Different reactions proceed to different extents before equilibrium is reached. For example, the reaction on the right does not proceed to any significant extent in the forward direction. A great deal of reactant molecules are left unreacted and very little product is formed.

Weak acids, such as ethanoic acid, commonly known as vinegar, are good examples of reactions that reach equilibrium without any significant formation of product. Nearly all the molecules of ethanoic acid remain unreacted while only 1% of the molecules react with the water to form hydronium ions according to the equation below.

The reaction on the left however, does show a strong tendency to react and the amount of product formed at equilibrium is significant.

Strong acids, such as hydrochloric acid, are good examples of reactions that proceed to a significant extent to the right. Nearly all the molecules of hydrochloric acid react with the water to form hydronium ions according to the equation below.