Equilibria

4.22 recall that some reactions are reversible and are indicated by the symbol ⇌ in equations


4.24 understand the concept of dynamic equilibrium


4.25 predict the effects of changing the pressure and temperature on the equilibrium position in reversible reactions.


A reversible reaction is one that can occur in both directions. It means that the reaction can go both forwards and backwards-products can change back into reactants.

It is given by this symbol.

For example ammonia is made by a reversible reaction.

                        N₂ + 3H₂ ⇌ 2NH₃

Reversible reactions occur in both directions. Nitrogen and Hydrogen can make ammonia but ammonia can break down into nitrogen and hydrogen again. Each reversible reaction has a state of dynamic equilibrium, where there is an equal amount of products and reactants being made so it would look like there is no net reaction (no overall reaction)-forwards and backwards reactions is happening at the same rate. 

Le Chatelier's Principal states that:

If a chemical system at equilibrium experiences a change in concentration, temperature, volume, or partial pressure, then the equilibrium shifts to counteract the imposed change and a new equilibrium is established.

Basically: the equilibrium will shift left or right to reduce this change. When they say the equilibrium shifts to the right, it means it is in favour of the forward reaction. Moving to the right=more products produced. And vice versa.

So with ammonia: there are 4 moles of gas in the reactants, but 2 moles of gas (ammonia) in the products. So the reactants have more pressure as there are more molecules. If you increase the pressure, you move the equilibrium to the right hand side, producing more ammonia, because you are trying to decrease the pressure. And the products have less pressure because there are only 2 moles there.

Temperature: If we increase the temperature the reaction will favour the endothermic reaction (the one that takes in heat). In this case the reverse reaction (making nitrogen and hydrogen) is endothermic and would increase. The equilibrium is shifting to the left hand side.

So therefore, to increase the yield of ammonia we can increase the pressure and decrease the temperature.

Increasing the temperature favours the endothermic reaction.

Decreasing the temperature favours the exothermic reaction.