equilibrium shifts right, raising the hydrogen ion concentration; the body compensates by breathing out carbon dioxide, which shifts the equilibrium back. This biological buffer is a clear real-world demonstration that adding or removing a species shifts an equilibrium in a predictable direction.

Try this

Q1. Define dynamic equilibrium. [2 marks]

• Cue. Forward and backward reactions occur at equal rates in a closed system, so concentrations stay constant.

Q2. For an exothermic forward reaction, state the effect of increasing temperature on the yield of product. [1 mark]

• Cue. The yield decreases, because equilibrium shifts in the endothermic (backward) direction.