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

Correct — C. Decomposition of gaseous ammonia on a hot platinum surface at high pressure is a zero order reaction, which is option (c).

2NH₃ → N₂ + 3H₂ runs on the platinum surface, and at high pressure that surface is fully saturated with adsorbed ammonia molecules.

Adding more ammonia cannot increase how much is adsorbed, so the rate stops depending on concentration. The rate law collapses to rate = k, and the order in NH₃ is zero.

Why the others are wrong

• (a)
  three — Third order would need the rate to vary with the cube of concentration. On a saturated platinum surface the rate does not vary with concentration at all.
• (b)
  two — Second order would mean rate ∝ [NH₃]², so doubling the pressure would quadruple the rate. Once the surface is saturated, doubling the pressure changes nothing.
• (d)
  one — First order is the low-pressure behaviour, before the surface is covered. The stem specifies high pressure, which is the condition that drives the order down to zero.

Order of reaction is an experimental quantity — the power to which a concentration is raised in the rate law. It need not match the stoichiometry of the balanced equation.

Surface-catalysed reactions give the clearest zero-order behaviour. At low pressure only part of the catalyst surface is covered and the rate rises with pressure.

Once the surface is saturated, the number of adsorbed molecules is fixed by the area available rather than by how much gas is in the vessel, so the rate becomes constant and concentration drops out of the rate law.

Key facts

• Decomposition of ammonia on a hot platinum surface at high pressure is a zero order reaction.
• For a zero order reaction the rate law is rate = k and the rate is independent of reactant concentration.
• Decomposition of hydrogen iodide on a gold surface is another standard zero order example.
• Order of reaction is found experimentally and need not equal the stoichiometric coefficients.

Common traps

• Reading the order off the balanced equation 2NH₃ → N₂ + 3H₂ and answering two.
• Skipping the words 'at high pressure', which are what make the order zero rather than one.

No directly related past PYQ was found.