In chemistry, equilibrium refers to a state in which the forward and reverse reactions of a chemical reaction occur at equal rates, resulting in no net change in the concentrations of reactants and products over time. It is characterized by a balance between the rates of the forward and reverse reactions.
There are three main types of equilibrium in chemistry:
Chemical Equilibrium: This type of equilibrium occurs in a reversible chemical reaction where the concentrations of reactants and products reach a constant value. It is represented by the equilibrium expression, which relates the concentrations of the species involved. An example is the Haber-Bosch process, where nitrogen and hydrogen react to form ammonia:
N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
Physical Equilibrium: Physical equilibria involve the equilibrium between different states of matter without any chemical reaction occurring. One common example is the equilibrium between the liquid and gas phases of a substance, such as water vapor and liquid water in a closed container.
Phase Equilibrium: Phase equilibria involve the equilibrium between different phases of a pure substance or a mixture. For example, the equilibrium between ice and water at 0°C and 1 atmosphere pressure is a phase equilibrium. Another example is the equilibrium between a liquid and its vapor phase, such as the boiling point of a substance.
It's important to note that equilibrium can also occur in other contexts beyond chemistry, such as in physics and economics, but the specific types mentioned above are commonly encountered in the field of chemistry.