When volume is changed in a equilibria reaction, why does the Kc value remain unchanged? D:

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I thought the Kc value would change because the equilibrium position changes but according to a question in the June 2011 exam, it stays the same? D:

Posted Sat 29th December, 2012 @ 16:33 by Kayleigh Nicholson

6 Answers

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By increasing just the overall volume, then the concentrations of each reagent in the reaction increases by the same amount (because concentration = moles/volume). Since Kc depends on the concentrations of the reagents, if they all increase by the same amount, there will be no change. Increasing the total volume does not necessarily alter the position of equilibirum as well. If you increased the volume (and hence decrease the concentration) of just one reactant (or product) then the position of equilibirum will shift to opose this change. However, if we say have the equilibrium A + B <-> C + D and we increase the concentration of A so that the position of equilibrium shifts to the right. The effect of this is to increase the concentration of C and D, so that shifts the position of equilibrium back to the left (and to the orriginal position). The position of equilibrium only shifts for a small amount of time before returning back. If we want to permanently shift the position of equilibrium, you would have to continually add more of A to maintain a higher concentration. Hope that helps, good luck with your exam :)

Answered Sat 29th December, 2012 @ 17:37 by Ross Edwards
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Kc isn't affect by anything other than temperature because Kc depends on whether the reaction gives out or takes in heat energy.
I don't really understand it myself, so I've just learnt that it's only affected by temp.
Sorry I couldn't be more help. What exam board are you doing?

Answered Sat 29th December, 2012 @ 16:48 by Molly
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ahhhh, I see :p I think I'll probaby just remember that then, thank you :)

AQA, what about you? :)

Answered Sat 29th December, 2012 @ 17:23 by Kayleigh Nicholson
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It's alright :)
Edexcel. I'm retaking AS aswell :S

Answered Sat 29th December, 2012 @ 17:28 by Molly
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ooo, is it quite similar to AQA? I have a revision book for chem but it's for all exam boards, so I sometimes see the parts that're ony edexcel :p

Same! And Biology AS :') What did you get last year?

Answered Sat 29th December, 2012 @ 17:36 by Kayleigh Nicholson
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I'm not sure, I think it has similar topics but in different exams. I got a D last year. Hopefully I can get a B to get into Uni :) You?

Answered Sat 29th December, 2012 @ 19:55 by Molly