When a chemical system is in dynamic equilibrium, there is a balance between forward and reverse processes. They are occurring at the same rate so that no overall change is taking place. There are several types:
- solubility equilibrium
Dissolved and undissolved solute are in equilibrium; the forward process (dissolving) is occurring at the same rate as the reverse process (precipitating).
CO2(g) ⇌ CO2(aq) (carbon dioxide in a pop can)
- phase equilibrium
Pure substances in different physical states are in equilibrium; opposite state changes are occurring at the same rate.
H2O(s) ⇌ H2O(l) (ice/water slush at 0 ºC)
- chemical reaction equilibrium
Reactants and products are in equilibrium; the forward reaction and the reverse reaction are occurring at the same rate.
CaCO3(s) ⇌ CaO(s) + CO2(g)
There are three conditions that must be met in order to achieve dynamic equilibrium in a chemical system:
- The forward and reverse processes occur at the same rate.
- The observable properties that depend on the total quantity of matter in the system, such as colour, pressure, concentration, and pH, are constant.
- The system is a closed system (energy can transfer to or from the surroundings, but matter cannot).
In an open system, a quantitative reaction can occur: virtually all of the limiting reactant is consumed and the maximum theoretical yield is achieved. In a closed system, the reaction is able to reach equilibrium. This limits the amount of product produced.