Why does ethane has a low melting point?

Answer 1

The relative ability of Ethane to boil depends upon the material it is compared too...

Compared to Helium, it's boiling point is relatively high.

Assuming you are comparing to a similar material such as ethanol...

The reason why it has a low boiling point in comparison to ethanol is because ethanol has a hydroxy/alcohol group which can for hydrogen bonds.

The only intermolecular force existing between ethane is dispersion forces (the weakest type of force), and therefore the melting point is much lower.

Answer 2

Nitrogen molecules are non-polar, so the only intermolecular bonds formed between molecules of Nitrogen are instantaneous dipole - induced dipole bonds, which are the weakest kind of intermolecular bonds. They are particularly weak in Nitrogen because it is a very small molecule, so has few electrons, so the polarity of the molecule in any one instant is small, leading to very weak id-id bonds.

Ethane is also non-polar, but is a larger molecule, so there are more electrons moving around, leading to a greater difference in polarity at any one instant, so the id-id bonds are stronger between ethane molecules than between Nitrogen molecules.

As the intermolecular bonds between Nitrogen molecules are weaker than those between ethane molecules, less energy is required to break these bonds, so the boiling point of nitrogen is lower than that of ethane,