Crude oil is separated into different fractions by fractional distillation.
Table 20.1 shows information about three of the molecules that are found in three different fractions.
Table 20.1
| Molecule | Formula | Boiling point (∘C^\circ\text{C}∘C) |
|---|---|---|
| octane | C8H18\text{C}_{8}\text{H}_{18}C8H18 | 126 |
| pentadecane | C15H32\text{C}_{15}\text{H}_{32}C15H32 | 270 |
| tetracosane | C24H50\text{C}_{24}\text{H}_{50}C24H50 | 391 |
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 molecules produced in fractional distillation into more useful molecules. The equation shows the cracking of hexadecane:
C16H34→C10H22+2C3H6 \text{C}_{16}\text{H}_{34} \rightarrow \text{C}_{10}\text{H}_{22} + 2\text{C}_3\text{H}_6 C16H34→C10H22+2C3H6 hexadecane→decane+2×molecule Y \text{hexadecane} \rightarrow \text{decane} + 2\times\text{molecule }\mathbf{Y} hexadecane→decane+2×molecule YState the name of molecule Y\mathbf{Y}Y, C3H6\text{C}_3\text{H}_6C3H6.
Molecule Y\mathbf{Y}Y has the general formula CnH2n\text{C}_n\text{H}_{2n}CnH2n. Hexadecane and decane are both alkanes. State the general formula of the alkanes.
Table 20.2 shows the percentage supply and percentage demand for some of the different fractions obtained from crude oil.
Table 20.2
| Fraction | Percentage supply (%) | Percentage demand (%) |
|---|---|---|
| LPG | 3 | 5 |
| petrol | 8 | 25 |
| naphtha | 10 | [blank] |
| kerosene | 13 | 9 |
| diesel oil | 20 | 26 |
| fuel oil | 46 | 25 |
Calculate the percentage demand for naphtha.
Suggest why fuel oil, rather than diesel oil, is cracked to obtain petrol.