RATE OF CHEMICAL REACTIONS
AVERAGE AND INSTANTANEOUS RATE OF CHEMICAL 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