What happens to the equilibrium when a reactant is removed?

When a reactant is removed, the equilibrium shifts to the left, favouring the reverse reaction to replace the lost reactant.

In a chemical reaction, equilibrium is the state where the concentrations of the reactants and products do not change with time. This happens when the rate of the forward reaction equals the rate of the reverse reaction. However, this equilibrium can be disturbed by changing the conditions of the reaction, such as concentration, temperature, or pressure. This is described by Le Chatelier's Principle, which states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change.

When a reactant is removed from the system, the equilibrium is disturbed. According to Le Chatelier's Principle, the system will respond by shifting the equilibrium to the left, to increase the concentration of the reactant that was removed. This is because the removal of a reactant decreases its concentration, creating an imbalance in the reaction. The system will try to restore the balance by favouring the reverse reaction, which produces more of the removed reactant.

For example, consider the reversible reaction: A + B ⇌ C + D. If some of A is removed, the equilibrium will shift to the left, to produce more A and B from C and D. This shift continues until a new equilibrium is established, where the rate of the forward reaction (producing C and D) equals the rate of the reverse reaction (producing A and B).

In conclusion, the removal of a reactant from a system at equilibrium causes the system to shift the equilibrium to the left, favouring the reverse reaction. This is a direct application of Le Chatelier's Principle, a fundamental concept in the study of chemical equilibrium.