Question
The acid-catalysed hydrolysis of ethyl acetate is a second-order reaction overall, yet it follows first-order kinetics experimentally. Explain why this reaction is called pseudo first-order. Write the rate law and show how the effective rate constant is derived.
(JEE Main 2023, similar pattern)
Solution — Step by Step
The true rate law is second-order:
The reaction is carried out in aqueous solution. Water acts as both solvent and reactant. Its concentration is approximately mol/L and barely changes during the reaction — even if all the ester (say 0.1 M) reacts, the water concentration drops from 55.5 to 55.4 M. That’s a change of less than 0.2%.
Since is effectively constant throughout the reaction:
where is the pseudo first-order rate constant.
The reaction now appears to depend only on raised to power 1 — hence, pseudo first-order.
If we plot vs time, we get a straight line with slope . This is exactly the signature of first-order kinetics:
The half-life is constant: , which is also a first-order characteristic.
Why This Works
The word “pseudo” means false. The reaction is fundamentally second-order (it depends on two concentrations), but because one reactant is in such overwhelming excess that its concentration doesn’t change, the kinetics simplify to first-order behaviour.
This trick isn’t limited to hydrolysis. Any bimolecular reaction where one reactant is in large excess will show pseudo first-order kinetics. The isolation method in kinetics experiments deliberately uses this — flood one reactant, measure the order with respect to the other.
Alternative Method
You can also identify pseudo first-order behaviour experimentally. If you double the initial ester concentration and the initial rate doubles, the observed order with respect to ester is 1. But if you somehow changed water concentration (using a non-aqueous solvent mix), you’d see the rate depends on water too — revealing the true second-order nature.
In JEE numericals, when they give you a pseudo first-order reaction, they’ll provide (the pseudo constant), not (the true constant). Don’t confuse the two. If asked to find the true rate constant, divide: .
Common Mistake
Students often say “the reaction is first-order because only one reactant’s concentration changes.” That’s the observation, not the reason. The actual reason is that water is in such large excess that its concentration remains practically constant. If you used a very small amount of water (say, just stoichiometric amount in a non-aqueous solvent), the reaction would show genuine second-order kinetics. The order doesn’t change — the apparent kinetics do.