4.17 - How conditions shift an equilibrium

What Equilibrium Position Means

The position of equilibrium describes the relative amounts of reactants and products in an equilibrium mixture. A shift to the right means a net forward reaction after a change; a shift to the left means a net reverse reaction. Compare the mixture before and after the change, rather than assuming that a right-hand shift means products must outnumber reactants.

For GCSE predictions, use this rule: when a condition is changed, the equilibrium moves in the direction that reduces or opposes the change. The system does not "choose" a direction; the rates of the forward and reverse reactions change until a new equilibrium mixture is reached.

That means an Edexcel Predict answer should normally include two things:

• the direction the equilibrium shifts, such as towards the products or towards the reactants
• the effect on the amount or yield of a named substance, if the question asks for it

For example, in a reversible reaction written as:

N2(g) + 3H2(g) ⇌ 2NH3(g)

shifting right forms additional ammonia. Shifting left uses up ammonia. When a substance has been added or removed, separate that immediate change from the subsequent reaction: removing product can trigger a right-hand shift without restoring all the product that was removed.

Changing Concentration

For dissolved substances and gases, change one concentration while keeping temperature and other relevant conditions fixed. After the addition or removal, close the system again. A concentration change is predicted by asking which direction would use up the added substance or replace the substance that has been removed.

If the concentration of a reactant is increased, the equilibrium shifts towards the products. This uses up some of the added reactant. If the concentration of a product is increased, the equilibrium shifts towards the reactants. This uses up some of the added product.

The reverse prediction also works. If a product is removed, the equilibrium shifts towards the products to replace some of what has been removed. If a reactant is removed, the equilibrium shifts towards the reactants.

For this equilibrium:

A(aq) + B(aq) ⇌ C(aq)

increasing the concentration of A shifts the position to the right, so more C is formed at the new equilibrium. Removing C also shifts the position to the right, because the system moves to replace some C.

Changing Pressure

Here, a pressure change means compressing or expanding a closed gas mixture at constant temperature. Pressure changes matter for equilibria involving gases. The prediction depends on the number of gas particles on each side of the reversible equation.

Increasing pressure shifts the equilibrium towards the side with fewer moles of gas particles. This helps oppose the pressure increase because the new equilibrium mixture contains fewer gas particles. Decreasing pressure shifts the equilibrium towards the side with more moles of gas particles.

Use the state symbols and coefficients to count only gases. In the Haber process equilibrium:

N2(g) + 3H2(g) ⇌ 2NH3(g)

the left side has 4 moles of gas particles and the right side has 2 moles of gas particles. Increasing pressure shifts the equilibrium to the right, so the equilibrium mixture contains more ammonia. Decreasing pressure shifts the equilibrium to the left, so less ammonia is present at equilibrium.

If both sides have the same number of moles of gas particles, a pressure change does not shift the equilibrium position. There is no side that reduces the pressure more than the other.

Changing Temperature

Temperature predictions depend on whether the forward reaction is exothermic or endothermic. An increase in temperature favours the direction that absorbs heat; a decrease favours the direction that releases heat. Consider temperature separately from other changes.