i encountered this question in my chem tutorial : Explain why when ammonia gas is passed over a platinum gauze, the rate of decomposition into nitrogen and hydrogen gas is independent of the partial pressure of ammonia but at VERY LOW pressures, the rate is directly proportional to the partial pressure of ammonia.

has it got something to do with reaction kinetics and order of reaction? someone please help. thanks thanks thanks =]


Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.

I think (I've only just woken up, so may well be talking entire gibberish) that Hydrogen Gas is adsorbed to the platinum surface much more favourably than ammonia or nitrogen, therefore at higher pressures, the rate is proportional to the rate of desorption of Hydrogen from the surface.
At low pressure, this rate will be faster than the rade od adsopbtion of ammonia, so the overall rate will be partial pressure dependent.
At high pressure the rate of desopbtion is much less than the rate of adsorption of ammonia, so as soon as a Hydrogen molecule leaves the surface, there will be an ammonia molecule ready to pounce on the vacancy.

Does that make sense?

Hi allan_chemist, thanks alot for replying. But i don't really understand your explanation. I have several questions regarding your answer.

1. Why is ammonia (and not hydrogen nor nitrogen) more readily adsorbed onto platinum?

2. Do you mean to say that at higher pressures, ammonia molecules are more closely packed and have a higher tendency to collide onto the Pt surface and be adsorbed? If so, could you further explain what partial pressure of ammonia has to do with the entire issue?

Sorry for the trouble and thanks once again! =D

It's Hydrogen that sticks to the Pt more strongly than ammonia or Nitrogen. I'm not sure of the reasons.

At high pressure, the Hydrogen on the surface is in no rush to leave (lets say it is being pushed on by the pressure).
So regardless of the partial pressure of Ammonia, there are just very few sites available on the surface of the Pt. the rate is dependent on the rate of loss of Hydrogen, which is not related to the partial pressure of ammonia.

At low pressure, the hydrogen comes off much faster, and there are loads of sites on the platinum available for the ammonia to stick to. Because of this, the rate is propotional to the partial pressure of ammonia - if it is higher, more can stick to the surface and react, if it is low, less can stick to the surface and react.

I don't think you need the hydrogen in your explanation at all (at first their isn't any even).

In my opinion it's just a regular heterogeneous catalyst question. The reaction rate is normally determined by the capacity of the catalyst. No matter how many molecules you have moving around there is just a distinct amount of molecules that can bond to the catalyst. So their is a constant reaction rate. If the concentration of ammonia decreases then the catalyst is not filled up totally anymore and the reaction rate depends on the ammonia concentration.

^^ Or you could just say that ^^

I failed AEA Chemistry because of kinetics questions :(

WebElements: the periodic table on the WWW []

Copyright 1993-20010 Mark Winter [The University of Sheffield and WebElements Ltd, UK]. All rights reserved.