Equilibrium DA - (CCEA)

• In a reversible reaction the products can react to turn back into the reactants.

• The direction of a reversible reaction can be changed by altering the reaction conditions.

• A dynamic equilibrium is formed in a reversible reaction when the rates of the forward and reverse reaction are equal and the amounts of the reactants and the products remains constant.

In many chemical reactions the reactants are converted into products and then the reaction stops.

For example:

2Mg + O₂ → 2MgO

In this reaction, once the magnesium oxide has formed, it cannot go back to reform magnesium and oxygen.

This is a non-reversible reaction.

Other chemical reactions are reversible reactions.

In these reactions, the products, once formed, can change back into the original reactants.

For example:

N₂ + 3H₂ ⇌ 2NH₃

In this reaction, once the ammonia (NH₃) has formed, it can ‘go back’ to reform nitrogen and hydrogen.

Reversible reactions are shown with reversible arrows (⇌).

The reversible arrow has two ‘half-arrows,’ one pointing in each direction.

Changing the conditions of a reversible reaction, such as temperature, pressure or amount of a reactant or product, can change the direction of the reaction.

Hydrated copper(II) sulfate is blue in appearance.

When it is heated, the water of crystallisation is removed, leaving anhydrous copper(II) sulfate which is white in appearance.

Equation: hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water

CuSO₄.5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(l)

This reaction is reversible, as the hydrated copper(II) sulfate is reformed if water is added to anhydrous copper(II) sulfate.

When a reversible reaction happens in a closed system (closed container) , a dynamic equilibrium can be achieved.

In a dynamic equilibrium:

• the rates of the forward and reverse reactions are equal.

• the amounts of reactants and products remains constant.

For example, consider a reversible reaction where gas A reacts to form a different gas B:

An everyday illustration of dynamic equilibrium is a person trying to walk up an escalator that is moving downwards.

The person appears to be stationary (not moving), but this is just because the rate of their walking up is equal to the rate the escalator is moving downwards.