The mathematical expression that relates the rate of a reaction to the concentrations of the reactants is known as the rate expression, rate equation or rate law.
For the reaction \(A + B \rightarrow \text{Products}\), the rate equation is
\[\text{Rate} \propto [A][B]\]
\[\text{Rate} = k[A][B]\]
k is known as the rate constant or specific reaction rate. The units of the rate constant are \((\text{mol})^{1-n}\,(\text{L})^{n-1}\,\text{s}^{-1}\), where n is the order of the reaction.
The rate law for any reaction cannot be predicted by merely looking at the balanced chemical equation, i.e., theoretically, but must be determined experimentally.
Several chemical reactions take place in a sequence of steps, and the overall rate of the reaction is governed by the slowest step, called the rate determining step.
For a reaction, the rate expression is Rate = k[A][B]2/3. The order of the reaction is:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 5/3
The order of a reaction is the sum of the powers of the concentration terms in the rate law.
Order with respect to A: [A] appears with power 1, so the order in A is 1.
Order with respect to B: [B] appears with power 2/3, so the order in B is 2/3.
Order of the reaction: 1 + 2/3 = 5/3.
A fractional order is allowed, because order is found from the experimental rate law.
NCERT (2026-27 edition):
3.2.3 Order of a Reaction – “… the sum of powers of the concentration of the reactants in the rate law expression is called the order of that chemical reaction.”
“Order of a reaction can be 0, 1, 2, 3 and even a fraction.”