What are alkanes and alkenes, and why is cracking used?
Alkanes and alkenes, the test for an alkene, the combustion of hydrocarbons, and cracking to make smaller useful molecules.
A focused answer to the WJEC GCSE Science Double Award Unit 5 topic on organic chemistry, covering alkanes and alkenes and the bromine-water test, the combustion of hydrocarbons, and cracking to make smaller, more useful molecules.
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What this dot point is asking
WJEC Double Award Unit 5 wants you to describe alkanes and alkenes and the test for an alkene, the combustion of hydrocarbons, and cracking.
Alkanes
Alkanes are fairly unreactive but burn well, which is why they are used as fuels. They are the main hydrocarbons in the fractions of crude oil.
Alkenes
The double bond makes alkenes more reactive than alkanes. Alkenes are used to make polymers (plastics) and other chemicals.
The test for an alkene
Combustion of hydrocarbons
When a hydrocarbon burns completely in plenty of oxygen, it produces carbon dioxide and water and releases energy. With too little oxygen, incomplete combustion produces toxic carbon monoxide and soot (carbon). This is why hydrocarbon fuels need a good air supply to burn cleanly and safely.
Making polymers from alkenes
A major use of alkenes is to make polymers (plastics). Because they have a reactive double bond, many small alkene molecules (monomers) can join together to make a very long chain molecule called a polymer, in a reaction called polymerisation. For example, many ethene molecules join to make poly(ethene) (polythene), used for plastic bags and bottles. This is the opposite of cracking: cracking breaks big molecules into small ones, while polymerisation joins small ones into big ones. Knowing that alkenes are the building blocks of plastics is a common exam link.
The problem with plastics
Most polymers are not biodegradable, meaning microorganisms cannot break them down, so plastic waste can stay in the environment for a very long time, causing pollution. Burning plastics releases carbon dioxide and can produce toxic gases. The problem can be reduced by recycling plastics, using less, and developing biodegradable plastics. This links the useful products of organic chemistry to their environmental impact, which is often asked about in evaluation questions.
Cracking
Try this
Q1. What type of bond makes a hydrocarbon unsaturated? [1 mark]
- Cue. A carbon-carbon double bond (C=C).
Q2. What does cracking produce that is used to make plastics? [1 mark]
- Cue. Alkenes.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksDescribe the difference between an alkane and an alkene, and how you would test which is which.Show worked answer →
A Unit 5 describe question worth 4 marks. Reward: alkanes are saturated (only single bonds between carbons), while alkenes are unsaturated (they have a carbon-carbon double bond) (2); to test, add bromine water: with an alkene the orange bromine water is decolourised (turns colourless), while with an alkane it stays orange (2). Markers credit the saturated/unsaturated difference and the bromine water test with the colour changes. A common error is to get the colour change the wrong way round.
WJEC style4 marksExplain why cracking is carried out and what is produced.Show worked answer →
A Unit 5 explain question worth 4 marks. Reward: there is more demand for small, useful molecules (such as petrol) than the supply from crude oil, and a surplus of large molecules (1); cracking breaks large hydrocarbon molecules into smaller, more useful ones (1); using heat and a catalyst (1); it produces smaller alkanes (used as fuels) and alkenes (used to make plastics/polymers) (1). Markers credit the demand for small molecules, breaking large into small, and the products. A common error is to say cracking joins molecules together.
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