In the rate equation
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 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.
Another property of a reaction called molecularity helps in understanding its mechanism. The number of reacting species (atoms, ions or molecules) taking part in an elementary reaction, which must collide simultaneously in order to bring about a chemical reaction is called molecularity of a reaction.
Assuming that the following reaction is an elementary reaction, what is its molecularity?
2FeCl3 + SnCl2 → SnCl4 + 2FeCl2
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 3
The passage states that molecularity is the number of reacting species (atoms, ions or molecules) taking part in an elementary reaction that must collide simultaneously. For an elementary reaction this number is read from the equation:
FeCl3: 2 molecules
SnCl2: 1 molecule
Total: 2 + 1 = 3 reacting species, so the reaction is termolecular (molecularity 3).
NCERT (2026-27 edition):
3.2.4 Molecularity of a Reaction – “Trimolecular or termolecular reactions involve simultaneous collision between three reacting species, for example, 2NO + O2 → 2NO2”