Like speed of a motor car (i.e. the change in position or distance covered in a unit time), the speed or rate of a chemical reaction can be defined as ?the change in concentration of a reactant or a product in unit time.?
We know that as the reaction proceeds towards equilibrium, the concentration of any one of the reactants decreases and consequently the concentration of any one of products increases. So, rate of reaction may be expressed in terms of rate of disappearance of any one of reactant or rate of appearance of any one of the product.
Let us consider the following reaction,
The rate of disappearance of reactants and the rate of appearance of product can be expressed as:
Where, Δ[A], Δ[B] and Δ[C] are the change in molar concentrations of A, B and C respectively in the time interval Δt .
?ve sign with reactants indicates that its concentration decreases with time while +ve sign with the product indicates the increase in its concentration with time.
Since the time can be expressed in minutes, hours, days or years (for radioactive reactions), the unit of rate of reaction may change accordingly. For gaseous reactions, concentration is expressed in partial pressures, hence the unit of rate of reaction is atm S-1.
Instantaneous rate of reaction: Above expression of rate only describes the average rate of reaction which cannot be used to predict the rate of a reaction at a particular instant.
?Instantaneous rate of reaction(rinst) is the rate at a particular moment of time.? To determine rinst we consider the average rate (rav) in the smallest time interval dt (when ).
Mathematically, for a reaction
Fig. Illustration of average rate and instantaneous rate of reaction
Example 1: The decomposition of N2O5 in CCl4 at 318K takes place according to the following equation,
The initial and final concentrations of N2O5 in a time interval of 184 minutes are 2.33 mol L-1 and 2.08 mol L-1. Calculate the rate of this reaction in terms of hours, minutes and seconds. Also find out the rate of production of NO2 during this period.