Fractional distillation of crude oil — AQA A-Level Chemistry
Test yourself on Fractional distillation of crude oil with AQA A-Level practice questions.
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Fractional distillation of crude oil explained
Two words, each doing work. Hydrocarbon means the molecule contains only carbon and hydrogen.
Read the full explanation
Saturated means every carbon to carbon bond is a single bond, so the molecule holds as many hydrogen atoms as it possibly can. The general formula of an open-chain alkane is CₙH₂ₙ₊₂, while cycloalkanes, having closed the ring, are CₙH₂ₙ. Every carbon has four σ bonds arranged tetrahedrally at 109.5°. Because carbon and hydrogen have almost the same electronegativity the bonds are essentially non-polar, and the only forces between molecules are London forces. That explains why alkanes are so unreactive, since there is no δ⁺ carbon for a nucleophile to attack, and why boiling point rises steadily with chain length.
Petroleum is a mixture consisting mainly of alkane hydrocarbons that can be separated by fractional distillation.
Crude oil is a mixture of alkanes of many chain lengths, and because they have different boiling points they can be separated by fractional distillation. The oil is heated until it vaporises and is fed into a tall column that is hot at the bottom and cooler towards the top. Vapours rise until they reach a tray cool enough for them to condense, and the liquid is drawn off there. Long chain molecules have more electrons and a greater surface area of contact, so their London forces are stronger, their boiling points higher, and they condense low down as fuel oil and bitumen. Short chains condense near the top as refinery gases and petrol, with naphtha and kerosene between. Each fraction is still a mixture.
Your focus
- Define saturated hydrocarbon, explaining both words separately.
- Deduce the molecular formula of any open-chain alkane or cycloalkane from its number of carbon atoms.
- Explain the low reactivity of alkanes in terms of bond strength and the absence of bond polarity.
Show all 6 objectives
- Describe how a fractionating column separates crude oil using a temperature gradient.
- Explain the order in which fractions are collected by relating chain length to London forces and boiling point.
- State why each fraction is a mixture rather than a single compound.
Fractional distillation of crude oil exam tips
Marking Points
- one mark for hydrocarbon meaning a compound of carbon and hydrogen only
- one mark for saturated meaning that all the carbon to carbon bonds are single bonds
- one mark for the general formula CₙH₂ₙ₊₂ for an open-chain alkane
- one mark for the tetrahedral shape at each carbon with a bond angle of 109.5°
- one mark for vaporising the crude oil and feeding it into a column with a temperature gradient
- one mark for a fraction condensing at the height where the temperature falls below its boiling point
- one mark for linking a higher boiling point to a longer chain and stronger London forces
- one mark for naming fractions in the correct order of boiling point up the column
Examiner Tips
- 💡Define both words separately if the question offers two marks for the term.
- 💡Check whether the alkane is a ring before applying a general formula.
- 💡Link low reactivity to non-polar bonds and to strong C–C and C–H bonds, not just to being stable.
- 💡Name London forces explicitly when explaining the boiling point order, since intermolecular forces alone is weaker.
- 💡Say that forces between molecules are overcome, not that bonds inside molecules break.
- 💡Give the fractions in order and attach a use to each if the question asks for one.
Common Mistakes
- defining saturated as containing no double bonds without saying between carbon atoms
- applying CₙH₂ₙ₊₂ to a cycloalkane, which is CₙH₂ₙ
- giving the bond angle in an alkane as 120° or as 90°
- describing alkanes as polar because C–H bonds have slight polarity, when the molecules are effectively non-polar
- saying each fraction is a pure compound rather than a mixture collected over a range of boiling points
- putting the hot end at the top of the column
- explaining higher boiling points by stronger covalent bonds, when no covalent bonds break on boiling
- calling the process simple distillation, which separates only two well separated components