Q.Define instantaneous rate of reaction. Explain pseudo first order reaction with suitable example.

Instantaneous rate is the rate at one exact moment; pseudo first order behaviour arises when a large excess of one reactant masks the true order.

Instantaneous rate: The rate of a reaction at a particular instant of time, obtained by taking the limit of the average rate as the time interval $\Delta t$ approaches zero, i.e. as the slope (tangent) drawn to the concentration-vs-time curve at that point: $rate = -\dfrac{d[R]}{dt} = \dfrac{d[P]}{dt}$.

Pseudo first order reaction: A reaction whose actual rate law involves two (or more) reactants, but which is experimentally observed to follow first order kinetics because one reactant is present in such large excess that its concentration remains virtually unchanged during the reaction. Example: acid-catalysed hydrolysis of ethyl acetate in aqueous solution:

$CH_3COOC_2H_5 + H_2O \xrightarrow{H^+} CH_3COOH + C_2H_5OH$ …