When a piece of sodium is dropped into water, a violent reaction occurs resulting in the formation of sodium hydroxide and hydrogen gas.
2Na(s) + 2H2O(l) à 2NaOH (aq) + H2
However, it is not possible to carry out the reverse reaction under any known experimental conditions, i.e. , reduction of aqueous sodium hydroxide by hydrogen to form sodium and water cannot be achieved.
The reactions in which the products do not react to give back reactants are called irreversible reactions.
Now let us consider the reaction between metallic iron and steam.
3fe(s) +4H2O(g) à Fe3O4(s) + 4H2(g)
If this reaction is carried out in an open vessel, then the hydrogen gas escapes and the reaction proceeds to completion provided the sufficient amount of steam is supplied. The reverse of this reaction, i.e. , reduction of iron oxide (Fe3O4) to iron by hydrogen gas can also be achieved rather easily in the laboratory.
Fe3O4(s) + 4H2(g) à 3Fe(s) + 4H2O(g)
To carry out this reaction hydrogen gas is passed over heated iron oxide. Again, if this reaction is carried out in an open vessel the whole of Fe30 4 is reduced to iron because one of the products, i.e., water escapes as steam.
The reactions in which products can react to give back reactants are called reversible reactions.
A reversible reaction is represented by putting two arrows pointing in opposite directions between the formulae of the reactants and the products as shown below:
Some more examples of reversible reactions are