Global challenges Study Guide

Crude oil is a naturally occurring fossil fuel formed from the remains of ancient marine organisms over millions of years under high pressure and temperature. It is a complex mixture primarily composed of hydrocarbons, which are organic compounds containing only hydrogen and carbon atoms. These hydrocarbons have varying lengths of carbon chains, leading to a wide range of properties and uses. Crude oil is not used directly but is separated into useful fractions through a process called fractional distillation. Each fraction contains hydrocarbons with a similar boiling point range. Examples of these fractions include gasoline (petrol), diesel, kerosene, and bitumen. Beyond its use as a fuel, crude oil is critically important as a primary 'feedstock' for the petrochemical industry. A feedstock is a raw material used to supply a chemical process or industrial plant. The petrochemical industry uses these fractions to produce a vast array of synthetic materials and chemicals. For instance, some fractions are cracked (broken down) into smaller, more reactive molecules called alkenes, which are then used as monomers for polymer production (e.g., plastics like poly(ethene) and poly(propene)). Other petrochemical products include artificial fibres, solvents, fertilisers, and pharmaceuticals. Understanding crude oil's composition and its role as a feedstock is fundamental to appreciating its economic significance and its impact on modern society.

Electrolysis is a chemical process that uses electrical energy to drive non-spontaneous redox reactions. It is a crucial method for extracting reactive metals that cannot be reduced by carbon, such as aluminium, sodium, and magnesium, from their ores. These metals are typically found as compounds (e.g., oxides) in their ores, which are very stable. Carbon reduction is only effective for metals lower in the reactivity series than carbon. For highly reactive metals, carbon is not a strong enough reducing agent to displace the metal from its ore. Therefore, electrolysis provides the necessary energy to break these strong ionic bonds.

The process works by dissolving or melting the metal compound, usually an oxide, to allow its ions to move freely. For example, in aluminium extraction, purified aluminium oxide (Al2O3) is dissolved in molten cryolite (Na3AlF6) at about 950°C. This molten mixture acts as the electrolyte. A direct electric current is then passed through the electrolyte using electrodes. The positively charged metal ions (cations) are attracted to the negative electrode (cathode), where they gain electrons (are reduced) to form the pure molten metal. Simultaneously, the negatively charged non-metal ions (anions), such as oxide ions, are attracted to the positive electrode (anode), where they lose electrons (are oxidised) to form a non-metal, often oxygen gas. In the case of aluminium extraction, the oxygen produced at the carbon anodes reacts with the carbon to form carbon dioxide, causing the anodes to gradually wear away and requiring regular replacement. Electrolysis is an energy-intensive process due to the high temperatures required to keep the electrolyte molten and the continuous electrical energy input, making it an expensive method of metal extraction.