Concept:Reactions catalyzed by solid surfaces often exhibit unique kinetics at high concentrations when the surface becomes completely saturated with reactant molecules.
Formula:$$ 2\text{NH}_{3(g)} \xrightarrow{\text{W (Tungsten)}} \text{N}_{2(g)} + 3\text{H}_{2(g)} $$
Solution:- When ammonia decomposes on a solid tungsten catalyst, it must first adsorb onto the tungsten surface.
- At moderate to high pressures, the entire active surface of the tungsten is completely covered by ammonia molecules.
- Adding more ammonia gas cannot increase the rate because there are no available binding sites on the catalyst.
- Because the rate becomes completely independent of the gaseous ammonia concentration, it acts as a classic zero-order reaction.
Why other options are incorrect:At extremely low pressures (where the surface is mostly empty), it acts as first order, but standard exam context implies the saturated zero-order condition, which is a textbook hallmark.
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