# Rate of a reaction

The speed at which a chemical reaction proceeds. It is often expressed in terms of either the concentration (amount per unit volume) of a product that is formed in a unit of time or the concentration of a reactant that is consumed in a unit of time. Alternatively, it may be defined in terms of the amounts of the reactants consumed or products formed in a unit of time. For example, suppose that the balanced chemical equation for a reaction is of the form

A + 3B = 2Z

The rate of reaction varies with time.

Rate of reaction = decrease in the concentration of a reactant / Time interval = increase in the concentration of a product / Time interval

Consider the reaction:

PCl5 ——-> PCl3 + Cl2

Suppose in a time interval ∆t, decrease in concentration of PCl5 is ∆ [PCl5] and increase in the concentration of PCl3 and Cl2are ∆ [PCl3] and ∆ [Cl2] respectively, where square brackets indicate molar concentrations in moles / litre of the substances involved. Hence,

Rate of reaction = – ∆ [PCl5] / ∆t = + ∆ [PCl3] / ∆t = + ∆ [Cl2] / ∆t

Units of the Rate of Reaction:

As concentration is usually expressed in moles / litre and the time is taken in seconds or minutes, the unit of rate of reaction is moles litre-1 sec-1 or moles litre-1 min-1