Question

A reaction is $50\%$ complete in two hours and $75\%$ complete in $4$ hours. What is the order of the reaction? Give reason.

Answer 1

Half-life is given as 2 hours as 50% is completed in 2 hours

If 75% reaction is completed then 25% of the compound remains which is half of 50%, and it takes (4–2=2) 2 hours to become half of 50 %, i.e. 25%

Therefore half-life of this reaction remains constant. We know that half-life of a first-order reaction is constant, that is, it does not depend on the concentration of the reactant.

Hence given reaction is the first order reaction.