why does oxygen have lower boiling point than hydrogen chloride when relative molecular masses are similar?

Answer 1

Boiling point is the temperature at which the vapor pressure of a liquid is equal to the atmospheric pressure. Raising the temperature of a liquid increases the vapor pressure. The higher the vapor pressure of a liquid already is, the lower the temperature required for it to boil. Oxygen has a lower boiling point than hydrogen because it has a higher vapor pressure. The reason for this is because there are less intermolecular forces holding oxygen together than there are intermolecular forces holding together hydrogen chloride. In oxygen, there are weak London forces and nonpolar-covalent bonds. In hydrogen chloride, there are London forces AND a polar-covalent bond.

Answer 2

Oxygen has a lower boiling point than hydrogen chloride because it is a non-polar molecule, while hydrogen chloride has a very polar bond, and it can also participate in hydrogen bonding with itself. oxygen's non-polarity leaves only London Dispersion forces to attract the molecules to each other, which in this case are very weak because oxygen is so light.

Answer 3

Hydrogen Chloride has a slight polarity. Because the chlorine is a strongly electronegative element it attract electrons in the covalent bond with hydrogen towards itself. Thereby denuding the hydrogen atom of its electron cloud. The hydrogen will then seek out a region of high electron density eg. another HCl molecule and be attracted towards the chlorine. This is a 'hydrogen bond' , usually found in water.
This attraction between a hydrogen atom of one molecule and the chlorine atom of another molecule brings the molecules closer together. Hence as the temperature lowers the molecules form a liquid.
However, with an oxygen molecule (o2) all the atoms are oxygen and have the same degree of electronegativity. Hence attractions between atoms/molecules are neutralised and so oxygen condenses at a lower temperature than HCl.

Answer 4

Boiling point of a molecule is not determined by its mass, but rather the types of bonding that exist between the molecules.

H-Cl (hydrogen chloride) has what are called hydrogen bonds, these are the strongest type of intermolecular forces and so much more energy is required to break them than for oxygen where the forces between them are only weak Van der Waals forces.

For more detail for this see:
http://www.scienceaid.co.uk/chemistry/fundamental/inter.html

Answer 5

OK...Hydrogen bonding can only occur between molecules with O-H or N-H bonds (or F), which clearly shows that HCl molecules do not have H-bonding as their intermolecular attractions. What they DO have is dipole-dipole attractions since the bond in HCl is a polar covalent bond. The bond in molecular oxygen is true covalent and therefore the intermolecular attractions are weak London or van der Waals forces (temporary dipoles) which are much weaker than dipole-dipole attractions. That's why O2 has a lower boiling point than HCl.

Answer 6

Hydrogen bonding between the hydrogen on one molecule and the chlorine on another, I should think. There will be little attraction between molecules of oxygen.

Answer 7

Ehhh, I dont think that the H-Cl bond is polarized enough to have hydrogen bonding. But there are still dipole-dipole interactions that hold the H-Cl molecules together.

Answer 8

Someone suggested weak hydrogen bonding, that would be my guess too.