real gases pressure and volume

[2010premed]

So I know that the volume of a real gas never decreases/increases as much as the ideal gas, but is there a general rule like this for pressure? Or does it depend on other factors, such as "a" in Pobs+an^2/V^2

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[Rabolisk]

So I know that the volume of a real gas never decreases/increases as much as the ideal gas, but is there a general rule like this for pressure? Or does it depend on other factors, such as "a" in Pobs+an^2/V^2

I don't understand your first proposition...

 

[jch1989]

I have a question sorta related to this... I just wanted to double check:

Real gases have volume greater than ideal gases because in the ideal gas law, it is assumed that gases have no molecular volume?

And real gases have pressure less than ideal gases?

 

[methylethyl88]

For an ideal gas, it is assumed that gas molecules have no volume. For real gases though, those gas molecules do take up volume so the actual volume is never as much as you would predict for the ideal gas.

Example: if you double the pressure with all else constant, then for an ideal gas, the volume should be halved. However, for a real gas, the volume of the molecules themselves don't change. Therefore, doubling the pressure should cause the volume to be a little more than half.

For real gases and pressure, the a factor you mentioned in "Pobs+an^2/V^2" depends on whether the molecules have an attractive or repulsive force on each other. If the molecules are attracted to each other, then a is positive because the molecules will not collide with the wall of the container as much since they are attracted to each other. Therefore, the observed pressure will be a little low so the "an^2/V^2" is positive. If the molecules were repulsive, then a would be negative. This makes sense because if the repulsive molecules would repel each other and thus hit the walls of the container more often than we would expect.

 

[Rabolisk]

For an ideal gas, it is assumed that gas molecules have no volume. For real gases though, those gas molecules do take up volume so the actual volume is never as much as you would predict for the ideal gas.

Example: if you double the pressure with all else constant, then for an ideal gas, the volume should be halved. However, for a real gas, the volume of the molecules themselves don't change. Therefore, doubling the pressure should cause the volume to be a little more than half.

For real gases and pressure, the a factor you mentioned in "Pobs+an^2/V^2" depends on whether the molecules have an attractive or repulsive force on each other. If the molecules are attracted to each other, then a is positive because the molecules will not collide with the wall of the container as much since they are attracted to each other. Therefore, the observed pressure will be a little low so the "an^2/V^2" is positive. If the molecules were repulsive, then a would be negative. This makes sense because if the repulsive molecules would repel each other and thus hit the walls of the container more often than we would expect.

Your explanation and example contradict each Other. There's a lot of misinformation here that should be cleared up. First let's define volume of a gas is the volume occupied by the gas, which includes empty space. The volume of a real gas is not always bigger than the volume of an ideal gas; it depends on pressure. At moderately high pressures, the intermolecular attraction brings the gas atoms/molecules together, resulting in smaller than expected volume. At really high pressure though, the volume occupied by the molecules matter, and the total volume is higher than expected. The range for "moderate" and "really" high differ for each gas. A graphical representation is a PV curve.

 

[fizzgig]

just one more way of writing it out...

the van der waals equation has (P+stuff)(V-stuff)=nRT. real gases occupy volume, so to get ideal gas volume you need to subtract a little bit. real gases have intermolecular forces decreasing pressure, so to get ideal gas pressure you need to add a little bit.

if you look at the ratio PV/nRT it should be equal to 1 of course.

if it is greater than 1, the increase in volume due to molecule vol is dominating over the decrease in pressure due to intermolec forces.
if it is less than 1, the decrease in pressure is dominating over the increase in volume.

 

[2010premed]

What do you mean the colume of a real gas is not always bigger than the volume of an ideal gas?Doesn't the equation Vreal - nb = V ideal say that real gases have a bigger V?

 

[Rabolisk]

What do you mean the colume of a real gas is not always bigger than the volume of an ideal gas?Doesn't the equation Vreal - nb = V ideal say that real gases have a bigger V?

Well... it depends on how you look at it. What I mean by Videal is the volume that one would expect to get from the perfect gas law, given a state of variables (n, P, T). For example, if you were given 1 mole of gas at 1 atm and at 273K, you would calculate a certain volume, or Videal. What I meant was that this calculated Videal is not always less than the actual volume one would measure in an experiment.

I know that some say that volume of a real gas is always bigger than the volume of an ideal gas because a real gas requires some space for its molecules. However, that is assuming that the volume taken up by empty space is the same. This would be true if there were no IMFs.

 

[2010premed]

I just looked at my notes and they actually say Vcontainer - nb=videal

is this right?

 

[Rabolisk]

I just looked at my notes and they actually say Vcontainer - nb=videal

is this right?

Yes, but I hope you understand what I meant too.