Assertion : The decomposition of gaseous ammonia on a hot platinum surface is a zero order reaction at high pressure.
For a zero order reaction the rate of reaction is independent of initial concentration.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reason are true but reason is not the correct explanation of assertion.
C. If assertion is true but reason is false.
D. If both assertion and reason are false.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Assertion**: The assertion states that the decomposition of gaseous ammonia on a hot platinum surface is a zero-order reaction at high pressure. This means that at high pressures, the reaction rate does not depend on the concentration of ammonia.

2. **Analyzing the Reaction**: The decomposition of ammonia can be represented by the equation:
\[ 2NH_3(g) \rightarrow N_2(g) + 3H_2(g) \]
At high pressures, the reaction environment changes, affecting how the reactants interact with the catalyst (platinum).

3. **Saturation of the Catalyst**: At high pressures, the platinum surface becomes saturated with ammonia molecules. This saturation means that there are enough ammonia molecules on the surface that adding more ammonia does not increase the reaction rate.

4. **Rate of Reaction**: For a zero-order reaction, the rate of reaction is constant and does not depend on the concentration of the reactants. Mathematically, the rate can be expressed as:
\[ \text{Rate} = k \]
where \( k \) is the rate constant. This indicates that the rate remains unchanged regardless of the concentration of ammonia.

5. **Conclusion on the Assertion**: Since the rate of reaction is indeed independent of the concentration of ammonia at high pressure, the assertion is true.

6. **Understanding the Reason**: The reason states that for a zero-order reaction, the rate of reaction is independent of the initial concentration. This is a true statement about zero-order reactions.

7. **Evaluating the Explanation**: While the reason is true, it does not specifically explain why the decomposition of ammonia on a platinum surface is a zero-order reaction at high pressure. The saturation of the catalyst is the key factor that leads to this behavior, which is not mentioned in the reason.

8. **Final Conclusion**: Both the assertion and the reason are true, but the reason does not adequately explain the assertion. Therefore, the correct answer is that both the assertion and reason are true, but the reason is not the correct explanation of the assertion.

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