Which of the following is a correct listing in terms of increaded boiling point and why?

Question

1) CH4 2) CH4 3) HF 4) CH4

Answer 1

HF > H2O > NH3 > CH4
Boiling depends upon the hydrogen bonding with the highly electronegative atom.
Here, Fluorine atom is more electro negative than Oxygen and so on.

Answer 2

CH4
H2O has a simple molecular structure consisting of discrete H2O molecules held together by hydrogen bonds. The atom O, is a highly electronegative atom contributing to the strong electrostatic forces of attraction between the H2O molecules. Much energy is required to overcome these forces, thus H2O has the highest boiling point.

NH3 has a simple molecular structure consisting of discrete NH3 molecules held together by hydrogen bonds. The atom, N, although also a highly electronegative atom, but of a lower elecronegativity than O, contributes to the relatively strong electrostatic forces of attration between the NH3 molecules. Much energy is also needed to overcome these forces, as such NH3 also have a high boiling point but one being lower than H2O.

HF has a simple molecular structure consisting of discrete HF molecules held together by hydrogen bonds. HF has a lower boiling point as compared to H2O and NH3 due to the average of one hydrogen bond per molecule as compared to two of H2O and NH3.

CH4 has a simple molecular structure consisting of discrete CH4 molecules held together by weak van der Waals' forces. Hence, less energy is need to break the weak intermolecular forces, resulting in CH4 having the lowest boiling point.

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