UPSC Combined Geo-Scientist, Mains · 2020 · Mains Paper II

Q1(g). Reaction order and rate constant

Question

Initial concentrations 2×10⁻³ and 1×10⁻³ mol L⁻¹ give rates 2.40×10⁻⁴ and 0.6×10⁻⁴ mol L⁻¹ s⁻¹, respectively. Determine order and rate constant.

The order is two and k=60.0 L mol⁻¹ s⁻¹.

Let r=k[A]ⁿ. The concentration ratio is 2 and the rate ratio is 4, so 2ⁿ=4 and n=2.
From the first experiment, k=(2.40×10⁻⁴)/(2.0×10⁻³)²=60.0 L mol⁻¹ s⁻¹.
The second experiment gives (0.6×10⁻⁴)/(10⁻³)²=60.0 as well, checking consistency. The result is the order with respect to the specified reactant under the stated conditions.

The full model answer

As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.

CoversReaction orderrate constantChemical kinetics