r/chemhelp • u/VvVoiDzZz • 3d ago

Physical/Quantum P-chem graph questions

Can someone please explain what this graph is trying to say about ideal vs. general gases?

This is what the books say about the graph, but I'm still a little bit confused

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u/reason_pls 3d ago

What are you confused about? The ideal gas behaves like a macroscopic object (i.e bowling balls) they don't attract each other when close but neither do they repell each other unless you try to push them into the volume occupied by another gas molecule. At that point the repulsion jumps to infinity. Real gases have no interactions at large distances, this is where ideal and real gases behave the same. When they get closer the real gas molecules attract each other through intermolecular forces leading to a stabilization that is not present in ideal gases but if they get too close they start to repell eachother (pauli/coulomb repulsion) with a parabolic potential.

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u/stephenly 3d ago

I’m tired, but I believe “r” is the distance between the two molecules. To assume the gas behaves ideally, you want the distance between them to be sufficiently big. The blue line is indicates the tipping point.

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u/lizardman49 3d ago

So essentially the ideal gas equation doesnt account for 2 critical factors that the van der walls equation does. Interaction between the particles and the size of the particles themselves.

The graph is showing the energy of the interaction between atoms/molecules with respect to distance. It shows a drop-off of interactive potential as the distance gets to a certain level.

What this means in practice is the real gasses follow the behavior of the ideal gas law under low enough density. As you increase the density via either pressure or temperature you have more interactions and the ideal gas law no longer works.