[nigel433]

"Chemistry", by Moore et al, claims that when ethanol boils IN AIR, on the lab bench,
some WORK is done "because the system expands and pushes back the atmosphere".
This strikes me as complete rubbish, and indeed contrary to Dalton's Law of partial
pressures. Surely, molecules of ethanol merely insinuate themselves among other
molecules in the air? As far as the ethanol is concerned the vapor pressure is essentially
zero all the time. (There is no macro push such as you MIGHT have with a piston.)

And, as far as I am aware, simple expansion of a gas into a vacuum does not change the temperature of the gas at all.

[Borek]

Try to boil the ethanol twice. Once, it is in a balloon that has to expand (let's assume it is perfectly elastic, so no work on stretching is done), second time, there is no balloon. I guess you mean that just the presence of rubber makes physics of both cases completely different?

[nigel433]

Not the "presence" of the rubber, but the spatial configuration of the rubber.
You are talking about making a piston pump which I already mentioned and
distinguished as a situation. Of course it makes a difference, as it segregates
the new gas, and turns an irreversible process into a reversible one.

The issue is not whether there is some energy transfer from the ethanol gas
expanding into the atmosphere, but whether this is properly described as transfer
by WORKING or by HEATING. Since the transfer is by particles striking particles,
seems like heating to me.

The more subtle point is whether it is correct to say of a mixture of gases that
one gas exerts a pressure on the other gas. Dalton is unequivocal. He say NO.

[Borek]

The issue is not whether there is some energy transfer from the ethanol gas
expanding into the atmosphere, but whether this is properly described as transfer
by WORKING or by HEATING. Since the transfer is by particles striking particles,
seems like heating to me.

OK, let's remove the rubber, but put the ethanol at the bottom of a long thin tube. It won't mix with the air above easily, so it will be quite well separated, and obviously it will work as a piston engine - just without a piston.

This is not much different from what is happening with CO₂ evolving during fermentation (or famous CO₂ and soap bubbles experiment), it is known to lie down and not mix easily with the air (as long as there is no forced motion of the air).

There are some more subtle points involving entropy change which can be a driving force of evaporation when there is no rubber, but I don't feel safe on these grounds

[MrTeo]

The issue is not whether there is some energy transfer from the ethanol gas
expanding into the atmosphere, but whether this is properly described as transfer
by WORKING or by HEATING. Since the transfer is by particles striking particles,
seems like heating to me.

Ok, let's analyze the situation using the 1st law of thermodynamics. An amount of heat Q is given (Q>0) to the gas and we know that ethanol doesn't change its temperature because it's boiling, so we can write:




but as ∆T=0 we get:



So there's a positive work done by the gas on the environment (a heating would increase U).

[nigel433]

This has certainly aroused some interesting points.

To Mister Borek

By the terms of the experiment, the delta U becomes undefined as the pressure
of the ethanol tends to zero and the volume tends to infinity (i.e. the vapour
dilutes indefinitely).

To Mr Teo

You are defining a different experimental condition. In any case, the ethanol WILL
eventually mix and then the initial work (by macroscopic push) will become heat
(by microscopic push).