Why is carbon disulfide a liquid?

Question

Explain why, at room temperature, carbon dioxide is a gas but carbon disulfide is a liquid.

Answer 1

No dipole moment in either molecule, because they are linear. However, van der Waals forces involving sulfur are stronger than those involving oxygen, hence the higher boiling point. This is the normal trend going down the periodic table if no other factors are involved.

Answer 2

There are two reasons that I can think of for the difference in boiling points (-78C vs 45C): mass and intermolecular interactions.

Mass: CS2 is heavier than CO2 (almost twice as heavy), so on that alone it will have a higher boiling point. Check out ethane and butane: ethane boils at -90, butane at 0. The only major difference between those molecules is that butane weighs more; they have the same basic types of bonds.

Intermolecular forces: The C-S bond is weaker than the C-O bond (due to worse orbital energy matching), and the electron cloud around S is held less tightly than that around O. These traits combine to make the CS2 molecule more polarizable, which leads to temporary dipole- temporary dipole interactions and raises the boiling point. This is kind of analogous to water (mucho very strong interactions) boiling way higher than methane (weighs about the same, but not as strong of interactions).

Based on the boiling point difference of over 120C, I'd say both these factors are in play here. Doubling the mass alone doesn't raise the BP that much (see ethane and butane).

Answer 3

Stronger intermolecular forces, due to greater dipole forces.