Physical Properties (Edexcel International AS Chemistry)

Branching

• The larger the surface area of a molecule, the more contact it will have with adjacent molecules
• The surface area of a molecule is reduced by branching 
• The greater its ability to induce a dipole in an adjacent molecule, the greater the London (dispersion) forces and the higher the melting and boiling points
• This point can be illustrated by comparing different isomers containing the same number of electrons:

Boiling points of molecules with the same numbers of electrons but different surface areas

Number of electrons

• The greater the number of electrons (or the greater the molecular mass) in a molecule, the greater the likelihood of a distortion and thus the greater the frequency and magnitude of the temporary dipoles
• The dispersion forces between the molecules are stronger and the enthalpy of vaporisation, melting and boiling points are larger
• The greater boiling points of the noble gases illustrate this factor:

As the number of electrons increases more energy is needed to overcome the forces of attraction between the noble gases atoms

Graph showing the increase in boiling point as the number of electrons increases

Alcohols

• Hydrogen bonding occurs between molecules where you have a hydrogen atom attached to one of the very electronegative elements - fluorine, oxygen or nitrogen
• In an alcohol, there are O-H bonds present in the structure
• Therefore hydrogen bonds set up between the slightly positive hydrogen atoms (δ+ H) and lone pairs on oxygens in other molecules
• The hydrogen atoms are slightly positive because the bonding electrons are pulled away from them towards the very electronegative oxygen atoms
• In alkanes, the only intermolecular forces are temporary induced dipole-dipole forces
• Hydrogen bonds are much stronger than these and therefore it takes more energy to separate alcohol molecules than it does to separate alkane molecules
• Therefore, the boiling point of alkanes is lower than the boiling point of the respective alcohols
• For example, the boiling point of propane is -42 ^oC and the boiling point of propanol is 97 ^oC

Hydrogen Halides 

• The boiling points of the hydrogen halides are as follows

• The boiling points of the rest of the hydrogen halides increase as the molecules become larger
• The extra electrons allow greater temporary dipoles and so increase the amount of London dispersion forces between the molecules
• Hydrogen fluoride also has hydrogen bonding between the HF molecules
• The bond is very polar so that the hydrogen has a significant amount of positive charge and the fluorine a significant amount of negative charge. In addition, the fluorine has small intense lone pairs
• Hydrogen bonds can form between the hydrogen on one molecule and a lone pair on the fluorine in its neighbour