Crude oil is separated into different fractions by fractional distillation.
Table 1 shows information about three of the hydrocarbon molecules found in three different fractions.
Table 1
| Molecule | Formula | Boiling point (∘C^\circ\text{C}∘C) |
|---|---|---|
| hexane | C6H14\text{C}_{6}\text{H}_{14}C6H14 | 69 |
| tetradecane | C14H30\text{C}_{14}\text{H}_{30}C14H30 | 254 |
| octacosane | C28H58\text{C}_{28}\text{H}_{58}C28H58 | 432 |
Which of these three molecules would be separated highest up the fractionating column? Explain your answer using ideas about molecular size and boiling point.
Cracking breaks down large hydrocarbons produced in fractional distillation into smaller, more useful molecules. The chemical equation shows the cracking of octadecane:
C18H38→C12H26+3C2H4 \text{C}_{18}\text{H}_{38} \rightarrow \text{C}_{12}\text{H}_{26} + 3\text{C}_2\text{H}_4 C18H38→C12H26+3C2H4 octadecane→dodecane+3×molecule Z \text{octadecane} \rightarrow \text{dodecane} + 3 \times \text{molecule }\mathbf{Z} octadecane→dodecane+3×molecule ZState the name of molecule Z\mathbf{Z}Z, C2H4\text{C}_2\text{H}_4C2H4.
Molecule Z\mathbf{Z}Z has the general formula CnH2n\text{C}_n\text{H}_{2n}CnH2n. Octadecane and dodecane are both alkanes. State the general formula of the alkanes.
Table 2 shows the percentage supply and percentage demand for some of the different fractions obtained from crude oil.
Table 2
| Fraction | Percentage supply (%) | Percentage demand (%) |
|---|---|---|
| LPG | 4 | 6 |
| petrol | 9 | 28 |
| naphtha | 12 | 8 |
| kerosene | 14 | [blank] |
| diesel oil | 22 | 27 |
| fuel oil | 39 | 21 |
Calculate the percentage demand for kerosene.
Suggest why fuel oil, rather than diesel oil, is cracked to obtain petrol.