Question

Which of the following is a zero order reaction?

• A. Decomposition of \( \text{H}_2\text{O}_2 \) catalysed by iodide in alkaline medium
• B. Artificial radioactive decay of unstable nuclei
• C. Decomposition of \( \text{N}_2\text{O}_5 \)
• D. Decomposition of \( \text{N}_2\text{O} \)
• E. Decomposition of gaseous ammonia on a hot platinum surface

Hint:

Surface catalyzed reactions often show zero-order kinetics when catalyst surface is fully occupied.

Answer 1

Answer: (E)

Step 1: Recall definition of zero order reaction.
In a zero order reaction: \[ \text{Rate} = k \] i.e., rate is independent of concentration of reactant.

Step 2: Identify typical conditions.
Zero order reactions usually occur: - On catalyst surfaces
- When surface becomes saturated

Step 3: Analyze given options.
- \( \text{H}_2\text{O}_2 \) decomposition → generally first order
- Radioactive decay → first order
- \( \text{N}_2\text{O}_5 \) decomposition → first order
- \( \text{N}_2\text{O} \) decomposition → not zero order

Step 4: Examine surface catalysis.
Decomposition of ammonia on hot platinum occurs on catalyst surface.

Step 5: Explain zero order nature.
At high concentration, surface gets fully occupied → rate becomes constant and independent of concentration.

Step 6: Identify correct reaction.
Thus, ammonia decomposition on platinum surface is a classic zero-order reaction.

Step 7: Final conclusion.
\[ \boxed{\text{Decomposition of gaseous ammonia on a hot platinum surface}} \] Therefore, correct option is \[ \boxed{(5)} \]