Question

The rate of the reaction, 2NO + Cl$_2$ $\rightarrow$ 2NOCl is given by the rate equation, rate = k[NO]$^2$[Cl$_2$]. The value of the rate constant can be increased by

• A. increasing the concentration of NO.
• B. increasing the temperature.
• C. increasing the concentration of the Cl$_2$.
• D. doing all of these.

Hint:

Key Exam Tip:
Factors Affecting Reaction Rate vs. Rate Constant:
• Concentration of Reactants: Affects the rate of the reaction but not the rate constant (k).
• Temperature: Affects both the rate and the rate constant (k). Higher temperature generally leads to a higher rate constant.
• Catalyst: Affects the rate by lowering the activation energy, thus increasing the rate constant (k). In this question, only temperature and a catalyst (not provided as an option) affect the rate constant.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
This question is a repetition of Q3. It tests the distinction between "Reaction Rate" and "Rate Constant (\( k \))".
Step 2: Detailed Explanation:
The rate law is \( Rate = k \cdot [Reactants]^n \).
Changing the concentration (\( [NO] \) or \( [Cl_2] \)) changes the value of the Rate, but it does not change the value of the proportionality constant \( k \).
The rate constant \( k \) is a function of:
1. Temperature: (Arrhenius Equation \( k = Ae^{-E_a/RT} \))
2. Catalyst: (By changing activation energy \( E_a \))
Step 3: Detailed Explanation:
Increasing the temperature increases the kinetic energy of the molecules and the frequency of effective collisions, which mathematically increases the value of \( k \).
Step 4: Final Answer:
Increasing concentration only affects the rate. To increase the rate constant, one must increase the temperature.