Question

Which would you expect to have the higher melting point(or boiling point): C8H18 or C4H10? Explain

Answer 1

Octane (`C_8H_18`)

Explanation:

Melting/boiling points are measures of the thermal energy required to break the intermolecular forces (IMFs) in a substance; these are attractions between the molecules of a substance, such as dispersion forces, dipole-dipole interactions, and hydrogen bonding.

The stronger the IMFs are, the harder it is for the molecules to 'loosen' and thus transition state from solid to liquid, and then to gas; in the case of melting/boiling point, we'll consider the differences in the states of matter as the amount of 'space' between their molecules, as we know solids are tightly packed, liquids loosely so, and gases even further spread apart.

Both octane (`C_8H_18`) and butane (`C_4H_10`) are non-polar hydrocarbons; what I mean by this is that the bonds between carbon and hydrogen are non-polar due to an insignificant electronegativity difference, and thus the entire molecule has no dipole moment.

In the case of purely non-polar compounds, dispersion forces are exhibited; the strength of these forces increases with increasing molar mass; dispersion forces are based off of the orbit of electrons, temporarily creating positive and negative ends, dipoles, in the molecule. The higher the molar mass, the more electrons a compound will contain, and thus the stronger the dispersion forces will be as the temporary dipoles will be more distinct.

Molar mass of octane: `114.224 gmol^-1`

Molar mass of butane: `58.12gmol^-1`

Hence, octane would have the stronger dispersion forces, requiring more thermal energy to break these forces, resulting in a higher melting and boiling point.

Another point to note is the fact that melting point is influenced by the 'packing' of molecules in the solid; if they can pack more tightly together their IMFs will be stronger and thus have a higher melting point.