Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started 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 nonstandard 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

Yes it does. ΔG° = ΔH°  TΔS°. To a first approximation, ΔH° and ΔS° may usually be treated as constant with temperature, over a nottoowide temperature range, but ΔG° definitely changes. In an expression like ΔG° = RTlnK_{eq}, 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°/RT^{2}
so the variation of K with T depends on ΔH°, not ΔS°.