Answer. Chemical reactions, which proceed in both directions i.e. forward as well as backward directions, are called reversible reactions. In these reactions the original reactants react to form the products and products also react with themselves to give back the original reactants. These reactions are represented by double arrow.
Let us discuss a reaction between hydrogen and iodine. Because one of the reactant, iodine is purple, while the product hydrogen iodide is colorless. Therefore, proceedings of the reaction are easily observable. On heating hydrogen and iodine vapours in a closed flask, hydrogen iodide is formed. As a result purple colour of iodine fades as it reacts to form colorless hydrogen iodide such as, shown in figure below.
H2(g) + I2(g) 2HI
This reaction is termed, as forward reaction.
On the other hand, when only hydrogen iodide is heated in a closed flask, purple colour appears because of formation of iodine vapours. Such as
2HI(g) ———> H2(g) + I2(g)
In this case hydrogen iodide acts as reactant and produces hydrogen and iodine vapours. This reaction is reverse of the above. Therefore, it is termed as reverse reaction.
When both of these reactions are written together as a reversible reaction, they are represented as:
H2(g) + I 2(g) 2HI(g)
Establishment of a reversible reaction
Graphical explanation of reversible reaction
In a reversible reaction, at last dynamic equilibrium is established. At initial stage the rate of forward reaction is very fast and reverse reaction is taking place at a negligible rate. But gradually forward reactions slow down and reverse reaction speeds up. Eventually, both reactions attain the same rate: it is called a dynamic equilibrium state as show in graph.