Global challenges Study Guide

Crude oil is a complex mixture of many different hydrocarbons, which are compounds made of hydrogen and carbon atoms. These hydrocarbons have different boiling points, a property that allows them to be separated. Fractional distillation is the process used to achieve this separation. The crude oil is first heated to a very high temperature, typically around 350-400 °C, causing most of it to vaporise. This hot mixture of liquid and vapour is then pumped into the bottom of a tall fractionating column. Inside the column, there is a temperature gradient, meaning it is hottest at the bottom and coolest at the top. As the hot vapours rise up the column, they cool and condense at different levels based on their boiling points. Hydrocarbons with high boiling points (larger molecules) condense at lower levels where it is hotter, while hydrocarbons with low boiling points (smaller molecules) continue to rise higher before they cool enough to condense at cooler, upper levels. The condensed liquids, called fractions, are then collected at various levels as separate products. This fundamental process allows crude oil to be refined into useful products like petrol, diesel, kerosene, and bitumen, each with a specific range of boiling points and molecular sizes.

Addition polymerisation is a process where many small, identical molecules, called monomers, join together to form a very large molecule, called a polymer. This type of polymerisation specifically occurs with monomers that are alkenes, meaning they contain a carbon-carbon double bond (C=C). During addition polymerisation, the carbon-carbon double bond in each monomer breaks open, allowing the individual monomer units to link together in a long chain without the loss of any atoms. The resulting polymer has a repeating unit that contains all the atoms of the original monomer, but with the double bond replaced by single bonds to adjacent monomer units.

To deduce the structure of an addition polymer from a simple alkene monomer, you essentially remove the double bond from the monomer and show single bonds extending from the carbons that were originally part of the double bond. These single bonds then connect to the next repeating unit. For example, if the monomer is ethene (CH₂=CH₂), the double bond breaks. The polymer, poly(ethene), will have a repeating unit of -CH₂-CH₂- connected by single bonds to form a long chain. We often represent the repeating unit in square brackets with an 'n' subscript to indicate many such units. For chloroethene (vinyl chloride), CH₂=CHCl, the polymer is poly(chloroethene) or PVC, with a repeating unit of -CH₂-CHCl-.

Conversely, to identify the monomer from an addition polymer's structure, you need to locate the repeating unit within the polymer chain. Once identified, you can recreate the original double bond by placing it between the two carbons that were involved in forming the polymer chain from that repeating unit. For example, if you see a polymer chain made of repeating -CH(CH₃)-CH₂- units, the monomer would be propene, CH₂=CH(CH₃). It is crucial to remember that addition polymerisation uses alkenes, so the monomer will always have a carbon-carbon double bond.