# chemical kinetics

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.