Physical Systems
When a liquid has reached its boiling point a significant number of particles will have enough energy to escape from the liquid state and form vapour by evaporating.
At the same time, some of these vapour molecules will collide with the surface of the liquid, lose energy and become liquid by condensing.
There will come a time when the rate of evaporation is equal to the rate of condensation and at this point there's no net change in the amounts of liquid and gas present. The system has reached equilibrium.
Chemical Systems
In a chemical system we are dealing with reversible reactions. There's the forward reaction (reactants to products), backward reactions (products to reactants) and the reverse reaction.
When a chemical system reaches equilibrium the reverse reaction equal the backward reaction and there's no net change observed even though both reactions are still occurring. The concentrations of both reactants and products remain constant over time and this is referred to as the equilibrium mixture.
Characteristics Of The Equilibrium State
At equilibrium state the rate of the forward reaction is equal to the rate of the backward reaction.
Feature of equilibrium state:
- Equilibrium is dynamic
The reaction hasn't stopped but both forward and backward reactions are still occurring.
- Equilibrium is achieved in a closed system
A closed system prevents exchange of matter with the surroundings, so equilibrium is achieved where both reactants and products can react and recombine with each other.
- The concentrations of reactants and products remain constant at equilibrium
They are being produced and destroyed at an equal rate.
- At equilibrium there's no change in macroscopic properties
This refers to observable properties such as colour and density. These don't change as they depend on the concentrations of the components of the mixture.
- Equilibrium can be reached from either direction
The same equilibrium mixture will result under the same conditions, no matter whether the reactions is started with all reactants, all products, or a mixture of both.
Even though the concentrations of reactant and product are constant and equilibrium, this doesn't imply that they're are equal. Most commonly there will be a higher concentration of either reactant or product in the equilibrium mixture, depending both on the reaction and on the conditions.
The proportion of reactant and product in the equilibrium mixture is referred to as its equilibrium position. Reactions where the mixture contains more products are said to 'lie to the right', and reactions with more reactants are said to 'lie to the left'.
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