Reversible reactions

Depending on the extent of reaction, reactions are of two types: reversible and irreversible reactions. Reactants, written on the left hand side of a chemical equation, proceed in their reactions. The products are written on the right hand side of the chemical equation. This is indicated by the direction of the arrow in a chemical equation. This is a direct reaction.

For example when steam is passed over hot iron, hydrogen gas and ferroso‐ferric oxide are produced.

3Fe  + 4H2””/  Fe3O4 + 4H2 ‐‐‐‐‐‐‐‐[Direct reaction]

In some chemical reactions, as soon as the products are formed they begin to react among themselves and actually a backward or reverse reaction starts.

Fe3O4 + 4H2 ””/  3Fe + 4H2O ‐‐‐‐‐‐‐‐[Reverse reaction]

It is interesting to note that if carried out in a open vessel, the reaction cannot go in the backward direction because, one of the products of the reaction, the hydrogen gas escapes. But if the reaction is carried out in a closed vessel, the reaction becomes reversible, because when hydrogen gas is passed over hot ferroso‐ferric oxide, steam and iron are produced.

These two reactions may be represented by a single equation as

Two Reactions in one Equation

Thus a chemical reactions that proceeds in two directions is known as a reversible reaction.