Average and instantaneous rate of reaction

Average rate of chemical reaction

It may be defined as the change in concentration of a reactant or product of a chemical reaction in a given interval of time. So

Let us take an example to understand this. When acidified hydrogen peroxide (H₂O₂) is added to a solution of potassium iodide (KI) then iodine is liberated.

H₂O₂ (aq) + 2KI (aq) + H₂SO₄ (aq)  2H₂0 (l) + I₂ (aq) + K₂SO₄ (aq)

Here initially the concentration of iodine is zero. But with the passage of time, it increases and the reaction solution becomes brownish. Concentration of iodine can be measured at different intervals of time by titration against sodium thiosulphate.

If concentration of iodine rises from 0 to 10^-5 mol L^-1 in 10 seconds.

Instantaneous rate of reaction

It may be defined as the change in concentration of a reactant or product of a chemical reaction at a given instant. So, we can calculate the instantaneous reaction rate of above reaction at any instant by using the following formula
       Instantaneous reaction rate = 

Where d[I₂] = small change in concentration of iodine

and        dt  = small change in time

Test your understanding and answer these questions:

1. What is average rate of reaction?
2. What is instantaneous rate of reaction?