Difference between K and K sub c?


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Chem_Mod
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Difference between K and K sub c?

Postby Chem_Mod » Sat Oct 29, 2011 6:50 pm

In regards to equilibrium constants, what is the difference between K and K sub c? In addition why do we have to use K for the Van't Hoff equation?

Chem_Mod
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Re: Difference between K and K sub c?

Postby Chem_Mod » Sat Oct 29, 2011 6:51 pm

K can mean many things. However, in equations where we take a logarithm of K, it must be unitless. Otherwise it would be meaningless, as a logarithm is just an inverse of an exponentiation. So K is actually expressed in the unitless activities of species. Still, activity is not that useful a concept in the lab, where we work in concentrations. So the thing is, if you want to use something like the van't Hoff equation, you need to be in the unitless K, which really is the only K that makes sense given the form of the equation. But obviously the same basic concepts of K are the same for Kc, which you would expect as activity is closely related to concentration.

Rachel Lin 2D
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Re: Difference between K and K sub c?

Postby Rachel Lin 2D » Sat Jan 30, 2016 2:32 pm

So it would be acceptable at times to use K sub c in the Van Hoff equation? When do you know when to use which?

Erin Murashige 1L
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Joined: Fri Sep 25, 2015 3:00 am

Re: Difference between K and K sub c?

Postby Erin Murashige 1L » Sun Jan 31, 2016 2:46 pm

Yes I think you can use Kc in the Van't Hoff equation. In most cases, just K will suffice but if the problem deals with concentration then Kc can be used. Hope this kind of helps.

Cindy Chen_2I
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Re: Difference between K and K sub c?

Postby Cindy Chen_2I » Sun Jan 31, 2016 10:22 pm

Then why does the textbook specifically stated in section 11.11 (Not entirely sure this is the correct section because I have the fifth edition) that we must use K, not Kc for the van't Hoff equation?


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