Gas Expansion/Compression v. Endo/Exothermic

[justadream]

Gas Expansion/Compression



Can someone reconcile these statements?



1) Gas expansion is endothermic. Gas compression is exothermic (these are straight from TBR)

2) Gas loses energy as it expands (and raises the piston in PV work). Thus, gas gains energy as it is compressed (these are from TPR).

---

Members don't see this ad.

 

[dnrs]

The difference between the two exists because in the first case, the system is the surroundings, whereas in the second case the system is the gas itself.

 

[diroten]

So you mean that gas expansion is endothermic to the surroundings, and exothermic wrt the actual system b/c the gas releases heat to the system which absorbs it? Isn't that kind of vague then?

 

[AwayFromReality]

The way I think of it is this way

When going from liquid to gas we are adding heat (boiling). At a molecular level molecules in the liquid level are getting farther apart from each other (expanding) thus liquid to gas is expansion which is endothermic. Same principle applies in the gas phase. If you go from compressed gas to expanded gas the process would be endothermic. The reverse is true for exothermic

In terms of the piston system, I think they might be talking about Potential Energy in that case. When the piston is lowered volume decreases and pressure is increased. The molecules of gas are close together and they don't want to be thus the gas has a higher potential energy. When the piston is raised, volume is increased and pressure decreased. The gas molecules have more space to get away from each which they like therefore potential energy of the gas is decreased.

The enthalpy (delta H) for compressed/expanded gas is different than its potential energy.

Side note: If you remember the carnot heat pump/engine process they are basically converting gas molecules into liquid molecules by compressing the gas therefore releasing heat and vice versa. If the gas is compressed to a point where it is still gas (and not liquid) a lower amount of heat is released (because exothermic) but the potential of gas turning into liquid is very high.

 

[davjan]

1)
pv=nrt; volume directly proportional to temperature; if you want to increase volume of gas, then you need to increase the temperature of the gas(system) - increasing temperature of the system(energy) = your gas sucking in temerature = is Endothermic.

pv=nrt; relationship b/w volume and temp is still the same (directly proportional). so, you decrease volume by decreasing temperature which is negative change in temperature = negative change in energy = pulling out energy from the gas system = your gas spitting out energy/temperature = exothermic

2)
when your gas do work on the environment (expanding by change in gas volume against environmental pressure), you are expending system's energy which goes into the env. So, gas expands - gas spends energy onto the atmosphere

when the atmosphere presses against the balloon, and the balloon contracts (-△v) then the atmosphere spent its energy into the gas system and the gas system is sucking in that spent energy = gas system gained energy(work)