Chemistry 8.5 - 8.6 - Fraction properties and alkanes

Molecule size and fraction properties

Most of the hydrocarbons in crude-oil fractions are members of the alkane homologous series. Most matters: crude oil is a complex mixture, so do not claim that every molecule in every fraction is an alkane.

The complete trend is easiest to follow in one direction:

Boiling point: larger hydrocarbon molecules have stronger intermolecular attractions because they contain more atoms and electrons and can make more contact with neighbouring molecules. More energy is needed to overcome these attractions, so their boiling points are higher. Boiling does not break the strong covalent bonds inside each molecule.

Viscosity: viscosity is a liquid's resistance to flow. Stronger attractions between larger molecules make it harder for them to move past one another, so larger liquid hydrocarbons are more viscous. More viscous means flows less easily; it does not mean more dense.

Ease of ignition: smaller hydrocarbons have lower boiling points and form vapour more readily. Their vapour can mix with oxygen in the air, so they are generally easier to ignite. Larger hydrocarbons are less volatile and harder to ignite. Ease of ignition describes how readily burning starts; it is different from how much energy a fuel releases.

Comparing fractions fairly

A teacher can compare risk-assessed synthetic fractions using a clean glass slope. This is a suggested investigation, not a mandatory core practical. At the same room temperature, release the same small volume at a marked starting line and time its flow to a second line. Keep volume, slope angle, travel distance, surface cleanliness and the timing endpoint constant. Longer flow time means greater viscosity; it does not measure density.

Repeat each fraction at least three times and calculate a mean flow time. Check any anomalous result before deciding whether to exclude it. A results table should include the fraction, its boiling range, individual flow times, mean flow time and any teacher-supplied ignition observation. Plot mean flow time vertically against typical carbon number horizontally to test the trend. Repeats reduce random variation; they do not correct a consistently wrong slope angle or changing sample temperature.

Synthetic fractions are flammable and may be harmful. Use small teacher-approved samples, eye protection and appropriate ventilation, and keep samples away from flames. Any distillation or ignition demonstration requires the teacher's substance-specific method and risk assessment; do not heat an unknown mixture or a sealed apparatus. Industrial fractional distillation uses repeated vaporisation and condensation, so a short laboratory apparatus is only a simplified model.