Rate of reaction
Consider a general reaction: aA + bB → cC + dD
where a, b, c and d are the stoichiometric coefficients of reactants and products. The rate expression for this reaction is Rate ∝ [A]x[B]y
where exponents x and y may or may not be equal to the stoichiometric coefficients (a and b) of the reactants. Above equation can also be written as Rate = k[A]x[B]y
Sum of these exponents, i.e., x + y gives the overall order of a reaction whereas x and y represent the order with respect to the reactants A and B, respectively.
\( \displaystyle -\frac{d[\mathrm{R}]}{dt}=k[\mathrm{A}]^x[\mathrm{B}]^y \)
The above equation is known as differential rate equation, where k is a proportionality constant called rate constant. The equation, which relates the rate of a reaction to concentration of reactants is called rate law or rate expression.
In the rate law expression reaction rate is given in terms of molar concentration of reactants with each term raised to some power, which may or may not be same as the stoichiometric coefficient of the reacting species in a balanced chemical equation. For example:
2NO2(g) + O2(g) → 2NO3(g)
The rate of this reaction is measured as a function of initial concentrations either by keeping the concentration of one of the reactants constant and changing the concentration of the other reactant or by changing the concentration of both the reactants. The results obtained are tabulated in table below:
| Experiment | Initial [NO2]/mol L−1 | Initial [O2]/mol L−1 | Initial rate of formation of [NO3]/mol L−1s−1 |
| 1 | 0.25 | 0.25 | 0.090 |
| 2 | 0.50 | 0.25 | 0.360 |
| 3 | 0.25 | 0.50 | 0.180 |
| 4 | 0.50 | 0.50 | 0.720 |
Among the following, choose the CORRECT statement?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Molecularity is applicable for elementary reaction only and the given reaction has molecularity three.
Molecularity is the number of reacting species that must collide at the same time in an elementary (single-step) reaction.
Taken as one step, \( \ce{2NO2} \) + \( \ce{O2} \) → \( \ce{2NO3} \) needs two \( \ce{NO2} \) molecules and one \( \ce{O2} \) molecule to collide together, so its molecularity is three, as in 2NO + \( \ce{O2} \) → \( \ce{2NO2} \), a termolecular reaction. This agrees with the experimental rate law, Rate = k[NO2]2[O2], whose order is also 2 + 1 = 3.
Why the other options are incorrect
Option (2) Molecularity is applicable for complex reactions only and the given reaction has molecularity three — Incorrect. It is the other way round; molecularity applies only to elementary reactions.
Option (3) Molecularity is applicable to elementary and complex reactions — Incorrect. For a complex reaction as a whole, molecularity has no meaning; only its individual steps have one.
Option (4) Molecularity and order can be fractional, zero and whole number — Incorrect. This is true only of order; molecularity is always a whole number (1, 2 or 3).
Note: It seems that there is a mistake in the original NTA paper, which prints 2NO2(g) + O2(g) → 2NO3(g) when it should read 2NO(g) + O2(g) → 2NO2(g).
NCERT (2026-27 edition):
Molecularity of a Reaction – …
