Question

Which of the following statements are correct?

(i) Law of mass action and rate law expressions are same for single step reaction.
(ii) Order of the slowest elementary reaction of a complex reaction gives the order of the complex reaction.
(iii) Both order and molecularity have normally a maximum value of 3.
(iv) Molecularity of a complex reaction $A+2B\to C$ is 3.

• A. i, ii, iii
• B. ii, iii, iv
• C. ii, iv
• D. All are correct

Answer 1

The correct option is A i, ii, iii

The statements i, ii, iii are correct.
(i) Law of mass action states that "the rate of chemical equation is directly proportional to the product of the active mass of the reactants, with each active mass term raised to the power equal to its respective stoichiometric coefficient in a balanced equation with a constant temperature."

Whereas in a rate law expression:-The concentration of reactants is raised to the power of something which may or may not be equal to the stoichiometric coefficient.
In a single-step reaction, the power to which the concentration of reactants is raised is equal to their stoichiometric coefficients.

Hence, Law of mass action and rate law expressions are the same for single-step reaction.
(ii) Order of the slowest elementary reaction of a complex reaction gives the order of the complex reaction. The slowest step is the rate-determining step.
(iii) Both order and molecularity have normally a maximum value of 3. The probability of the collision of 4 or more molecules is rare.
(iv) It is wrong to say that the "Molecularity of a complex reaction $A+2B\to C$ is 3". This is because the concept of molecularity is applicable only for elementary reactions. The molecularity of the complex reaction is not decided.