Reversible and Irreversible Reactions

NH₄Cl ⇌ NH₃ + HCl

When you see a reaction written down using a double arrow between the reactants and products, it means that it is reversible. A single arrow indicates an irreversible reaction.

When talking about reversible reactions, you will hear the phrase dynamic equilibrium used. This describes the situation in a closed system when the forward and reverse reactions are taking place at exactly the same rate - the concentration of products and reactants remain the same. In a closed system, no products or reactants can escape or be lost from the reaction vessel - it is fully sealed. At equilibrium, the reactions are both still happening but everything is balanced. This balance can be changed either by altering the conditions or by adding/removing a reactant or product.

In any reversible reaction it is the forward portion that we want, so we must change the conditions to be as favorable as possible. You studied this in detail when you were learning about the Haber process. If the forward reaction is exothermic for example, the reverse reaction will be endothermic by exactly the same amount. Increasing the temperature means that there is more heat available which helps the reverse reaction (remember that endothermic reactions absorb energy (usually heat) from the surroundings, so if there is more heat energy available it can be more readily absorbed). We say that the equilibrium shifts to the left. In other words, the backwards reaction is favored and less product is made. Cool down the system and the opposite will be true as it will be easier for the forward exothermic reaction to lose energy to the surroundings.

Try this quiz and see how well you understand reversible and irreversible chemical reactions.