Where do fuels and plastics come from, and what are hydrocarbons?
Crude oil, hydrocarbons and the alkanes, fractional distillation and the uses of fractions, the properties of hydrocarbons and combustion, and cracking to produce alkenes and more useful products.
A focused answer to the AQA GCSE Combined Science: Trilogy Organic chemistry topic, covering crude oil and hydrocarbons, the alkanes, fractional distillation and the uses of fractions, combustion, and cracking to make alkenes.
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What this topic is asking
AQA wants you to describe crude oil as a source of hydrocarbons, give the alkanes and their general formula, describe fractional distillation and the uses of fractions, explain trends in hydrocarbon properties and combustion, and describe cracking to make alkenes.
Crude oil and the alkanes
Alkanes are saturated hydrocarbons (they contain only single carbon-to-carbon bonds, so they hold the maximum number of hydrogen atoms) with the general formula . The first four are methane (), ethane (), propane () and butane (). You can use the general formula to find the formula of any alkane: for (pentane) the formula is .
Fractional distillation and properties
As hydrocarbon chains get longer, the intermolecular forces between molecules increase, so the molecules become more viscous (thicker), less volatile, less flammable and have higher boiling points. Hydrocarbons are used mostly as fuels because their combustion releases a lot of energy. Complete combustion (plenty of oxygen) produces only carbon dioxide and water; incomplete combustion (limited oxygen) can produce carbon monoxide (toxic) and soot.
Cracking and alkenes
Cracking breaks long-chain hydrocarbons into shorter, more useful ones. The two methods are catalytic cracking (passing vapour over a hot catalyst) and steam cracking (mixing with steam and heating to a high temperature). Cracking is economically important because demand for short-chain hydrocarbons (such as petrol) is much greater than the amount obtained from distillation, so cracking converts the surplus long chains into valuable products and matches supply to demand.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20194 marksExplain how fractional distillation separates crude oil into useful fractions, referring to the boiling points of the hydrocarbons.Show worked answer →
A Chemistry Paper 2 explanation. Reward: crude oil is a mixture of hydrocarbons with different chain lengths and therefore different boiling points. The oil is heated so it evaporates and the vapours enter the fractionating column, which is hot at the bottom and cooler at the top. As the vapours rise they cool, and each fraction condenses at the level where the temperature matches its boiling point. Hydrocarbons with longer chains have higher boiling points and condense low down, while shorter chains condense higher up; the fractions are tapped off at different heights. Markers credit the temperature gradient in the column, condensing at the boiling point, and the link between chain length and boiling point.
AQA 20214 marksDescribe the test that distinguishes an alkene from an alkane, and explain why cracking long-chain hydrocarbons is economically important.Show worked answer →
A Chemistry Paper 2 question on organic reactions. Reward the bromine-water test: add orange bromine water to each; an alkene decolourises it (turns it from orange to colourless) because it is unsaturated and reacts across the carbon-to-carbon double bond, whereas an alkane (saturated) leaves it orange. For cracking: it breaks long-chain hydrocarbons, for which demand is low, into shorter, more useful molecules such as petrol and into alkenes used to make polymers, so it matches supply to demand and turns less useful fractions into valuable products. Markers reward the correct colour change with the saturated and unsaturated reasoning, and the supply-and-demand point.
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