Reversible Reactions
Most chemical reactions only proceed in one direction. They cannot be reversed. For example, sodium hydroxide reacts with hydrochloric acid to produce sodium chloride and water.
NaOH (aq)+ HCl(aq) → NaCl(aq)+ H2O(l)
This reaction cannot be reversed. It is not possible to make sodium chloride react with water to form sodium hydroxide and hydrochloric acid.
Some reactions can be reversed; they are said to be reversible. The reaction for the manufacture of ammonia is a reversible reaction. This is indicated by the sign “→”
forward reaction
N2 (g)+ 3H2 (g) → 2NH3 (g)
backward reaction
The change shown by the arrow →is called the forward reaction. This s the reaction of nitrogen and hydrogen to form ammonia. The change shown by the arrow →is called the backward reaction. This is the decomposition of ammonia back to nitrogen and hydrogen. The forward and reactions take place at the same time. That is, as nitrogen backward ammonia formed and hydrogen combine to form ammonia, some of the ammonia formed decomposes into nitrogen and hydrogen again.
Reversible reactions do not go to completion, when the reaction appears to be over, some nitrogen and hydrogen remain. For the Haber process reaction, about 10% of the reactants remain.
At the end of the reaction, a mixture reactants and products is present. The of The mixture is said to be in equilibrium.
Note: At equilibrium, the forward and reactions do stop they continue, but at the same speed. Hence, there is no overall change in the amounts of reactants and products.
The amounts of reactants and products at equilibrium can be altered by changing the conditions of the reaction, such as temperature and pressure.