Gibb's free energy

umdnjmed

10+ Year Member
5+ Year Member
I find the idea of an exergonic, endothermic reaction quite perplexing, even though I know it is possible, sine delta S is also considered when calculating delta G (hence the reaction ultimately ending up as exergonic) If a reaction takes up heat (a form of energy) as an endothermic reaction then has to release energy as an exergonic reaction, why did it take up the heat in the first place? was the heat used as activation energy?

kasho11

Activation energy, kinetics, is separate from thermodynamics, gibb's free energy.

I can't think of any specific reactions but going from a more ordered system to a more disordered system would imply an input of heat to give the molecules enough KE to spread out and the resulting increase in entropy would be significant enough to be an overall spontaneous reaction.

chiddler

5+ Year Member
I find the idea of an exergonic, endothermic reaction quite perplexing, even though I know it is possible, sine delta S is also considered when calculating delta G (hence the reaction ultimately ending up as exergonic) If a reaction takes up heat (a form of energy) as an endothermic reaction then has to release energy as an exergonic reaction, why did it take up the heat in the first place? was the heat used as activation energy?
exergonic means it has a negative free energy change, NOT release of heat.

for example,

&#916;G = -100 (spontaneous, exergonic)
&#916;H = +100 (endothermic, absorbs heat!)
and &#916;S = +100

&#916;G = &#916;H - T&#916;S
-100 = (100) - T(100)

=100 - 100T

Know where i'm going with this? Temperature must be large in order for the reaction to be exergonic. Exergonic is not the same as exothermic.

so i'm not sure if you just typo'd or if you don't know this. ask if you need.

OP
U

umdnjmed

10+ Year Member
5+ Year Member
Activation energy, kinetics, is separate from thermodynamics, gibb's free energy.

I can't think of any specific reactions but going from a more ordered system to a more disordered system would imply an input of heat to give the molecules enough KE to spread out and the resulting increase in entropy would be significant enough to be an overall spontaneous reaction.
I couldn't have asked for a more direct answer to my question. Thank you.

OP
U

umdnjmed

10+ Year Member
5+ Year Member
exergonic means it has a negative free energy change, NOT release of heat.

for example,

&#916;G = -100 (spontaneous, exergonic)
&#916;H = +100 (endothermic, absorbs heat!)
and &#916;S = +100

&#916;G = &#916;H - T&#916;S
-100 = (100) - T(100)
=100 - 100T

Know where i'm going with this? Temperature must be large in order for the reaction to be exergonic. Exergonic is not the same as exothermic.

so i'm not sure if you just typo'd or if you don't know this. ask if you
need.
I reread the question but I don't see any place where I referred to an exergonic reaction as a release of heat.

chiddler

5+ Year Member
"then has to release energy as an exergonic reaction"

eh. nevermind i guess..

milski

1K member
5+ Year Member
Ice melting is a good example of exergonic, endothermic "reaction." It is also a fairly clear case of how change of temperature will change the sign of &#916;G and make it spontaneous or not.