Does a large covalent molecule generally have a low boiling point?

Answer 1

Boiling point of a molecular substance depends on the intermolecular forces - forces that attracts a molecule to its neighbours of the same kind.

For a covalent molecule, the possible intermolecular forces are

• dispersion forces
• dipole-dipole bonding and
• hydrogen bonding

As the strength and magnitude of the intermolecular forces increase, the boiling point increases because it becomes increasingly difficult to break the bonds and requires more energy for the same.

Since different types of intermolecular bonding are present, the answer to the question cannot be given in a single sentence.

Analyzing all types:

If, for large covalent molecules, the intermolecular forces increase, then the boiling point will increase.

If only dispersion forces are present, then as the number of electrons increases (and consequently the mass), so will the dispersion forces. Therefore, boiling points will be higher.

If only dipole-dipole bonding is present, then, as the molecule increases in size, the charge is dispersed in the molecule, strength of the polarity decreases and thus the intermolecular forces decreases. Therefore, boiling point decreases.

If only hydrogen bonding were present, the strength of the bond will depend on the halogen atom with which hydrogen forms a bond. It does not depend on the mass/size of the molecule.

However, dipole-dipole and hydrogen bonding can never exist in a substance on their own but with dispersion forces.

Therefore, as the mass/size of a covalent molecule increases, the dispersion forces increase and will lead to a higher boiling point independent of hydrogen bonding and the decrease in dipole-dipole bonding will be compensated by an equal or higher increase in dispersion forces.

Large covalent molecules do not have low boiling points (in comparison with small covalent molecules). But boiling point of covalent molecules, in general, is less than that of ionic molecules.

Answer 2

Large covalent molecules generally have high boiling points due to the strong covalent bonds between the atoms. The intermolecular forces in large covalent molecules are also stronger, leading to higher boiling points compared to smaller covalent molecules.