[kæmpekanon2017]

Hey

I have a few specific questions pertaining to enthalpy and Gibbs free energy.

1) Most texts I've read reach the conclusion that ΔH = q under the assumptions that a) pressure is constant and b) the only work performed by the system is expansion work. I am pretty sure I can follow the reasoning:

H = E + PV = q + w + PV
and
ΔH = ΔE + Δ(PV) = q + w Δ(PV)

Constant pressure is assumed, so

ΔH = q + w + pΔV

It is assumed that the only work exerted by the system is expansion work, so

ΔH = q -pΔV + pΔV

The sign is because work performed by the system is an output.

Thus,

ΔH = q

Now, my professer arrived at a different expression (and was altogether pretty rough in his explanation). He concluded that

ΔH = q + w'

where w' denotes useful work (or so I assume; my professer used a Danish word with the same meaning).

He largely left the above expressions out, and instead introduced w as



(this is the same as -w = ΔPV, right?)

Again, I assume the sign is due to the work being performed by the system, not on the system.

Then constant pressure is assumed, and then w' (useful work) is defined

-w' = pΔV

... or expansion work like above, I assume.

Then he concludes that ΔH = q + w'.

So why does work persist as a part of the expression? Shouldn't it cancel out with the original pΔV like in the other texts I've read?

2) So, useful work. I ran into it in 1), and when I look it/Gibbs energy up, the gist seems to be that a) useful work is the amount of non-expansion work performed by a system (which isn't consistent with the expressions in 1); my professor seems to define useful work w' as expansion work (i.e. pΔV) - how to make sense of this?) and b) Gibbs energy is the maximum value of useful work a system can perform.

So what exactly is useful work? Is it a part of the enthalpy definition in addition to being something that Gibbs energy describes?

3) I have a hard time understanding changes in Gibbs free energy intuitively. I understand that the sign of ΔG indicates the spontaneity/non-spontaneity of a process (and I've run into some pretty hand-wavey explanations of G itself that kind of make sense). If ΔG is negative for a chemical process, e.g. respiration/the combustion of glucose, -ΔG is the maximum amount of useful work it is possible to extract from the process (creating ATP for instance), yes? But why does the increase of entropy in the system influence the amount of maximum work exerted on the surroundings (ΔG = ΔH - TΔS)? I can't get it to make sense intuitively.

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Any help is appreciated. My native language isn't English, so if there are any physics/chemistry terms that don't make sense, let me know.