Thermodynamics Question

[thais]

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I just did a passage that talks about the manufacturing of ammonia.

N2(g) + H2(g) <-> 2NH3 (g)

change in enthalpy = -91.8KJ/mol

Then a question asks:
In order to maintain the maximum yield of ammonia when raising the temperature of the reaction, scientists should:

a. Maintain N2 and H2 at high pressure.
b. allow N2 and H2 to expand with the increased KE.
c. Increase the volume in the reaction chamber to favor the forward reaction.
d. Continuously remove the excess heat produced by the forward reaction.

Answer is A. I understand that it is A because the reactants have more moles of gas, thus it would shift the reaction to the right according to Le Chatelier's principle. However, I don't understand why D is wrong. I thought that when we have an exothermic reaction we count heat as a product. Thus, if we kept taking it out wouldn't we shift the reaction to the right as well?

Thanks.

 

[SaintJude]

Edit: Nevermind, I found the old thread that may help you!


http://forums.studentdoctor.net/showthread.php?t=818690

 

[thais]

Perfect! Thank you so much! 🙂

 

[SaintJude]

But, I'm not sure I understand completely why B is wrong.

Someone wrote :

Choice B is wrong. If the volume of the container could expand, there would be no change in pressure because the kinetic energy supplied by the increase in temperature is being used up to expand the volume of the container and thus cool it down until equilibrium is once again reached. In other words, this change won't shift the reaction in the forward direction (no increase of ammonia)

So the increase in KE is used to expand the volume of the container? How did they know this?

And are you using PV=nRT to determine the associated changes of decrease/increase in pressure or volume? Is it applicable to this problem (I thought PV=nRT is only applicable to ideal gases...)

 

[milski]

Expansion in general is going to push the reaction to the left. The reaction in top post actually is not balanced - it is missing 3 in front of H2.

PV=nRT is applicable for ideal gas only but real gasses don't work in a completely different way. There are small adjustments that have to be made for real experiments but they are just adjustments, not really a change in behavior. Even with real gasses doubling the pressure will roughly half the volume, not decrease it 10 times or just by 2 %.

 

[thais]

Yea the equation wasn't balanced. Sorry guys. It should've been:
N2(g) + 3H2(g) <-> 2NH3 (g)
I guess that makes sense now right SaintJude? Since when volume is expanded the reaction would shift to the side with more gas moles (left) decreasing the yield of NH3.

 

[MedPR]

I didn't read the thread that St Jude linked to, but D is wrong because you would eventually run out of reactants. Removing heat from an exothermic reaction will shift the reaction to the products side, but if you don't add in more reactants the forward reaction must stop eventually.