Exergonic/Exothermic

[paulsterio]

What's the difference between an exergonic reaction and an exothermic reaction.


Say like ATP + H₂O --> ADP + P_i + Energy is Exergonic, but what does that have to do with it being exothermic, do the two describe the same thing?


Also, what is the difference between ΔH and ΔG?

[Russ]

You can consider them pretty equivalent, but the difference is to do with higher level concepts (Gibbs Free Energy, entropy). DeltaG is a measure of the energy of a system etc. and whether it's increasing or decreasing - DeltaH is the same but for heat. Since heat is a form of energy, you can pretend they're the same if it makes it easier to understand.

There are two because a reaction can release X amount of heat but a different amount of energy overall due to interactions with other molecules etc.

[paulsterio]

Oh alright then, I kinda get it, so basically ΔG refers to all the energy which is released whereas ΔH refers specifically to the heat energy?

Thanks Russ

[Russ]

Not exactly, but I don't think the difference is relevant at first year, so thinking of it like that makes sense. tbh i would probably need it explained to me, I'm not a physicist

[paulsterio]

Well, I managed to find this, if anyone is still lost.



496. What is the difference between endergonic reactions and endothermic reactions? (Likewise for exergonic and exothermic) 

Endergonic and exergonic refer to free energy changes (delta G). Endothermic and exothermic refer to the changes in internal energy of molecules, measured as heat given off or taken up, delta H.
Delta g takes into account not only changes in internal enregy but also changes in entropy (delta S) that may accompany a reaction.
For most ordinary simple chemical reactions, the entropy factor is not great, so chemists usually talk about delta H.
For many biological reactions,the entropy factor is significant, so biochemists usually talk about delta G.
Delta G = delta H - (T x delta S ).
Despite the signs, which have to do with the way these terms are defined, delta G is the sum of the 2 effects.



Essentially I think there's this other thing called delta S which is basically the change in Gibbs free energy of a system, and delta H, which is the change in the heat content (enthalpy) of a system. Gibbs free energy measures the so-called "randomness" of a system. And delta G is basically the sum of these two effects. I really don't think the theory behind it is relevant in Biology (it's more physical chemistry related) but hmm, I think that's probably enough to understand the gist of what it's about.

[golden]

Well, I managed to find this, if anyone is still lost.



496. What is the difference between endergonic reactions and endothermic reactions? (Likewise for exergonic and exothermic) 

Endergonic and exergonic refer to free energy changes (delta G). Endothermic and exothermic refer to the changes in internal energy of molecules, measured as heat given off or taken up, delta H.
Delta g takes into account not only changes in internal enregy but also changes in entropy (delta S) that may accompany a reaction.
For most ordinary simple chemical reactions, the entropy factor is not great, so chemists usually talk about delta H.
For many biological reactions,the entropy factor is significant, so biochemists usually talk about delta G.
Delta G = delta H - (T x delta S ).
Despite the signs, which have to do with the way these terms are defined, delta G is the sum of the 2 effects.



Essentially I think there's this other thing called delta S which is basically the change in Gibbs free energy of a system, and delta H, which is the change in the heat content (enthalpy) of a system. Gibbs free energy measures the so-called "randomness" of a system. And delta G is basically the sum of these two effects. I really don't think the theory behind it is relevant in Biology (it's more physical chemistry related) but hmm, I think that's probably enough to understand the gist of what it's about.

Were you referring to entropy rather than Gibbs energy?

[paulsterio]

Whoops, yupp, my bad

[golden]

If Delta G = delta H - (T x delta S ), and delta S is always positive, does that mean there would be a situation where delta H is actually positive but delta G is negative?

[paulsterio]

I'm not sure to be honest However, I don't think it's possible for delta H to be positive whilst delta G is negative, it just doesn't seem to make much sense that you're taking in heat from the environment yet you're releasing energy.

I really don't understand it all that much though. It's probably best posed to someone who's doing Physical Chemistry or Thermodynamics rather than someone who's doing Biology

[moekamo]

yes its possible, think of an ice block melting, heat is absorbed to melt the ice, but entropy increases(liquids in general have higher entropy than solids).
So depending on the temperature it occurs at Delta G could be positive or negative(also depends slightly on the Delta H and Delta S values which I don't know and can't be bothered looking up since you cant really melt an ice block below 0 C).

i dont really have an intuitive understanding of gibbs free energy though, maybe someone else can help with that...