Question

Reaction order describes dependence of rate on

• A. concentration
• B. temperature
• C. pressure
• D. catalyst

Hint:

The order of a reaction can be zero, fractional, or an integer, whereas the molecularity of an elementary reaction must always be a positive integer.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks which parameter's effect on reaction rate is described by the "order of a reaction" in chemical kinetics.

Step 2: Key Formula or Approach: For a general chemical reaction: \[ aA + bB \rightarrow \text{Products} \] The rate law is mathematically represented as: \[ \text{Rate} = k [A]^x [B]^y \] where: \( k \) is the rate constant,
\( [A] \) and \( [B] \) are the molar concentrations of the reactants, and
\( x \) and \( y \) are the partial reaction orders with respect to reactants A and B.
The overall reaction order is defined as \( n = x + y \).

Step 3: Detailed Explanation:

• Concentration Dependence: The reaction order defines the relationship between the rate of a chemical reaction and the concentration of its reactants.
For example, in a first-order reaction (\( n = 1 \)), doubling the concentration of the reactant doubles the rate of the reaction.

• Experimental Nature: Reaction order is an experimentally determined quantity and cannot be deduced solely from the stoichiometric coefficients of the balanced chemical equation.

• Role of Other Variables:
-

Temperature (Option B) influences the rate constant \( k \) via the Arrhenius equation (\( k = A e^{-E_a/RT} \)), not the reaction order.
-

Pressure (Option C) primarily affects gas-phase reaction rates by changing the effective concentration.
-

Catalysts (Option D) lower the activation energy, changing the mechanism and rate constant, but do not define the basic concentration-dependent order.


Step 4: Final Answer:
Consequently, reaction order describes the dependence of reaction rate on concentration, matching Option (A).