# Compression of gases that liquefies

• I

## Main Question or Discussion Point

For example we had a closed system with water at 298K, so it's pressure is 0.031atm. To it we add a gas, A such that we get 1atm of total pressure. That gas undergoes a phase transition at 1.5atm and 298K. So initially, the partial pressure of water is a little smaller than 0.031atm due to the dissolution of some of that gas, and the rest of the pressure is from that gas.

As we compress this mixture of gases with 2atm of pressure what would happen? How would the A and water's partial pressure change?

Since A is able to increase its partial pressure further before liquefying, I feel that it should increase to 1.5atm. But that results in a decrease in the water's partial pressure because it's mole fraction in the liquid continues to drop further. So the total pressure will be 1.5atm from A, and less than 0.031atm from water. But then since we're applying 2atm of pressure, how does that equal out?

Something like the uploaded image but instead of air we replace it with a different gas.

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sophiecentaur
Gold Member
Are you actually describing a stable final situation here? The final pressure in the cylinder has to be 2Atm .

• russ_watters
Chestermiller
Mentor
Are you saying that pure A would undergo a phase transition at 1.5 atm and 298K?

Are you saying that pure A would undergo a phase transition at 1.5 atm and 298K?
Yes so in a phase diagram for A it would transition at 1.5atm. Does that matter if it's a mixture?
Are you actually describing a stable final situation here? The final pressure in the cylinder has to be 2Atm .
I get that the final internal pressure has to be 2atm. But I don't see how that can be achieved with the parameters I've set because A would liquefy once it's pressure is at 1.5atm, and the water would liquefy at pressure less than 0.031atm ad well.

Chestermiller
Mentor
Yes so in a phase diagram for A it would transition at 1.5atm. Does that matter if it's a mixture?

I get that the final internal pressure has to be 2atm. But I don't see how that can be achieved with the parameters I've set because A would liquefy once it's pressure is at 1.5atm, and the water would liquefy at pressure less than 0.031atm ad well.
Are the liquid phases of A and water miscible?

Are the liquid phases of A and water miscible?
Yes I would assume so for this scenario. So I guess I'd use Raoult's law for water and Henry's law for A to calculate their partial pressures at each point? So initially, when I first introduced A to the pure water system, how would I know how much it dissolves? We would just know that the total P is 1atm and initial P is 0.031atm for pure water vapour only.

russ_watters
Mentor
For example we had a closed system with water at 298K, so it's pressure is 0.031atm. To it we add a gas, A such that we get 1atm of total pressure. That gas undergoes a phase transition at 1.5atm and 298K. So initially, the partial pressure of water is a little smaller than 0.031atm due to the dissolution of some of that gas, and the rest of the pressure is from that gas.

As we compress this mixture of gases with 2atm of pressure what would happen? How would the A and water's partial pressure change?

Since A is able to increase its partial pressure further before liquefying, I feel that it should increase to 1.5atm. But that results in a decrease in the water's partial pressure because it's mole fraction in the liquid continues to drop further. So the total pressure will be 1.5atm from A, and less than 0.031atm from water. But then since we're applying 2atm of pressure, how does that equal out?

Something like the uploaded image but instead of air we replace it with a different gas.
It really sounds to me like you are just arguing with your own constraints. If the final pressure is 2atm because you specified it to be 2atm, then the final pressure is 2atm.

Perhaps what you really mean to specify (or find?) is a compression ratio...? I see your title says "compression", but then seemed to misuse the word in the description of the problem. Compression and pressurization are different things: compression is decreasing the volume by a specified amount whereas pressurization is increasing the pressure to a specified quantity.

Also, you didn't specify what happens, if anything, to the energy of compression. Should we assume adiabatic or isothermal?

Chestermiller
Mentor
Yes I would assume so for this scenario. So I guess I'd use Raoult's law for water and Henry's law for A to calculate their partial pressures at each point? So initially, when I first introduced A to the pure water system, how would I know how much it dissolves? We would just know that the total P is 1atm and initial P is 0.031atm for pure water vapour only.
You are assuming that the gas phase is an ideal gas mixture. That's fine.

But, for the liquid phase, if you use Raoult's law, you are assuming an ideal liquid mixture (that applies to both species), and, if you assume A obeys Henry's law for the liquid, you are assuming a non-ideal liquid mixture model with a known activity coefficient. What liquid phase mixture model are you assuming (a) Raoult's law, (b) Henry's law, or (c) a different non-ideal fluid mixture model?

It really sounds to me like you are just arguing with your own constraints. If the final pressure is 2atm because you specified it to be 2atm, then the final pressure is 2atm.

Perhaps what you really mean to specify (or find?) is a compression ratio...? I see your title says "compression", but then seemed to misuse the word in the description of the problem. Compression and pressurization are different things: compression is decreasing the volume by a specified amount whereas pressurization is increasing the pressure to a specified quantity.

Also, you didn't specify what happens, if anything, to the energy of compression. Should we assume adiabatic or isothermal?
Sorry for the wrong terminology used, I meant that the mixture is compressed with 2atm of pressure. But since gas A undergoes a phase transition at 1.5atm, and water undergoes a phase transition at 0.031atm in their pure states how can their gaseous pressures ever be equal to 2atm?

Edit: it should be a isothermal process where temperature stays at 298K

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You are assuming that the gas phase is an ideal gas mixture. That's fine.

But, for the liquid phase, if you use Raoult's law, you are assuming an ideal liquid mixture (that applies to both species), and, if you assume A obeys Henry's law for the liquid, you are assuming a non-ideal liquid mixture model with a known activity coefficient. What liquid phase mixture model are you assuming (a) Raoult's law, (b) Henry's law, or (c) a different non-ideal fluid mixture model?
I was taught that an ideal dilute mixture would have the solvent follow Raoult's law, and the solute follow Henry's law. So I presumed that A being the added gas should follow Henry's law.

But even if I were to use assume a Raoult's law for both, when I first introduced the gas water's mole fraction in the liquid phase should decrease so the pressure of 0.031atm should decrease. But I don't really know how to tell how much it decreases by and likewise for A, I'm unsure how much of it dissolves as well. So I know that P(H2O(g))<0.031atm and P(A(g))>0.969atm but is there a way to know for sure their actual pressures?

And even if we do know it, when we compress this mixture with 2atm of pressure how would the system change with this pressure? I presumed that A could continue to increase in pressure up til it hit 1.5atm, because that's it's vapor pressure at 298K. But for water it's vapour pressure is 0.031atm at 298K, but since we increased the pressure of A to 1.5atm, by Raoult's law, it's pressure should decrease even more because more A would have dissolved right? I'm quite clueless as to how the system changes when such a scenario happens to be honest.

Chestermiller
Mentor
Sorry for the wrong terminology used, I meant that the mixture is compressed with 2atm of pressure. But since gas A undergoes a phase transition at 1.5atm, and water undergoes a phase transition at 0.031atm in their pure states how can their gaseous pressures ever be equal to 2atm?

Edit: it should be a isothermal process where temperature stays at 298K
Well, what does Raoult's law predict for the mole fractions of water and A in the liquid at 1 atm pressure?

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sophiecentaur
Gold Member
how can their gaseous pressures ever be equal to 2atm?
Because you have said it is in your initial description. Something may need to change state or dissolve . . . . . It's not an ideal situation.

Well, what does Raoult's law predict for the mole fractions of water and A in the liquid at 1 atm pressure?
P(H2O(g))=P*(H2O(g))X(H2O) and P(A)=P*(A)X(A), but we only know the values of P*(H2O) and P*(A) so I don't get how we can predict their mole fractions though.

Well, what does Raoult's law predict for the mole fractions of water and A in the liquid at 1 atm pressure?
Oh, wait we can tell it from the graph right? So we would know that the exact pressures of water and A at a total pressure of 1 atm? So it will look something like this right? So we can get the mole fractions of either component?

But continuing on, when we compress the gas with 2atm of pressure the mole fraction of water goes to 0? I don't really understand how the process continues on from here though.

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Chestermiller
Mentor
We first have to specify how much water and how much A are present total. So, please specify. That way we can determine how much of each species is present in each phase.

We first have to specify how much water and how much A are present total. So, please specify. That way we can determine how much of each species is present in each phase.
Let's say we have 20 moles of H2O in the liquid phase and 5 moles of A in the liquid phase initially. And from the graph, the partial pressure of water would be 0.0248atm and A would be 0.9752atm. Would this be alright?

russ_watters
Mentor
Sorry for the wrong terminology used, I meant that the mixture is compressed with 2atm of pressure.
That's still the wrong terminology. Again, 2atm is a pressure, not a compression amount.
But since gas A undergoes a phase transition at 1.5atm, and water undergoes a phase transition at 0.031atm in their pure states how can their gaseous pressures ever be equal to 2atm?
They can't, but you could have all compressed liquid.

Chestermiller
Mentor
Let's say we have 20 moles of H2O in the liquid phase and 5 moles of A in the liquid phase initially. And from the graph, the partial pressure of water would be 0.0248atm and A would be 0.9752atm. Would this be alright?
No. The partial pressure of A would be (0.2)(1.5)=0.3 atm, and the total pressure would be 0.3248 atm. What number of moles of A would you need in the liquid for the total pressure to be 1 atm? Also, you would need to specify the total number of moles of vapor at 1 atm if we are going to compress the system and quantify what's happening next.

Let me guess. You've been studying multicomponent phase equilibrium, but you haven't actually worked any practice problems yet.

No. The partial pressure of A would be (0.2)(1.5)=0.3 atm, and the total pressure would be 0.3248 atm. What number of moles of A would you need in the liquid for the total pressure to be 1 atm? Also, you would need to specify the total number of moles of vapor at 1 atm if we are going to compress the system and quantify what's happening next.

Let me guess. You've been studying multicomponent phase equilibrium, but you haven't actually worked any practice problems yet.
Haven't even gone there yet, we just got introduced to the topic of mixing but I've been thinking a lot to make sure I understood what's going on in the lectures. Sometimes I feel like I overthink too much but I'm just unsure if what I'm overthinking will be looked at further in the module or no, which leads me to these types of situations.

But I'd need 37 moles of A to get 1 atm total pressure. 37/57.14x1.5=0.9752atm.

That's still the wrong terminology. Again, 2atm is a pressure, not a compression amount.

They can't, but you could have all compressed liquid.
Hmm, so everything would just liquefy?

Hmm, so everything would just liquefy?
Yes.
An actual close example might be neopentane.
Boils at 101 kPa at 9,5 Celsius. Vapour pressure at 20 Celsius is 146 kPa.
So if you compress a cylinder of neopentane at isothermal conditions (compression heat freely conducted away) of just over 20 Celsius, it would condense at 1,5 bar till all of it is liquid, and then you can compress the liquid to 2 bar.
Neopentane is a low polarity alkane, so not miscible with water, though.

Chestermiller
Mentor
Haven't even gone there yet, we just got introduced to the topic of mixing but I've been thinking a lot to make sure I understood what's going on in the lectures. Sometimes I feel like I overthink too much but I'm just unsure if what I'm overthinking will be looked at further in the module or no, which leads me to these types of situations.

But I'd need 37 moles of A to get 1 atm total pressure. 37/57.14x1.5=0.9752atm.
37 moles of A is incorrect. I calculate 38.76 moles of A. Please re-analyze the problem and show how you can get the right answer.

Chestermiller
Mentor
Hmm, so everything would just liquefy?
That's what we are going to prove by doing some calculations. But, first you need to show that you can get the right answer for the case of 1 atm pressure.

37 moles of A is incorrect. I calculate 38.76 moles of A. Please re-analyze the problem and show how you can get the right answer.
If we had 20 moles of water, (0.031)(20/20+b)+(1.5)(b/20+b)=1atm so solving for B gives us 38.76 moles. Would be this correct?

Chestermiller
Mentor
If we had 20 moles of water, (0.031)(20/20+b)+(1.5)(b/20+b)=1atm so solving for B gives us 38.76 moles. Would be this correct?
Excellent. Now you're starting to understand.

So the total number of moles of liquid you have is 58.76. Now, to do the calculation we wish to carry out, you are going to need to specify the number of moles of vapor present in the cylinder at 1 atm total pressure. This is because, when we compress the system, the total number of moles of liquid plus vapor present in the system doesn't change. So please specify the number of moles of vapor present at 1 atm. Then, please determine (from Raoult's law) the number of moles of water and of species A present in the vapor at 1 atm. Then determine the total number of moles of water and the total number of moles of species A present in the cylinder (vapor plus liquid).