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My textbook says that when ammonia decomposes on a platinum surface at high pressure, the reaction is 0-order. This is because at high pressures, the metal surface gets saturated with ammonia molecules thus adding more ammonia will not change the rate of the reaction.

However, high pressure does not necessarily mean a large number of molecules per unit volume, instead, the temperature might be the reason for the high pressure, so the molecules per unit volume will remain the same.

If we do a thought experiment with a small number of molecules at a high temperature colliding with a metal surface then we can see there is still some space for additional molecules. It will no longer be a 0-order reaction despite the high pressure.

Is my textbook wrong?

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  • $\begingroup$ your textbook is essentially correct. I guess your textbook relies on the fact that you are working in conditions when you saturate the surface of the catalyst. If the density of molecule is not enough to saturare the surface, of course you will not observe zero order kinetics. Somebody more familiar with the process could confirm this, but I guess your textbook is decribing the conditions that the catalyst is normally used and it is given you an example of a zero order process. $\endgroup$
    – PAEP
    Commented Jul 9, 2023 at 16:53
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    $\begingroup$ You seem confused. The point is that this reaction occurs on a surface not in a volume. it is easy to fill all the surface sites where the reaction can occur if the pressure is high enough. Hence there is no relationship with the bulk concentration. You can't force more molecules into the available reaction sites when they are already saturated. $\endgroup$
    – matt_black
    Commented Jul 10, 2023 at 10:02
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    $\begingroup$ @HarjotDhillon The original claim doesn't say the reaction will always be zero order. Your logic is correct. But the point of mentioning the conditions was to point out that the surface would be saturated. Under much lower pressure the reaction would not be zero order. The point is that, under conditions where the surface is saturated, the reaction is zero order. $\endgroup$
    – matt_black
    Commented Jul 10, 2023 at 20:03
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    $\begingroup$ Chemistry frequently uses simple models from boundary conditions. So look at it from the opposite point of view. "High pressure" means that the catalyst is saturated, and "low pressure" means that it is not. "High pressure" is easy to model since it is zero order. "Low pressure" on the other hand could be almost any order. $\endgroup$
    – MaxW
    Commented Jul 11, 2023 at 9:07
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    $\begingroup$ It is also generally assumed in catalytic reactions that the amount of catalyst is much less than the reactant $\endgroup$
    – Andrew
    Commented Jul 12, 2023 at 18:55

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Well textbook mostly tries to explain a concept in a broader and simple way rather than diving deep into different conditions in which reactions could occur (without giving much details of the conditions in which reaction is occurring). I don't know if our textbooks are same , but in the topic chemical kinetics this is mentioned as just an example of zero order reaction. While studying such type of reactions in which information is incomplete, try to take that the other properties of system (which are not mentioned) at STP. This could help you to get the meaning of 'high pressure' given in the statement and in what context the example is trying to explain you zero order reactions.

I hope my answer helps

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