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Topic: Equilibrium constant temperature dependence  (Read 2079 times)

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Offline Big-Daddy

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Equilibrium constant temperature dependence
« on: July 03, 2013, 03:46:22 PM »
If we have two equilibrium constants [itex]K_1[/itex] and [itex]K_2[/itex], measured at [itex]T_1[/itex] and [itex]T_2[/itex] respectively, for a certain reaction, should there not be a ΔG° relationship between them? If ΔG°=-R*T*ln(K) then it follows that ln(K2/K1)=-(ΔG°/R)*(1/T2-1/T1) as far as I can see (just divide K2 by K1). But then if you try replacing with ΔG°=ΔH°-T·ΔS°, ΔS° falls out and we end up with ln(K2/K1)=-(ΔH°/R)*(1/T2-1/T1). How can both formulae be correct - doesn't that suggest ΔG°=ΔH°, which is not necessarily (almost never) true?

Offline curiouscat

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Re: Equilibrium constant temperature dependence
« Reply #1 on: July 31, 2013, 04:48:15 AM »
If we have two equilibrium constants [itex]K_1[/itex] and [itex]K_2[/itex], measured at [itex]T_1[/itex] and [itex]T_2[/itex] respectively, for a certain reaction, should there not be a ΔG° relationship between them? If ΔG°=-R*T*ln(K) then it follows that ln(K2/K1)=-(ΔG°/R)*(1/T2-1/T1) as far as I can see (just divide K2 by K1). But then if you try replacing with ΔG°=ΔH°-T·ΔS°, ΔS° falls out and we end up with ln(K2/K1)=-(ΔH°/R)*(1/T2-1/T1). How can both formulae be correct - doesn't that suggest ΔG°=ΔH°, which is not necessarily (almost never) true?

You are assuming ΔG° is invariant with Temperature? That'd be a HUGE bug in your derivation right there (I think).

This itself is wrong: ln(K2/K1)=-(ΔG°/R)*(1/T2-1/T1)

Offline Big-Daddy

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Re: Equilibrium constant temperature dependence
« Reply #2 on: August 09, 2013, 09:04:58 PM »
Sorry, big mistake by me. Yeah, I see where I went wrong now. Thanks

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