What is the real explanation for entropy and its role in chemical reactions?

[Senex01]

The heat absorbed by a system as you go from one thermodynamic state to anoyther, depends on the path you take. There is no function Q such that the difference of Q between the two states will give you the heat.

Does that mean that there is no such thing as enthalpy? in terms of the heat given off by a reaction?

 

[Senex01]

That's how Carnot did it. He showed that the entropy change between a final and intital state does not depend on which path you take.

I thought you just said it did depend on the path you took?

 

[Count Iblis]

Does that mean that there is no such thing as enthalpy? in terms of the heat given off by a reaction?

Enthalpy change equals the absorbed heat when the pressure is kept constant. So, while enthalpy change is path independent, it will only give you the absorbed heat for specific paths.

 

[Count Iblis]

I thought you just said it did depend on the path you took?

Heat is path dependent, entropy and enthalpy are path independent.

 

[Senex01]

OK. Thank you. I think I am getting it.

If it's okay, I want to keep going though.

So the entropy ( = heat change / temperature (q / T ) ) is identical from one state to another. But the heat change itself could vary according to particular cases, according to the path they take from one state to another.

In what way would one path differ from another? You mentioned pressure, that sounds like a pretty relevant difference, which would affect the heat gain/loss.

I assume if a sequence of reactions, some exothermic some endothermic, would count as a different path if the sequence varied, but at constant pressure the net heat change would be identical from the initial state to the final state. Right? The same for the rapidity of the change. These scenarios are actually both very common in cellular systems, as you know.

What else would constitute a different path?

 

[Senex01]

Actually I suppose the answer to my last question is pretty obvious. The way one path will have a different result to another path will be if it happens at a different temperature. (Duh!). Obviously differing pressures will affect temperature.

Obviously, chemical reactions in a living cell will generally happen under conditions of constant pressure and temperature. Not sure what the effect of different catalysts would be, but I would guess that wouldn't make a difference to the heat loss/gain.

 

[Senex01]

So Gibbs energy of a reaction is the energy that the reaction releases at a certain temperature: that energy is available as work.

dG = dH - TdS

where dH is the energy released by the reaction
TdS is a term which shows the energy that will not be released by the system at this temperature.

 

[Senex01]

No-one is obliged to ever respond, especially to a stroppy old bugger like me. But I'm assuming that the posts nobody has come back to me on, are probably not too far wrong: like the one about entropy being the inverse of the sum/average energy differences of parts of the system. Actually, looking at the gibbs energy equation, I'm not sure about that one now.

Well, I am genuinely grateful for everyone who has responded so far Count i, Mapes, atyy and all of you.