GCSE Chemistry AQA 8462

Reversible reactions

In some chemical reactions, the products of the reaction can react to produce the original reactants. Such reactions are called reversible reactions and are represented:

$A + B ⇌ C + D$

The direction of reversible reactions can be changed by changing the conditions.

For example:

$\text{ammonium chloride} \xrightleftharpoons[cool]{heat} \text{ammonia} + \text{hydrogen chloride}$

Energy changes and reversible reactions

If a reversible reaction is exothermic in one direction, it isendothermic in the opposite direction. The same amount of energyis transferred in each case. For example:

$\text{hydrated copper sulphate (blue)} \xrightleftharpoons[exothermic]{endothermic} \text{anhydrous copper sulphate (white)} + \text{water}$

Equilibrium

When a reversible reaction occurs in apparatus which prevents the escape of reactants and products, equilibrium is reached when the forward and reverse reactions occur at exactly the same rate.

**The effect of changing conditions on equilibrium

The relative amounts of all the reactants and products at equilibrium depend on the conditions of the reaction.

If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract the change.

The effects of changing conditions on a system at equilibrium can be predicted using Le Chatelier’s Principle.

Students should be able to make qualitative predictions about the effect of changes on systems at equilibrium when given appropriate information.**

**The effect of changing concentration

If the concentration of one of the reactants or products is changed, the system is no longer at equilibrium and the concentrations of all the substances will change until equilibrium is reached again.

If the concentration of a reactant is increased, more products will be formed until equilibrium is reached again.

If the concentration of a product is decreased, more reactants will react until equilibrium is reached again.

Students should be able to interpret appropriate given data to predict the effect of a change in concentration of a reactant or product on given reactions at equilibrium.**

**The effect of temperature changes on equilibrium

If the temperature of a system at equilibrium is increased:

• the relative amount of products at equilibrium increases for an endothermic reaction
• the relative amount of products at equilibrium decreases for an exothermic reaction.

If the temperature of a system at equilibrium is decreased:

• the relative amount of products at equilibrium decreases for an endothermic reaction
• the relative amount of products at equilibrium increases for an exothermic reaction.

Students should be able to interpret appropriate given data to predict the effect of a change in temperature on given reactions at equilibrium.**

**The effect of pressure changes on equilibrium

For gaseous reactions at equilibrium:

• an increase in pressure causes the equilibrium position to shift towards the side with the smaller number of molecules as shown by the symbol equation for that reaction
• a decrease in pressure causes the equilibrium position to shift towards the side with the larger number of molecules as shown by the symbol equation for that reaction.

Students should be able to interpret appropriate given data to predict the effect of pressure changes on given reactions at equilibrium.**