Predicting equilibrium shifts from concentration data

At equilibrium, the forward and reverse reactions continue at equal rates, so concentrations remain constant. Changing the concentration of a reactant or product disturbs the equilibrium. The system then shifts in the direction that opposes the change, until a new equilibrium is reached.

To predict the shift, first identify the substance whose concentration was changed and whether it was increased or decreased. Then choose the reaction direction that would use up an added substance or replace some of a removed substance.

Concentration changes and equilibrium shifts

• Reactant increased: the equilibrium shifts towards products, using up some of the added reactant.
• Reactant decreased: the equilibrium shifts towards reactants, replacing some of the removed reactant.
• Product decreased: the equilibrium shifts towards products, replacing some of the removed product.
• Product increased: the equilibrium shifts towards reactants, using up some of the added product.

A shift towards products means the forward reaction is favoured as the system moves to its new equilibrium: reactants are used up and products are formed. A shift towards reactants means the reverse reaction is favoured: products are used up and reactants are formed.

Interpreting a concentration change

Consider the equilibrium $$\ce{N2 + 3H2 <=> 2NH3}$$. If nitrogen is added, its concentration first increases. The system then shifts towards ammonia, using up some nitrogen and hydrogen and forming more ammonia.

The prediction describes changes after the disturbance, as the system moves towards the new equilibrium. For example, the added nitrogen decreases from its immediately increased concentration, but the shift does not necessarily restore its original concentration. The pattern predicts the direction of change, not the exact final concentrations.

Concentration changes and equilibrium position

In a reversible reaction, reactants form products in the forward reaction, while products form reactants in the reverse reaction. At equilibrium, both reactions continue at equal rates. The concentrations of reactants and products therefore remain constant, although they are not necessarily equal to one another.

If the concentration of a reactant or product is changed, the system is no longer at equilibrium. The forward and reverse reactions no longer occur at equal rates, so there is a net change in the direction that tends to reduce the effect of the disturbance. This is described as a shift in equilibrium position.

Direction of the equilibrium shift

• Adding a reactant shifts the equilibrium forwards. Some of the added reactant is used to form more products.
• Removing a product shifts the equilibrium forwards. More reactants are converted into products, replacing some of the product removed.
• Removing a reactant shifts the equilibrium in reverse. Some products are converted back into reactants, replacing some of the reactant removed.
• Adding a product shifts the equilibrium in reverse. Some of the added product is converted back into reactants.

A shift does not mean that the reaction in the other direction stops. Both reactions continue, but one is faster than the other while the concentrations adjust.

Eventually, the forward and reverse reactions reach equal rates again. A new equilibrium is established, with a new set of constant concentrations. The shift reduces the effect of the concentration change; it does not necessarily reverse it completely.