C2aL16 Equilibrium

Key Words

Closed System - Nothing can come in or go out.

Dynamic - forward and backward reactions are going on all the time.

Equilibrium - balance

Forward - reaction makes products

Products - new substances made in a reaction

Reactants - substances that react in the reaction.

Reversible reaction - reaction that can go forwards or backwards.

Grade E

Last lesson we saw how some reactions are reversible.  That means that as well as going forwards, the reaction can go backwards:

reactant 1 + reactant 2  ® product 1 + product 2  (forwards)

product 1 + product 2  ® reactant 1 + reactant 2 (backwards)

For example:

ammonium chloride D ammonia + hydrogen chloride

NH₄Cl  D NH₃ + HCl

If there is a closed system, nothing goes in or out.  If there are forward reactions going on at the same time as there are backward reaction, we get a balance, which we call equilibrium.  The balance point is affected by:

• temperature
• pressure.

If the balance point is changed, the amount of reactants and the amounts of products will change.

Grade C

When equilibrium is reached in a closed system, it does not mean that there are no reactions; instead the forward reactions happen at the same rate as the backward.  This is called dynamic equilibrium.

We can change the equilibrium point:

• If the reaction is exothermic (gives out heat), the equilibrium would change so that there are more reactants than products.  So there is no point in heating an exothermic reaction to help it along.
• If the reaction is endothermic (takes in heat), the equilibrium would change so that there are more products than reactants.  So there is every point in heating an endothermic reaction to help it along.

This reaction is exothermic:

hydrogen + chlorine D  hydrogen chloride

H₂ + Cl₂ D 2HCl

If we reduce the temperature, more hydrogen chloride is produced.

If we have reactions that involve gases, the pressure has an effect. 

• If we increase the pressure, the equilibrium favours the side with fewer gas molecules;

• If we reduce the pressure, the equilibrium favours the side with more gas molecules.

Consider this decomposition reaction:

calcium carbonate D calcium oxide + carbon dioxide

CaCO_3(s) D CaO_(s) + CO_2(g)

If we reduce the pressure, we get more carbon dioxide and more calcium oxide.  So when lime is being made, they ventilate the kilns to pull the reaction along.

Catalysts only affect the rate of reaction, but NOT the equilibrium position.

Grade A

Sulphuric acid is made by dissolving sulphur trioxide in water.  Sulphur trioxide is made in this reaction:

2SO_2(g) + O_2(g) D 2SO_3(g)

It is carried out at 450 ^oC with a vanadium (V) oxide catalyst:

• If we increase the pressure, the equilibrium moves to the side that has fewer gas molecules, i.e. sulphur trioxide.

• The reaction is exothermic, so increasing the temperature would move the equilibrium in favour of the reactants.

• The catalyst speeds up the reaction, but does not affect the equilibrium.

If you go on to A level chemistry, you will discover more about this when you study Le Chatelier's Principle.