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Author Topic: Quick Qemistry Questions  (Read 375 times)

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Offline TorterrableTopic starter

Quick Qemistry Questions
« on: January 29, 2014, 06:02:26 PM »
Having three "Q"s in a row was too tempting.

I have some conceptual questions that I've been struggling with, and I'd love it if anyone could help me.

So, we have reaction quotient Q. It is affected by pressure, concentration of reactants and products, and volume.

How do volume and pressure affect Q given a chemical equation A+B<->C where A, B, and C are gasses?

Offline Kythia

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Re: Quick Qemistry Questions
« Reply #1 on: January 29, 2014, 07:01:53 PM »
You've almost answered your own questions, to be honest.  Kinda looks like you might understand it but just be a little uncertain? 

We have 1 mole of A, 1 mole of B initially.  Let's stop splitting "volume" and "pressure" into two things and view them as flip sides of the same coin - by keeping the amount of each initial gas the same V and P are aspects of the same issue, right?  V is inversely proportional to P.

So.  What happens to the mixture when V is increased and P decreased or vice versa? In what my old chemistry teacher used to call "billiard ball terms", if that makes sense. 

Offline TorterrableTopic starter

Re: Quick Qemistry Questions
« Reply #2 on: January 29, 2014, 07:50:44 PM »
The main problem is, in an explanation on a quiz or a test, I would have to relate a change in volume or pressure to Q or K (at least I think I would) and then use that as an explanation. Just saying "Le Chatelier's principle" and explaining what it is is not enough. I know what happens to K when heat is added, and I know the effect on Q when concentration is changed, but pressure and volume don't seem to directly affect either of those variables.

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Re: Quick Qemistry Questions
« Reply #3 on: January 29, 2014, 08:03:40 PM »
Stick with me.  V and P both directly alter both of those variables.

Think, as I say, about the microscopic effects of a change in V or P.  What is heat on a microscopic level?  What do you actually mean when you say "concentration".

In essence, an increase in V causes the gas(es) to expand.  Which, as you know, is identical to a decrease in temperature, all else being equal.  So the equilibrium constant shifts. An increase in P is functionally identical to an increase in volume concentration for the purposes of this question - albeit an equal one for both reagents - which moves the reaction quotient.

Offline TorterrableTopic starter

Re: Quick Qemistry Questions
« Reply #4 on: January 29, 2014, 08:13:01 PM »
Ah, I think I get it now. Are there any equations that can help prove this, or is this just accepted as fact?

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Re: Quick Qemistry Questions
« Reply #5 on: January 29, 2014, 08:42:51 PM »
Well, the temperature aspects are just an extrapolation from the Ideal Gas Laws while the concentration aspects are kinda true by definition if you think about what is meant by "concentration" - or, rather, think about why the concentration of a reagent affects Q or K.

It's just thinking about what an increase or decrease in V and P actual means, in microscopic terms, and what Q actually stands for/represents.  Anything that affects the number of interactions between a molecule of A and a molecule of B is going to affect Q.

Offline TorterrableTopic starter

Re: Quick Qemistry Questions
« Reply #6 on: January 29, 2014, 09:46:36 PM »
Thank you very much for your help; I understand now, I think.

I have one more question that just popped up; I figure I could just direct it to you since you seem to know. Under what circumstances can you use successive approximation? The professors merely say that it is "when you can ignore the x" due to the fact that it's an insignificantly small value, but what are some examples where this may occur?

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Re: Quick Qemistry Questions
« Reply #7 on: January 29, 2014, 09:49:36 PM »
Ah, now you've got me I'm afraid.  Not a term I've come across before.  I think I have my notes from A-level chemistry still so I can have a look and see if anything shows up, but off the top of my head I don't have a Scooby.  Sorry.

Offline TorterrableTopic starter

Re: Quick Qemistry Questions
« Reply #8 on: January 29, 2014, 11:12:23 PM »
It's okay; don't go to any trouble on my account. Honestly, I should probably just watch the recorded lectures, but since you were knowledgeable on the subject, I thought I'd simply ask first.

Thank you.