Question

Assertion (A) Decomposition of gaseous ammonia on a hot platinum surface is a zero order reaction at high pressure.
Reason (R) At high pressure, the metal surface gets saturated with gas molecules.

• A. Both $A$ and $R$ are correct and $R$ is the correct explanation of $A$.
• B. Both $A$ and $R$ are correct, but $R$ is not the correct explanation of $A$.
• C. $A$ is correct but, $R$ is incorrect.
• D. $R$ is correct but, $A$ is incorrect.

Answer 1

Answer: (A)

It is a zero order reaction at high pressure.
$2 NH _{3}(g) \xrightarrow[\text { Pt catalyst }]{1130\, K } N _{2}(g)+3 H _{2}(g)$
Rate $=k\left[ NH _{3}\right]^{0}=k$
In this reaction, $Pt$ acts as a catalyst the metal surface gets saturated with gas molecules at high pressure.
Thus, a further change in reaction conditions does not alter the amount of ammonia on the surface of the catalyst making rate of the reaction independent of its concentration.