Under certain conditions, the 2nd order kinetics can be well approximated as first order kinetics. These Pseudo-1st-order reactions greatly simplify quantifying the reaction dynamics.
Consider reaction where A and B reacts to form P: A+B→P Assume we already know the rate eqn:
Rate = k[A]1[B]1
Overall order = 2
Definition of Pseudo-order When an experiment is carried out with: [A] = 0.0100 mol dm–3 [B] = 0.100 mol dm–3 10 times more concentrated If the reaction were to go to completion, [B] remaining would be 0.099 ≈ 0.100 mol dm–3 i.e. [B] remains constant throughout the experiment.
Rate = k[A]1[B]1 For expt where [B] >> [A],
[B] = constant Rate = (k[B]1) [A]1
Rate = k’[A]1 where k’ = k[B]1
Experimental results will show that the overall order is 1 instead of 2.
That’s pseudo 1st order reaction.