GCSE Chemistry revision

Reversible reactions

Reversible reactions and dynamic equilibrium

AQA 4.6.2.1 Foundation & Higher

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In most chemical reactions, a single forward arrow (→) indicates a one-way reaction where reactants form products and cannot revert back.
A reversible reaction is shown by a double arrow (⇌), meaning the reaction can proceed in both the forward and backward directions.
An example of a reversible reaction is ammonium chloride breaking down into ammonia and hydrogen chloride, and then reforming again.

1. Introduction AQA

In most chemical reactions, a single forward arrow (→) indicates a one-way reaction where reactants form products and cannot revert back.
A reversible reaction is shown by a double arrow (⇌), meaning the reaction can proceed in both the forward and backward directions.
An example of a reversible reaction is ammonium chloride breaking down into ammonia and hydrogen chloride, and then reforming again.

The forward reaction describes reactants converting into products, whilst the backward reaction describes products converting back into reactants.
At the start of a reversible reaction, the forward reaction is fast because there is a high concentration of reactants and no products yet.
As reactants are used up and products build up, the forward reaction slows down and the backward reaction speeds up.

2. Forward and Backward Reactions AQA

The forward reaction describes reactants converting into products, whilst the backward reaction describes products converting back into reactants.
At the start of a reversible reaction, the forward reaction is fast because there is a high concentration of reactants and no products yet.
As reactants are used up and products build up, the forward reaction slows down and the backward reaction speeds up.

In any reversible reaction, if the forward reaction is exothermic, then the backward reaction must be endothermic, and vice versa.
The thermal decomposition of hydrated copper sulphate to anhydrous copper sulphate and water is an endothermic forward reaction.
Heating blue hydrated copper sulphate crystals drives the forward (endothermic) reaction, producing white anhydrous copper sulphate powder.
Adding water to anhydrous copper sulphate drives the backward (exothermic) reaction, reforming the blue hydrated copper sulphate crystals and releasing energy.

3. Exothermic and Endothermic Directions AQA

In any reversible reaction, if the forward reaction is exothermic, then the backward reaction must be endothermic, and vice versa.
The thermal decomposition of hydrated copper sulphate to anhydrous copper sulphate and water is an endothermic forward reaction.
Heating blue hydrated copper sulphate crystals drives the forward (endothermic) reaction, producing white anhydrous copper sulphate powder.
Adding water to anhydrous copper sulphate drives the backward (exothermic) reaction, reforming the blue hydrated copper sulphate crystals and releasing energy.