Crude oil, fuels and pollutantsEdexcel International A Level Chemistry: Revision notes

Crude oil is a mixture of hydrocarbons, mostly alkanes. It is separated by fractional distillation: the oil is heated and the vapour passes into a column that is hottest at the bottom and coolest at the top.

Each molecule rises until the temperature is below its boiling point, where it condenses and is drawn off. Longer chains have more electrons and a larger surface area, so London forces between molecules are stronger, the boiling point is higher and the fraction is collected lower down. Short-chain fractions (refinery gas, petrol) leave near the top; diesel and the residue leave near the bottom.

The process separates but does not change the molecules, and it gives more of the heavy fractions than are in demand.

Cracking breaks long-chain alkanes from the heavy fractions into shorter, more useful molecules: a shorter alkane and an alkene. Demand for petrol and for alkenes (polymer feedstocks) is greater than the supply from distillation.

• Thermal cracking uses high temperature and pressure and mainly gives alkenes.
• Catalytic cracking uses a zeolite catalyst at about 450 °C and gives branched alkanes and aromatic hydrocarbons as well.

Example: $C_{10}H_{22} \rightarrow C_8H_{18} + C_2H_4$. Equations must balance for both C and H.

Reforming converts straight-chain alkanes into branched alkanes and cyclic or aromatic hydrocarbons of a similar number of carbon atoms, using a platinum catalyst. The number of carbons stays the same, and hydrogen is a by-product of ring formation.

Examples: $C_6H_{14} \rightarrow C_6H_{12} + H_2$ (cyclohexane) and $C_6H_{14} \rightarrow C_6H_6 + 4H_2$ (benzene).

Branched and cyclic hydrocarbons burn more smoothly in an engine, so reforming improves the quality of petrol.

Alkanes burn in plenty of air to give carbon dioxide and water: $C_8H_{18} + 12\tfrac{1}{2}O_2 \rightarrow 8CO_2 + 9H_2O$. In a limited supply of oxygen, incomplete combustion gives carbon monoxide or carbon particulates (soot) instead.

Other pollutants from fuels:

• Oxides of nitrogen (NO, NO₂) form when nitrogen and oxygen from the air react at the high temperature in an engine: $N_2 + O_2 \rightarrow 2NO$.
• Sulfur dioxide forms from sulfur compounds in the fuel: $S + O_2 \rightarrow SO_2$.
• Unburned hydrocarbons leave the engine unchanged.
• Carbon particulates (soot) are produced by incomplete combustion.

Carbon monoxide is toxic because it binds to the iron in haemoglobin much more strongly than oxygen does. The blood then carries much less oxygen to the tissues, causing headaches, unconsciousness and death. It is colourless and odourless, so it is not noticed.

Oxides of nitrogen and sulfur dioxide dissolve in rain water to form nitric acid and sulfuric acid (via sulfurous acid), so the rain becomes acidic. Acid rain damages plants, kills aquatic life and corrodes buildings and metals.

Crude oil is finite, so fuels from it are not sustainable, and burning them releases carbon dioxide, a greenhouse gas that absorbs infrared radiation and drives climate change. Alternative fuels are developed to conserve resources and cut emissions.

A fuel is carbon neutral if the CO₂ released on burning equals the CO₂ taken in when it was made.

• Petrol: not neutral; it releases carbon locked away for millions of years.
• Bioethanol: from fermented crops; the CO₂ absorbed by photosynthesis offsets the CO₂ released, so approximately neutral, but fossil energy for farming and distillation reduces the benefit.
• Hydrogen: burns to water only ($2H_2 + O_2 \rightarrow 2H_2O$); neutral only if made using renewable electricity, not from methane.