10.06.1 Reversible Reactions

Syllabus

- In some chemical reactions, the products of the reaction can react to produce the original reactants.
- These are called reversible reactions & are represented: A + B ⇌ C + D
- The direction of reversible reactions can be changed by changing the conditions. Eg. The Ammonium Chloride equilibrium (Top right)

- If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction.
- The same amount of energy is transferred in each case. For example: the Copper Sulphate equilibrium (right)
- 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.

What does this mean?

In Year 8 we learn that Physical Changes are easily reversed and Chemical Changes are irreversible.

This is not always true.

Nitrogen + Hydrogen ⇌ Ammonia

N2(g) + 3H2(g) ⇌ 2NH3(g)

The ⇌ symbol means that the reaction is reversible - it can go forward and backwards.

If we started with a mixture of Nitrogen and Hydrogen the forward reaction would be fast because the collision frequency between reactants would be high.

The rate of reaction for the backward reaction would be 0 because there is no Ammonia to collide yet.

Later, the collision frequency for the reactants would get lower as they are used up - the forward reaction rate slows.

As the concentration of Ammonia increases its collision frequency will increase and so will the backward rate of reaction.

Eventually, the two rates will become equal to each other.

Both the forward and backward reaction will continue but the concentrations of reactants won't change because the two reactions will be cancelling each other out.

This is dynamic equilibrium.

But a Reversible Reaction can't reach Equilibrium unless it is in a sealed container.

Copper Sulphate Equilibrium

Hydrated Copper(II) Sulphate ⇌ Anhydrous Copper(II) Sulphate+ Water

CuSO4.5H2O(s) ⇌ CuSO4(s) + 5H2O (g)

If we heated blue hydrated Copper Sulphate in a dry, closed container the water of crystalisation would evaporate quickly - fast forward reaction.

There would be no backward reaction because there would be no water vapour and no anhydrous Copper Sulphate.

As anhydrous Copper Sulphate is formed, so is water vapour. The backwards reaction begins.

But the forward reaction slows as we run short of hydrated Copper Sulphate.

Eventually, the two reaction rates become the same - we have reached dynamic equilibrium.

For an animation of the above equilibrium see below.

Ammonium Chloride Equilibrium

Ammonium Chloride ⇌ Ammonia + Hydrogen Chloride

NH4Cl (s) ⇌ NH3(g) + HCl(g)

In the same way, heating Ammonium Chloride in a closed container would cause it to break down into two colourless gases that can recombine to make Ammonium Chloride.

Eventually the two reactions will happen at the same rate and we would have reached dynamic equilibrium.

If we start with solid Ammonium Chloride an heat gently, the forward reaction is favoured and it begins to decompose into gaseous Ammonia and Hydrogen Chloride.

But as it rises, it cools.

Now the backward reaction is favoured and Ammonium Chloride should re-form on the sides of the flask as a white ring.