Le Chatelier's Principle

State Le Chatelier's Principle.

If a system at dynamic equilibrium is subjected to a change, the position of equilibrium shifts to oppose the change and restore equilibrium.

How does increasing the concentration of a reactant affect the position of equilibrium?

Adding more of a reactant increases its concentration in the system. To oppose this change, the equilibrium shifts to the right (towards the products) to use up the extra reactant. The concentration of products increases and the concentration of other reactants decreases until a new equilibrium is established. Similarly, removing a reactant shifts the equilibrium to the left to produce more of it.

How does increasing the concentration of a product affect the position of equilibrium?

Adding more of a product shifts the equilibrium to the left (towards the reactants) to oppose the increase by using up the excess product. Conversely, removing a product shifts the equilibrium to the right — this is the principle behind continuously removing a product to drive a reaction towards completion.

How does changing pressure affect the position of equilibrium, and what determines the direction of shift?

Changing pressure only affects equilibria involving gases. Increasing pressure shifts the equilibrium towards the side with fewer moles of gas, because moving in that direction reduces the total number of gas molecules and therefore reduces the pressure — opposing the increase. Decreasing pressure shifts equilibrium towards the side with more moles of gas. If there are equal numbers of gas moles on both sides of the equation, changing pressure has no effect on the position of equilibrium.

How does changing temperature affect the position of equilibrium, and why does the direction depend on whether the forward reaction is exothermic or endothermic?

Increasing temperature adds heat energy to the system. To oppose this, the equilibrium shifts in the endothermic direction — whichever direction absorbs heat — to use up the extra energy. Decreasing temperature shifts the equilibrium in the exothermic direction, to release heat and replace what has been lost. In practice: if the forward reaction is exothermic (ΔH negative), increasing temperature shifts equilibrium to the left (towards reactants), decreasing yield of products. If the forward reaction is endothermic (ΔH positive), increasing temperature shifts equilibrium to the right (towards products), increasing yield.

Apply Le Chatelier's Principle to the Haber Process: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92 kJ mol⁻¹. How does each change affect the yield of ammonia?

There are 4 moles of gas on the left and 2 on the right. The forward reaction is exothermic.

• Increase concentration of N₂ or H₂ → equilibrium shifts right → more NH₃ produced.

• Remove NH₃ as it forms → equilibrium shifts right → more NH₃ produced.

• Increase pressure → equilibrium shifts right (fewer gas moles) → more NH₃ produced.

• Decrease temperature → equilibrium shifts right (exothermic direction) → more NH₃ produced, but the rate of reaction also decreases.

How does a catalyst affect the position of equilibrium?

A catalyst does not change the position of equilibrium at all — the equilibrium mixture contains the same proportions of reactants and products whether a catalyst is present or not. This is because a catalyst increases the rate of both the forward and reverse reactions by exactly the same factor (by lowering the activation energy of both equally). Since both rates increase equally, the ratio of rates remains unchanged and the position of equilibrium is unaffected.

If a catalyst does not change the yield, why is it used in industrial processes?

A catalyst allows the equilibrium to be reached more quickly — it speeds up the rate at which the system reaches equilibrium without changing where that equilibrium lies. In industry, this means the same yield can be obtained in a much shorter time, making the process more economically efficient. The catalyst also allows lower temperatures to be used (since equilibrium is reached faster even at lower temperatures), which saves energy and reduces costs.