Liquid (Propane) escaping from cylinder: endothermic or exothermic?

Question

We had this on our exam. A propane cylinder contains liquid propane and when that escapes it goes into gas state. In normal cases liquid -> gas is endothermic, but this is more complex. The gas would escape and lose temperature (get colder) and the inside of the cylinder would lose pressure, correct? Wouldn't this be more of an equilibrium thing? Is it endothermic or exothermic at all? Please many details and citation so I can argue it to my teacher.

I need more proof to show my teacher. If it is endothermic, where is the energy coming from and where is it going (inside tank, or escaped gas)?

If the propane cylinder was in in space and the gas was released, how could it be endothermic?

Answer 1

This question does raise some interesting points of physics, and the asker is quite right to identify that the situation is rather different to just boiling water, for instance.

The first point to note (even though its obvious when you think about it) is that the Propane is only a liquid because of its pressure. At room temperature and pressure, it would be a gas. So to liquify it, it is compressed. As a gas is compressed, its boiling point generally decreases (I'm not sure if this is universally true, but I can't think of any exceptions!). A graph of presure against temperature, showing the line correponding to the change of state is called a phase diagram (incidently, knowledge of the phase diagram of a substance is vital in chemical engineering and materials processing)

When the cylinder is opened, and the pressure drops, the propane is now in a liquid state, and at atmospheric pressure it is now at a temperature above its boiling point, and so vaporises. But because its temperature is somewhat above its boiling point, the energy can actually come from its own specific heat - so the vapour can be cooler than the liquid was, but is still above its boiling point. This is different to the case where you are boiling water, because in that case everything takes place at room temperature, so you have to constantly provide latent heat to keep the water boiling.

The other main point here is that of equilibrium, and adiabatic vs. isothermal processes. When a state change like this takes place, it can either be adiabatic (exchanges no heat with its surroundings) or isothermal (it stays at the same temperature as its surroundings, taking in or giving out the appropriate quantity of heat).

In the example above, I've described the system as adiabatic , because the vapourisation is going to take place so quickly, that there won't be time for the heat to be exchanged and reaach thermal equilibrium. Since the vapour has cooled (its thermal, specific heat has been converted to latent heat), then if left over time at room temperature, it will take in heat, and so the process can be described as endothermic in that sense.

All of which goes to show how careful one has to be when thinking about energy and heat to define what the boundaries of the system are. And in teh real world, thinsg are in equilibrium far less often than textbooks would have you believe!

Answer 2

The question is not stupid. Answer 2 is only correct if you neglect the state change (e.g., release compressed air from a tank). Even if the propane leaves the tank in liquid form only, there is vaporization inside the tank to make up the volume loss. The overall process is endothermic.
EDIT: I think epidavros is objecting to the loose usage of the word 'endothermic', which strictly means absorbing heat from the surroundings. And it's true an adiabatic process doesn't absorb heat, by definition. However real-world processes are never perfectly adiabatic since they take place in nonzero time and involve imperfect insulation. We assume adiabaticity to simplifiy analysis of heat engines and compressors, but these machines only approach the ideal definition; they give off heat. Also, 'endothermic' is often used to describe a process that lowers the temperature of the material involved without reference to absorption of heat from the surroundings. But since some heat is bound to be absorbed in the described situation, the question can be considered a practical one, if not semantically correct.
About your teacher's questions: Any heat energy absorbed comes from the surroundings. In space, if it's a perfect vacuum and we ignore solar radiation, there is no available heat in the surroundings to absorb; the tank, the remainder of its original contents and the released liquid/gas will all approach a solid state at 0 K as the escaping gas disperses throughout empty space, carrying away the heat as molecular kinetic energy.

Answer 3

Liquid propane under pressure escaping a cylinder would go to gas and to do so would need energy.This phase change is endothermic...liquid to gas.This would continue until cylinder pressure equaled atmospheric pressure.Inside the cylinder the liquid loosing pressure would all finally become gas as well and soak up heat in the process..

Answer 4

Its a stupid question set by someone who does not understand thermodynamics.

If the expansion was adiabatic, the gas would cool on expanding as you say. No energy would be absorbed (thats what adiabatic means).

If the expansion was isothermal, energy wold be absorbed,

Answer 5

it is an endothermic process as latent heat of vaporisation is taken by the molecule of gas.