Question

A reaction is of zero order when the rate of reaction is

• A. Directly proportional to the concentration of reactant
• B. Inversely proportional to the concentration of reactant
• C. Independent of the concentration of reactant
• D. Independent of temperature and pressure

Hint:

Zero-order reactions are relatively uncommon but can occur under specific conditions, such as when a reaction is catalyzed by a surface that is saturated with reactant, or when a reaction is limited by the rate at which a reactant is supplied.

Answer 1

The Correct Option is C

Step 1: Understand the concept of the order of a reaction.
The order of a reaction with respect to a particular reactant is defined as the power to which the concentration of that reactant is raised in the rate law. The overall order of a reaction is the sum of the orders with respect to each reactant. The order of a reaction is experimentally determined and cannot be predicted from the stoichiometry of the balanced chemical equation. The rate law for a general reaction \(aA + bB \rightarrow products\) can be written as:
$$rate = k [A]^m [B]^n$$ where:
\(rate\) is the rate of the reaction.
\(k\) is the rate constant.
\([A]\) and \([B]\) are the concentrations of reactants A and B.
\(m\) is the order of the reaction with respect to A.
\(n\) is the order of the reaction with respect to B.
The overall order of the reaction is \(m + n\).
Step 2: Define a zero-order reaction based on its rate law.
A reaction is said to be of zero order with respect to a particular reactant if the exponent of its concentration in the rate law is zero. For a reaction that is zero order in a single reactant A, the rate law is: $$rate = k [A]^0 = k \times 1 = k$$ This equation shows that the rate of a zero-order reaction is equal to the rate constant \(k\) and does not depend on the concentration of the reactant A. If a reaction involves multiple reactants and is zero order overall, it means that the sum of the orders with respect to all reactants is zero. However, the question focuses on the fundamental characteristic of a zero-order reaction with respect to the concentration of the reactant mentioned. Step 3: Evaluate the given options based on the definition of a zero-order reaction.
(1) Directly proportional to the concentration of reactant: This describes a first-order reaction (rate \(\propto [reactant]^1\)).
(2) Inversely proportional to the concentration of reactant: This would have a rate law of the form \(rate = k / [reactant]\) or \(rate = k [reactant]^{-1}\), which is not a zero-order reaction.
(3) Independent of the concentration of reactant: This is the defining characteristic of a zero-order reaction, as shown by the rate law \(rate = k\). The rate remains constant regardless of how the concentration of the reactant changes (as long as some reactant is present).
(4) Independent of temperature and pressure: The rate of a reaction is generally dependent on temperature (through the rate constant \(k\), as described by the Arrhenius equation) and can also be affected by pressure, especially for gas-phase reactions. The order of a reaction describes the dependence of the rate on concentration, not directly on temperature or pressure.
Step 4: Conclude the condition for a zero-order reaction.
A reaction is of zero order when its rate is independent of the concentration of the reactant(s).