Reversible Reactions and Equilibria

Reversible Reactions

A reversible reaction is a chemical reaction that can proceed in both forward and backward directions.
The reactants can form the products, which in turn can react to produce the original reactants.
A reaction is shown to be reversible by the presence of a double-headed arrow in the chemical equation.

Characteristics of Reversible Reactions

Equilibrium is reached in a closed system where the concentrations of reactants and products remain stable over time.
At equilibrium, the forward and backward reactions continue to happen, but there is no overall change as the rates of these reactions are equal.
The position of equilibrium can be on the side of the reactants (left) or products (right) and it indicates the concentrations of substances in the balanced system.

Dynamic Equilibrium

A dynamic equilibrium occurs in a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction.
Although reactants are being converted into products and vice versa, there is no observable change in the system as the concentrations of reactants and products remain constant.
Both macroscopic properties (like pressure and temperature) and microscopic factors (like individual molecule behaviour) remain constant at dynamic equilibrium.

The Haber Process

The Haber process is an industrial method for synthesising ammonia, with nitrogen and hydrogen as reactants. It is a key example of a reversible reaction under dynamic equilibrium.
The optimised conditions for this process include a high pressure (to favour the forward reaction) but a relatively low temperature (to moderate the rate of reaction).

Le Chatelier’s Principle

Le Chatelier’s principle is used to predict the effect of a change in conditions on a chemical equilibrium.
According to this principle, if a dynamic equilibrium is disrupted by changing conditions, the system will adjust to restore the equilibrium.
If the concentration, temperature, or pressure is changed, the equilibrium position will shift to oppose that change.

Applying Le Chatelier’s Principle

Increasing the concentration of reactants or decreasing the concentration of products will shift the equilibrium to the right, forming more products.
Similarly, decreasing the concentration of reactants or increasing the concentration of products will shift the equilibrium to the left, forming more reactants.
An increase in temperature favours the endothermic direction, while a decrease in temperature favours the exothermic direction.
Increasing pressure will shift the position of equilibrium towards the side with the fewer moles of gas; conversely, reducing pressure causes the system to shift towards the side with the more moles of gas.

Importance of Understanding Equilibria

An understanding of reversible reactions and equilibria is fundamental in optimizing industrial processes like the Haber process, to maximise yield and conserve resources.
Equilibrium concepts also apply to biological systems, environmental chemistry, and the study of chemical reactions in general.