Question

Reactions of which order will show the rate to be independent of the concentration of the reactant ? Give one example of this order.

Hint:

In a zero-order reaction, a graph of concentration versus time is a straight line with a negative slope equal to \( -k \).
The units for a zero-order rate constant are \( mol \, L^{-1} \, s^{-1} \).

Answer 1

Concept:

• The order of a reaction is defined as the sum of the powers of the concentration terms of the reactants in the rate law expression.

• In certain specific chemical processes, the rate of the reaction does not change regardless of how much reactant is added or removed.

• These reactions are classified as zero-order reactions because the concentration term in the rate law is raised to the power of zero.

• Zero-order reactions typically occur when the reaction is limited by the availability of a catalyst surface or light intensity.
Step 1: Derive the mathematical dependency
Consider a general reaction: \( A \rightarrow \text{Products} \).
For a zero-order reaction, the rate law is written as: \[ \text{Rate} = k[A]^{0} \]
Since any number raised to the power of zero is 1, we get: \[ \text{Rate} = k \]
This shows that the rate of reaction is equal to the rate constant (\( k \)) and is independent of the molar concentration of reactant \( A \).

Step 2: Describe a practical example and the underlying reason
A standard example is the decomposition of ammonia gas on a hot platinum surface: \[ 2NH_{3}(g) \xrightarrow{Pt, \Delta} N_{2}(g) + 3H_{2}(g) \]
In this heterogeneous catalysis, the ammonia molecules are adsorbed on the surface of the platinum. At high pressures, the entire surface of the catalyst becomes completely covered (saturated) with ammonia molecules.
Once the surface is saturated, any further increase in the concentration (or pressure) of ammonia cannot increase the number of molecules reacting per unit time because there are no more free sites on the catalyst. Consequently, the rate remains constant. The final answer is Zero order reaction; example is decomposition of \( NH_{3} \) on Pt.