In the below given hypothetical equation
\(r = k[A]^x[B]^y\)
If the concentration of [B] is kept constant and concentration of [A] is increased to 3 times, the reaction rate increases 27 times, then the order of reaction with respect to A is
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 3
With [B] constant, the rate depends only on [A]x:
\( \displaystyle \frac{r_2}{r_1} = \left(\frac{3[A]}{[A]}\right)^x = 3^x = 27 = 3^3 \;\Rightarrow\; x = 3 \)
The reaction is third order with respect to A.
NCERT (2026-27 edition):
Order of a Reaction – “In the rate equation (3.4) Rate = k [A]x [B]y, x and y indicate how sensitive the rate is to the change in concentration of A and B. Sum of these exponents, i.e., x + y in (3.4) gives the overall order of a reaction whereas x and y represent the order with respect to the reactants A and B respectively.
Hence, 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. A zero order reaction means that the rate of reaction is independent of the concentration of reactants.”