Gen Chem- Conficting info in EK vs TBR

[missdoctor]

My EK book says Vreal>Videal because ideal gas molecules have no volume and thus must be less than the volumes of real gas molecules, wherea TBR says Vreal<Videal because real gases experience intermolecular forces pulling them closer together than ideal gases. Which is it?

Thanks !!

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[fizzgig]

both happen. if the molecular size effect outweighs the intermolecular force effect,you'll see an upward deviation in volume, vversa gives a lower than expected volume.

which one you see will be determined by the type of molecule and the pressure and temperature, i believe?

 

[ilovemcat]

My EK book says Vreal>Videal because ideal gas molecules have no volume and thus must be less than the volumes of real gas molecules, where as TBR says Vreal<Videal because real gases experience intermolecular forces pulling them closer together than ideal gases. Which is it?

Thanks !!

Real Gas molecules DO have volume, however, when using the Ideal Gas Law, a lot of assumptions are made. First, it's assumed that the volume of a gas(es) is negligible when the pressure is low (lots of free volume) and temperature is high (lots of kinetic energy). The gas molecules are free to fly all over the place.

So for example when a problem tells you that a certain type of gas is occupying a container of "X" volume, it doesn't take into account the net volume of the gas (because it's assumed to behave ideally ...unless they state otherwise).

However, for a non-ideal gas, the "true" volume would be "Total Vol. = Volume of Container - Volume of Gases." Therefore, Vreal would LESS than Videal (under non-ideal conditions).

Also, intermolecular forces (attraction / repulsion ) of gas molecules can affect the Pressure within a container. If two different gases attract each other than Preal < Pideal because they hit the container wall less. However, if the intermolecular forces repel each other, they hit the container wall more often so Preal > Pideal.

This is the whole idea behind Van der Waals equation.

I hope this helps.

 

[MCAT guy]

This is actually a very simple idea.

Ideal gas: assumed to take up NO volume
Real gas: takes up volume (has actual volume)

Therefore the volume of a real gas is LARGER (as it occupies space) than the volume of an ideal gas.

Wait, what in the world is BR saying then?

BR is saying, lets say we have a container of 1 L of gas. An ideal gas has 1L of free space left over (gas takes up no space). A real gas has 99% of that space left over (because it's taking up space).

So in one situation (EK), we say the gas takes up more volume. While the other (BR), we say that the volume (free space) is less for an ideal gas.

Same idea stated different ways.

(also both do happen, but the idea is that the real volume will definitely be greater). EK is the simpler choice here.

Btw, I have done now 5 FLs and around 300 passages and have not encountered this idea more than once or so. Just know that volume real > volume ideal. Done.