Question

Which of the following statement is incorrect?

• A. All natural radioactive processes follow first order kinetics.
• B. Thermal decomposition of HI on gold surface is a zero-order reaction.
• C. Decomposition of hydrogen peroxide catalysed by iodide in an alkaline medium is a first order reaction.
• D. The unit of second order rate constant is mol$^{-1}$L s$^{-1}$.
• E. The experimental rate expression of the reaction CHCl$_3$ + Cl$_2 \rightarrow$ CCl$_4$ + HCl is, Rate $=k[$CHCl$_3][$Cl$_2]$

Hint:

Chemistry Tip: Never assume rate law from balanced equation unless reaction is explicitly elementary.

Answer 1

Answer: (E)

Concept:
The rate law of a reaction is determined experimentally. It cannot be predicted only from stoichiometric coefficients unless the reaction is elementary.
Step 1: Check statement (A).
Radioactive decay obeys: $$\frac{dN}{dt}=-kN$$ This is first-order kinetics. So statement (A) is correct.
Step 2: Check statement (B).
Surface-catalysed reactions often become zero order when catalyst surface is saturated. Thermal decomposition of HI on gold surface is a standard example of zero-order reaction. So (B) is correct.
Step 3: Check statement (C).
Decomposition of H$_2$O$_2$ catalysed by iodide in alkaline medium is treated as first-order in standard syllabus context. So (C) is correct.
Step 4: Check statement (D).
For second-order reaction: $$\text{Rate}=k[\text{conc}]^2$$ Unit of rate constant: $$k=\frac{\text{mol L}^{-1}\text{s}^{-1}}{(\text{mol L}^{-1})^2} = \text{L mol}^{-1}\text{s}^{-1}$$ So (D) is correct.
Step 5: Check statement (E).
For reaction: $$\text{CHCl}_3+\text{Cl}_2 \rightarrow \text{CCl}_4+\text{HCl}$$ The experimentally observed reaction is photochemical chain reaction and does not follow: $$Rate=k[\text{CHCl}_3][\text{Cl}_2]$$ Hence this given rate law is incorrect.
Step 6: Final answer.
Therefore incorrect statement is: $$\boxed{\text{Option (E)}}$$ :contentReference[oaicite:1]{index=1}