The concept of dynamic equilibrium

Some reactions are able to go in two directions; forward and reverse. They are known as reversible reactions.

The forward and reverse reactions occur at the same time and never stop. As a result, they are called dynamic reactions.

When the rate of the forward reaction is equal to the rate of the reverse reaction the reaction is said to have reached equilibrium.

Example: the reaction of iron(III) ions with thiocyanate ions

Fe³⁺(aq) + CNS^-(aq)FeCNS²⁺(aq)

Pale yellow iron(III) ions react with colourless thiocyanate (CNS) to produce red iron thiocyanate.

When there are more products than reactants present the position of equilibrium lies to the right.

Example: in ester formation there are more products than reactants at equilibrium

CH₃COOH(l) + CH₃OH(l)CH₃COOCH₃(l) + H₂O

In this example of ester formation, ethanoic acid reacts with methanol to produce methylethanoate (the ester) and water. The equilibrium lies to the right.

When there are more reactants than products present the position of equilibrium lies to the left.

Example: in water, only a small proportion of the molecules have split to form ions at equilibrium:

H₂O(l)H⁺(aq) + OH^-(aq)

In this example of water, the equilibrium lies to the left. Only a few molecules have split to form ions.

It doesn't matter whether the reaction starts with 100% reactants or 100% products, the reaction will always reach the same equilibrium position.