Question

The rate law for the given chemical reaction is in the form rate = k[A][B]$^{\frac{3}{2}}$. This can be -

• A. Net chemical reaction
• B. An Elementary reaction
• C. Both
• D. Can't be predicted

Answer 1

Answer: (A)

Net chemical reaction

Answer 2

The rate law for a chemical reaction is an experimentally determined expression that relates the rate of reaction to the concentrations of reactants.

1. Elementary Reaction: An elementary reaction is a reaction that occurs in a single step. For an elementary reaction, the molecularity (number of reactant molecules involved in the step) is equal to the order of the reaction with respect to each reactant. Molecularity must always be an integer (1, 2, or 3, rarely higher). Therefore, for an elementary reaction, the exponents in the rate law must be integers, corresponding to the stoichiometric coefficients of the reactants in that elementary step. The given rate law is rate = k[A][B]^(3/2). Since the order with respect to B is 3/2 (a fractional value), this reaction cannot be an elementary reaction. You cannot have 3/2 molecules of B participating in a single step.

2. Net Chemical Reaction (Overall Reaction): A net chemical reaction represents the overall change from reactants to products, which may occur through one or more elementary steps. The rate law for a net chemical reaction is determined experimentally and does not necessarily correspond to its stoichiometry. It is often derived from the mechanism of the reaction, particularly from the rate-determining step. It is common for net chemical reactions to have rate laws with fractional orders, zero orders, or negative orders, especially when the mechanism involves multiple steps, intermediates, or pre-equilibria.

Given that the rate law has a fractional order (3/2 for B), it conclusively indicates that the reaction is not an elementary reaction. Such a rate law is characteristic of a complex reaction that proceeds through a multi-step mechanism, and thus represents a net chemical reaction.