Dynamic equilibrium

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).

CO_2(g) ⇌ CO_2(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.

CaCO_3(s) ⇌ CaO_(s) + CO_2(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.