Question

Which of the following is an incorrect statement?

• A. For a zero order reaction the rate of the reaction is independent of reactant concentration.
• B. In a first order reaction the half-life period does not depend on the initial reactant concentration.
• C. For a chemical reaction the rate constant increases with increase in temperature.
• D. In a zero order reaction plot of reactant concentration against time is a straight line with negative slope.
• E. In a first order reaction, the time required for 75% completion of the reaction is thrice the half-life period.

Hint:

In first order reactions: 50% → 1 half-life, 75% → 2 half-lives, 87.5% → 3 half-lives.

Answer 1

Answer: (E)

Concept: First order reaction: \[ t_{1/2} = \frac{0.693}{k} \]

Step 1: Check each statement.

• (A) Correct $\rightarrow$ Zero order rate independent of concentration
• (B) Correct $\rightarrow$ Half-life independent of initial concentration
• (C) Correct $\rightarrow$ Rate increases with temperature
• (D) Correct $\rightarrow$ Linear decrease for zero order

Step 2: Check statement (E).
For first order: \[ 75\% \text{ completion} = \text{2 half-lives} \] NOT 3 half-lives.

Step 3: Conclusion.
Statement (E) is incorrect.