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Midterm 2007 Q3B- Explanation of (delta)S total

Posted: Sat Feb 07, 2015 9:08 pm
by Amy Luong 1L
Would someone mind explaining why "delta S knot" is not a criterion for assessing the spontaneity of a reaction but the "delta S total rxn" shows that a reaction is spontaneous when it is greater than 0?

Re: Midterm 2007 Q3B

Posted: Sat Feb 07, 2015 9:58 pm
by Satvir Saggi 1I
According to the second law of thermodynamics, "A spontaneous change is accompanied by an increase in the total entropy of the system and its surroundings." Therefore, we can only determine if a reaction is spontaneous based off the total change in entropy.

Re: Midterm 2007 Q3B- Explanation of (delta)S total

Posted: Sun Feb 08, 2015 4:32 pm
by Chem_Mod
Satvir is correct. To be spontaneous, TOTAL entropy must be positive. If we don't want to care about the surroundings, then we must use deltaG. If deltaG at constant T and P of the system is negative, then it is also spontaneous.

Showing the equivalence of spontaneity conditions:






Re: Midterm 2007 Q3B- Explanation of (delta)S total

Posted: Wed Feb 11, 2015 1:47 pm
by Justin Le 2I
since deltaG = deltaH - TdeltaS, we can't determine if a reaction is spontaneous if we are simply given deltaS. We need more information to see if deltaG is less than 0