Physical Properties

Typical physical properties are boiling and melting points, density, etc.

The BOILING POINT is the temperature at which a liquid turns into gas. The molecules DON'T BREAK. They only stop attracting one another.

Graph of the boiling points of linear alkanes versus number of carbons — Boiling points of linear alkanes

Which is the alkane boiling at 100ºC? HEPTANE C₇H₁₆. How many atoms does it have? TWENTY-THREE. How many atoms does water have? Only three, but it also boils at 100ºC. Why?

The MELTING POINT is the temperature at which a solid melts and transforms into liquid. The molecules DON'T BREAK either. They only 'unpack' from the solid state and attract each other more weakly in the liquid state.

Graph of the melting points of linear alkanes versus number of carbons — Melting points of linear alkanes

Which alkane melts at 0ºC ? TRIDECANE C₁₃H₂₈. How many atoms does it have? FORTY-ONE. How many atoms does water have? Only three, but it also melts at 0ºC. Why?

The DENSITY allows us to know how compact a compound is at a given temperature.

Graph of the density of linear alkanes versus number of carbons — Density of linear alkanes

Which alkane has a density of 1 g/mL ? That value is not reached in the alkane series even with a large number of carbons. Water, with only three atoms, achieves that density. Why?

The experimental boiling point, melting point and density of water show that its molecules attract one another much more strongly than alkane molecules do. Alkanes need 23 atoms to reach the boiling point of water, and 41 to reach its melting point!

Dispersion (London) forces between two nonpolar molecules — Dispersion forces (London)
WEAK

Dipolar forces between two polar molecules — Dipolar forces
STRONG

Hydrogen bond between two molecules — Hydrogen bonding
VERY STRONG

The interaction among molecules (INTERMOLECULAR FORCES) is a balance between the repulsion of the external electrons and the attraction of the nuclei of one molecule upon the electrons of the neighboring one.

Dispersion forces (London) These are the weakest intermolecular forces, the only ones possible in alkanes.

The alkanes, lacking electronegative atoms, have an outer electron shell which is relatively dense and continuous. The repulsion of the electron clouds is important and the attraction between alkane molecules is thus very weak.

Boiling points of the C₅H₁₂ isomers: pentane 36.1 °C, 2-methylbutane 27.8 °C and 2,2-dimethylpropane 9.5 °C

The 'shape' of the molecules affects the contact surface. What do these experimental results suggest? The non-branched molecules, with a linear zigzag structure, attract one another more effectively, as their higher physical constants show.