Reversible reactions

What you need to know

Reflections and Exam tips

Reversible reactions

Some chemical reactions can go in both directions, this means that the product of a reaction can react to produce the reactants. This type of reaction is known as a reversible reaction and we use this symbol

In a closed system where nothing can escape, the rates of the forward reaction and backward reaction of a reversible reaction will be equal, at this point equilibrium has been reached.

In a dynamic equilibrium both the forward and backward reaction proceed at the same rate.

The Haber Process

The Haber Process is used to make ammonia.

The ammonia can be used to make fertilisers and other commercial chemicals.

There are two raw materials in the process: nitrogen from the air and hydrogen (from natural gas).

The reactants are purified and mixed in the ratio of 3:1 (hydrogen : nitrogen) proportions and are then passed over an iron catalyst at temperatures of around 450ºC and a pressure of approximately 200 atmospheres.

This is a reversible reaction and so some ammonia is broken down back into nitrogen and hydrogen.

The gases are passed through a condensor which liquifies the ammonia.

The liquid ammonia is removed from the unreacted gases and they are recycled.

The yield is below 20% but the production of ammonia is quick and no gases are wasted.

The equation for the reaction is:
N₂ + 3H₂ <--> 2NH₃

The reaction to produce ammonia is exothermic, so lower temperatures give higher yields.

In general, if a reaction produces larger volumes of gases:

1. an increase in pressure decreases yield
2. a decrease in pressure increases yield
   

In general, if a reaction produces smaller volumes of gases:

1. an increase in pressure increases yield
2. a decrease in pressure decreases yield

Understand that in a reversible reaction the energy change in the forwards direction is opposite to that in the reverse direction