A reversible reaction is one in which there is both a forward reaction (reactants are made into products) and a reverse reaction (products are broken to form reactants).

The sign ‘⇌’ is used to show that the reaction is reversible. 

Example:
N₂ + 3H₂  ⇌  2NH₃

Reversible reactions always result in a mixture of reactants and products being formed.

Reversible reactions in a closed systems reach equilibrium when the rates of forward and reverse reactions are constant. 

There are three factors that can affect the equilibrium position:

  • Temperature
  • Pressure
  • Concentration

Temperature

You first need to know whether heat is given out or taken in.

Example:
A + B  ⇌  C + D          ∆H = -200 kJ mol⁻¹

The negative sign in the ‘-200’ shows that the forward reaction (reactants forming products) is exothermic (heat if given out). 

The reverse reaction would be the same except that heat would be taken in i.e. ∆H = ‘+200’. 

If temperature is increased
According to According to Le Chatlier’s law, the position of equilibrium will move to counteract the change. The position of equilibrium will move so that temperature is reduced. 

If temperature is increased, the system would need to absorb more heat. Therefore, heat would need to be taken in. This would favour the endothermic reaction. Therefore, the position of equilibrium would shift to the left. 

Therefore, A and B would be produced. 

If temperature is decreased
According to According to Le Chatlier’s law, the position of equilibrium will move to counteract the change. The position of equilibrium will move so that temperature is increased. 

If temperature is decreased, the system would need to give out more heat. Therefore, heat would need to be given out. This would favour the exothermic reaction. Therefore, the position of equilibrium would shift to the right. 

Therefore, C and D would be produced. 

  • Increasing the temperature favours the endothermic reaction since heat would need to be taken in.
  • Decreasing the temperature favours the exothermic reaction since heat would need to be given out.

Pressure

This only applies to reactions involving gases.

Example:
A + 2B  ⇌  C + D  

If pressure is increased
According to According to Le Chatlier’s law, the position of equilibrium will move to counteract the change. The position of equilibrium will move so that pressure is reduced. 

Number of moles on the left:
No. of moles of A = 1
No. of moles of B = 2
Total no. of moles on the left = 1 + 2 = 3

Number of moles on the right:
No. of moles of C = 1
No. of moles of D = 1
Total no. of moles on the right = 1 + 1 = 2

Increasing the pressure favours the reaction which produces lesser moles of gases. Since number of moles produced is lesser in the forward reaction, the forward reaction is favoured. So C and D would be produced.

If pressure is decreased
According to According to Le Chatlier’s law, the position of equilibrium will move to counteract the change. The position of equilibrium will move so that pressure is increased. This can happen by producing more molecules.

Number of moles on the left:
No. of moles of A = 1
No. of moles of B = 2
Total no. of moles on the left = 1 + 2 = 3

Number of moles on the right:
No. of moles of C = 1
No. of moles of D = 1
Total no. of moles on the right = 1 + 1 = 2

Decreasing the pressure favours the reaction which produces more moles of gases. Since number of moles produced is more in the reverse reaction, the reverse reaction is favoured. So A and B would be produced.

Concentration

According to Le Chatlier’s law, if the concentration of a substance is changed, the equilibrium will shift to minimise the effect of that change.

If we add a chemical that is present on either side of the reaction equation, this will cause a shift in the equilibrium position.  

Example:
A + B  ⇌  C + D

Let us increase the concentration of B. 
According to the law, the position of equilibrium will shift to counteract this change. It will have to shift in such a way, that the concentration of B decreases again. 
It will react with A to turn it into C + D. 
The position of equilibrium moves to the right (towards the products).

Similarly, if we decrease the concentration of A, the equilibrium will move to the left (towards the reactants) to increase the concentration of A. 

Catalysts

Adding a catalyst has NO EFFECT on the position of the equilibria. 

This is because a catalyst speeds up the forward and the reverse reaction to the same extent. 

A catalyst can only speed up reactions.