Question regarding ideal and real gases

[jillzhou]

I'm using Kaplan and EK and I feel the stuff where they talk about real gases throws me off. In kaplan, i've read that as pressure increases, a gas becomes less ideal. then why is it that in EK 1001 General Chem, question 210, as the temperature of a container is decreased at constant temperature, the gas inside begins to behave less ideally. compared to the pressure predicted by the ideal gas law the actual pressure is most likely to be....
the answer says) B-lower, due to the intermolecular attractions among gas molecules.
shouldn't it be higher, if it behaves less ideally i'm assuming the pressure would increase and you're decreasing the volume of a container so wouldn't it intuitively make sense for the pressure to increase?

any help appreciated!

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[burntorange]

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[mehc012]

The pressure of a container is caused by the gas particles hitting the walls with a certain frequency and velocity. In an ideal gas, the only effect on movement is collisions - hitting each other and the walls. In a real gas, however, there are attractive forces between the gas molecules. This means that a real gas will have the tendency to 'clump' a bit more than an ideal gas. In other words, a molecule which would have proceeded unhindered to a full-speed collision with the wall of the container in the absence of IMFs will, under real conditions, be attracted to the other gas molecules (away from the container wall) and therefore collide at a lower speed. This means the pressure would be lower than predicted by Ideal Gas rules.

 

[ClemsonFirst]

To sum up the two previous posts, ideally, the gas molecules have no interaction and collide with the container at full speed creating pressure of 100%. When the gas starts behaving like a real gas, aka deviates from the ideal gas law (this occurs at high pressure OR low temperature), the gas molecules interact with each other reducing the speed of the molecules, so they collide with the container wall at less than optimal speed creating a pressure of less than the ideal 100%.

That is why the pressure under real condition is lower than expected. The pressure does increase, but we are asked about the relative pressure increase of the real gas compared to that of the ideal gas. The ideal has a higher pressure than the real gas because of the intermolecular force interaction.

 

[young_gohard]

I was also thrown off by this question because I learned repulsive forces dominate at higher pressures so I thought pressure would be higher than expected at some point. Most everything I've read says pressure is lower than expected for a real gas because of attractive forces, but I'm wondering if there's a point where the pressure would be higher than expected.

This is from the UC Davis ChemWiki site:

Repulsive forces: As a gas is compressed, the individual molecules begin to get in each other's way, giving rise to a very strong repulsive force acts to oppose any further volume decrease. We would therefore expect the PV vs P line to curve upward at high pressures, and this is in fact what is observed for all gases at sufficiently high pressures.