Physical properties and separation of hydrocarbons

Organic chemistry • Carbon compounds as fuels

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Why do larger hydrocarbons have higher boiling points?

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Larger hydrocarbons have more electrons and surface area, producing stronger dispersion forces that need more thermal energy to overcome.

Key concepts

What you'll likely be quizzed about

Boiling point and molecular size

Boiling point increases as hydrocarbon molecular size increases because larger molecules have greater surface area and stronger London (dispersion) forces. Stronger intermolecular attraction requires more thermal energy to convert the liquid into vapour. Branching reduces surface contact and therefore lowers boiling point compared with straight-chain isomers of the same molecular mass. External factors such as pressure affect measured boiling points; lower pressure lowers boiling points and higher pressure raises them.

Viscosity and molecular size

Viscosity increases as hydrocarbon molecular size increases because longer chains become more entangled and experience greater intermolecular resistance to flow. Stronger London forces between larger molecules also reduce molecular mobility, producing thicker, slower-flowing liquids. Temperature acts as a limiting factor: increasing temperature reduces viscosity by providing thermal energy that overcomes intermolecular forces and chain entanglement, causing even heavy hydrocarbons to flow more easily.

Flammability and molecular size

Flammability decreases as hydrocarbon molecular size increases because smaller molecules vapourise more readily at a given temperature and produce combustible vapour-air mixtures. Smaller hydrocarbons ignite and burn more easily because less energy is required for vaporisation and bond activation during combustion. Larger hydrocarbons tend to burn with soot and incomplete combustion because they produce heavier fragments and require higher temperatures for full oxidation. Environmental oxygen availability and mixture concentration influence flammability limits.

Fractional distillation: evaporation and condensation

Fractional distillation separates a mixture of hydrocarbons by their different boiling points. Heating causes the lowest boiling components to evaporate first; rising vapour passes up a fractionating column where cooler temperatures higher in the column cause higher-boiling vapours to condense earlier. Repeated evaporation and condensation across trays or packing enriches vapour in lower-boiling components towards the top and in higher-boiling components towards the bottom. Condensed fractions are drawn off at different heights where their condensation temperatures match the local column temperature.

Practical limitations and other factors

Molecular branching, functional groups (e.g., oxygen-containing groups), and molecular polarity alter boiling points and viscosity compared with simple hydrocarbons. Atmospheric pressure and column design determine separation efficiency in fractional distillation: lower pressure distillation reduces boiling points but can change relative separations. Column height, surface area of packing/trays and reflux ratio affect the number of theoretical plates and therefore the purity of separated fractions.

Key notes

Important points to keep in mind

Boiling point increases with molecular size because London forces strengthen with more electrons and surface area.

Branching lowers boiling point by reducing surface contact and dispersion forces.

Viscosity increases with molecular size due to chain entanglement and stronger intermolecular attraction.

Higher temperature reduces viscosity by overcoming intermolecular forces.

Flammability decreases with molecular size because larger molecules vapourise less easily and require more ignition energy.

Fractional distillation separates by repeated evaporation and condensation along a temperature gradient in a column.

Column efficiency depends on theoretical plates, packing/tray surface area and reflux ratio.

External pressure changes boiling points and can alter distillation behaviour; note reduced pressure for heat-sensitive fractions.

Polar functional groups and heteroatoms alter trends in boiling point and viscosity compared with pure hydrocarbons.

Products collect at column heights where local temperature equals their condensation temperature.