Useful hydrocarbons are obtained by the fractional distillation of crude oil, which itself has little or no direct use - Leaving Cert Chemistry - Question 6 - 2007

1. Pentane
2. 2-Methylbutane
3. 2,2-Dimethylpropane

2,2-Dimethylpropane would have the lowest octane number due to its highly branched structure, which tends to be less stable under combustion.

The branching in 2,2-Dimethylpropane leads to a lower surface area for reactions compared to the straight-chain pentane, resulting in lower combustion efficiency and hence a reduced octane number.

The balanced chemical equation is:

$C_5H_{12} + 8O_2 \rightarrow 5CO_2 + 6H_2O$

Naphtha molecules are generally smaller, containing fewer carbon atoms and less mass compared to gas oil molecules, which are larger, heavier, and have longer carbon chains.

The separation occurs in a distillation column. As crude oil is heated, it vaporizes and rises through the column. The temperature gradient allows lighter fractions like naphtha to condense and be collected at lower heights, while heavier fractions remain vaporized at higher temperatures.

Catalytic cracking is a process used in oil refining to break down larger hydrocarbon molecules into smaller, more valuable products using heat and a catalyst.

Catalytic cracking increases the yield of lighter hydrocarbons (like gasoline) from crude oil, catering to higher demand and improving the efficiency of fuel production.