# Define Rate of Reaction and also give its units.

The rate of reaction can be defined as the change in the concentration of any one of the reactants or products per unit time. The rate of the reaction depends on the molar concentrations of the reactants. The rate of the reaction means the speed with which the reaction takes place. This is expressed either in terms of decrease in the concentration of a reactant per unit time or increase in the concentration of a product per unit time. 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 Cl2 are ∆ [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
The minus sign along with the first term is used simply to show that the concentration of the reactant (PCl5) is decreasing while plus sign along with the other two term is used simply to show that the concentration of the products (PCl3 and Cl2) is increasing.

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

Category: Chemical Kinetics