Attainment of equilibrium in reversible reaction
Starting from the beginning of the reaction, represented bythe Y–axis (or when time = 0), the rate of forward reaction rises sharply, while the rate of the reversereaction decreases. This is due to the reaction consisting of pure reactants.
In order to advance the reaction, reactants are converted to products, and it is only until a large enough concentration of products are available,that the reverse reaction becomes a factor. It is at this point that we reach equilibrium, where the forward andreverse rates converge at the same point, forming the equilibrium state.
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 + 4H2O 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
Thus a chemical reactions that proceeds in two directions is known as a reversible reaction.