Irreversible and Reversible Reactions

A chemical reaction is a process in which one or more atoms or compounds (reactants) are converted into one or more different atoms or compounds (products).

Some reactions are ‘irreversible’, while others are ‘reversible’.

An irreversible reaction where the reactants are A & B and the products are C & D is represented by:

The single arrow indicates that this reaction is irreversible, meaning that the products cannot be converted back into the reactants. The single arrow also indicates that the reaction will ‘proceed to completion’ (there will eventually be no A and B remaining in the system).

Irreversible reactions are like baking a cake…

The ingredients (reactants) are mixed together and baked to form a cake (product), and, once the cake has been made, it is practically impossible to recover these reactants (eggs, flour, sugar, etc…) from the product…

However, for some reactions, the products are able to convert back into the reactants. These types of reactions are said to be reversible.

A reversible reaction with reactants A & B and products C & D is represented by:

The double half-headed arrow symbol indicates that this reaction is reversible.

Whether a reaction is reversible largely depends on the relative sizes of the forward reaction activation energy and the reverse reaction activation energy.

If the forward reaction has a very small activation energy, while the reverse reaction has a very large activation energy, then the reaction will be irreversible and only proceed in the forward direction.

If the activation energy for the forward and reverse reaction are both relatively small, then the reaction will be reversible.