The decomposition of gaseous Ammonia on a hot platinum surface is a _______________ order reaction at high pressure.

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- Heterogeneous catalytic decomposition of NH$_3$ on platinum proceeds via adsorption of NH$_3$ on surface active sites; the observed rate depends on surface coverage.
- At low pressure the rate is proportional to concentration (first order), because coverage increases with pressure.
- At high pressure the surface becomes saturated with adsorbed NH$_3$, so further increase in gas concentration does not change the rate; the rate becomes independent of $[\mathrm{NH_3}]$.
- Mathematically: $	ext{rate} = k[\mathrm{NH_3}]^0 = k$, which is the definition of a zero-order reaction.

Therefore the decomposition is zero-order at high pressure (option (2) ⇒ label b).

FINAL_ANSWER: b

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