Question

The following data were obtained for the decomposition of phosphine.

Initial pressure (mm of Hg)	707	500	79
Half-life	84	83.8	84

Find the order of reaction.

Answer 1

We know the relationship between half-life and initial concentration for different reaction orders:

Zero order = t_1/2 ∝ [A]₀

First order t_1/2 = `0.693/k` (independent of [A]₀)

Second order t_1/2 ∝ `1/[A]_0`

Despite significant variation in initial pressure (from 79 to 707 mm Hg), the half-life remains nearly constant around 84 seconds.

Since half-life is independent of initial pressure, this indicates that the reaction follows first-order kinetics.

Order of reaction = 1 (first order)