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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: jimmy2554 on February 11, 2020, 02:18:34 AM

Title: Does ∆G° Change with temperature?
Post by: jimmy2554 on February 11, 2020, 02:18:34 AM
I was always under the assumption that the "naught" symbol, whether it be delta H naught or delta S naught meant that the value was tabulated under standard conditions (i.e. 1 ATM, 293 K, etc). However, after doing a problem that asked for the Keq at a non-standard temperature and getting it wrong, I was told that I needed to calculate a new value of ∆G° using the new temperature. Doesn't that defeat the entire purpose of using the "naught" symbol? Does ∆G° change with temperature?

Thanks!
John
Title: Re: Does ∆G° Change with temperature?
Post by: mjc123 on February 11, 2020, 04:35:22 AM
Yes it does. ΔG° = ΔH° - TΔS°. To a first approximation, ΔH° and ΔS° may usually be treated as constant with temperature, over a not-too-wide temperature range, but ΔG° definitely changes. In an expression like ΔG° = -RTlnKeq, the ° sign means standard conditions at the temperature T, whatever that is.
It is worth noting that if lnK = -ΔG°/RT = -ΔH°/RT + ΔS°/R, then
d(lnK)/dT = ΔH°/RT2
so the variation of K with T depends on ΔH°, not ΔS°.