ideal/real gas

[chiddler]

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A U shaped tube is closed at one end so that a noble gas is trapped and compressed by some mercury that is poured from the open side. Standard setup. Question is what if a molecular gas instead of an atomic gas is used?

Answer: Because molecules are larger than atoms, they are less compressible. So for a given mercury amount, then the molecular gas will have more volume than the noble gas.

Why are they less compressible? I would have thought that after a threshold, intermolecular forces start becoming strong enough to reduce the volume more than a noble gas.

I remember reading that pressure of a real gas is less than an ideal gas and I understand this. But I also remember reading that high pressure and low temp create the least ideal gases because they facilitate intermolecular forces. So when do molecular attractions reduce volume more than the massless nature of an ideal gas?

 

[MedPR]

A U shaped tube is closed at one end so that a noble gas is trapped and compressed by some mercury that is poured from the open side. Standard setup. Question is what if a molecular gas instead of an atomic gas is used?

Answer: Because molecules are larger than atoms, they are less compressible. So for a given mercury amount, then the molecular gas will have more volume than the noble gas.

Why are they less compressible? I would have thought that after a threshold, intermolecular forces start becoming strong enough to reduce the volume more than a noble gas.

I remember reading that pressure of a real gas is less than an ideal gas and I understand this. But I also remember reading that high pressure and low temp create the least ideal gases because they facilitate intermolecular forces. So when do molecular attractions reduce volume more than the massless nature of an ideal gas?

Interesting question. I'm not sure if I'm right, but this is what I think.

Imagine putting 1mol H2O vapor in a balloon, and 1 mol H2 vapor in another balloon. At first you can squeeze both balloons pretty easily, since there is a lot of empty volume. However, since the H2O particles are much larger than the H2 particles, at some point the H2O(g) balloon will stop compressing, while the H2 balloon will continue to compress because the H2 particles are smaller, and thus can be more compacted. H2 particles can squeeze into little areas of open space more easily than H2O particles can.

Or, maybe this is easier to grasp (it is for me 🙂)

Take 50 peanut M&Ms and 50 plan M&Ms. The peanut ones are the molecular gas, and the plain are the atomic gas.

Smash them into a ball. Which ball is smaller?

 

[chiddler]

good examples, i think you're right. i think it would be easier to just think of it in static terms rather than dynamic. volume is larger period. the only time compression is significant enough is when it is near its critical point.

thanks.

still accepting criticism if anybody wants to comment.

 

[TXKnight]

A U shaped tube is closed at one end so that a noble gas is trapped and compressed by some mercury that is poured from the open side. Standard setup. Question is what if a molecular gas instead of an atomic gas is used?

Answer: Because molecules are larger than atoms, they are less compressible. So for a given mercury amount, then the molecular gas will have more volume than the noble gas.

Why are they less compressible? I would have thought that after a threshold, intermolecular forces start becoming strong enough to reduce the volume more than a noble gas.

I remember reading that pressure of a real gas is less than an ideal gas and I understand this. But I also remember reading that high pressure and low temp create the least ideal gases because they facilitate intermolecular forces. So when do molecular attractions reduce volume more than the massless nature of an ideal gas?

I think MedPR is right. imagine you have a box with large marbles and one with tiny marbles, as you make the walls of that box smaller (smaller V) there's a point where large marbles are all compressed and their "repulsion" or volume is prevents further compression, whereas the small marbles still have room to compress.
I think is convenient if you approach a problem with either the "real gas" or "ideal gas" approach, but not mix them together. It keeps things simple and less confusing. In this experiment it obviously talks about a "real life" experiment, thus, sticking to the "real gas" framework is adecuate.
Just as a note, rememeber that at STP all gases have the same volume...just thought i would throw that in there...🙄

 

[MedPR]

I never did completely understand the STP rule about volume. Is that for ideal gases only? 1mol of any gas at STP = 22.4L right?

How is that possible

 

[TXKnight]

I never did completely understand the STP rule about volume. Is that for ideal gases only? 1mol of any gas at STP = 22.4L right?

How is that possible

yes 1 mol is 22.4L at STP
I think is for real gases too (or so close, that is taken as that for our purposes) I'm not too sure but i think the reason why is that at that T,P one mol of any gas has the same average KE and larger molecules have more momentum than lighter ones but lighter ones more speed thus they collide more (or compensate for the weight with speed) or something like that. the point is that the average momentum is the same, thus creating the same volume when they collide with the walls of a container.

 

[MedPR]

yes 1 mol is 22.4L at STP
I think is for real gases too (or so close, that is taken as that for our purposes) I'm not too sure but i think the reason why is that at that T,P one mol of any gas has the same average KE and larger molecules have more momentum than lighter ones but lighter ones more speed thus they collide more (or compensate for the weight with speed) or something like that. the point is that the average momentum is the same, thus creating the same volume when they collide with the walls of a container.

I see what you're saying, but KE = m*v^2, so the higher velocity and mass aren't 1:1 so why would they cancel each other out?

 

[TXKnight]

I see what you're saying, but KE = m*v^2, so the higher velocity and mass aren't 1:1 so why would they cancel each other out?

yeah I remember thinking about that too when i read it on TBR, I don't rememebert he reason why. I will check on that.

I think i vaguely recall it may have to do with the root mean square speed...and the average energy of thesystem.....i will definitely check on that.

i think it may go something like V=sqrt(3RT/M)

 

[MrNeuro]

im pretty sure 22.4 L is only for ideal gases as it assumes
1. there are no IMFs
2. atom volume isn't significant
3. all collisions are elastic (conserves KE)

if you want to solve for the volume of a real gas you have to use van der Waal's equation.

correct me if im wrong