What are alkanes and alkenes, and why do we crack hydrocarbons?
Alkanes and alkenes as saturated and unsaturated hydrocarbons, their general formulae and combustion, the bromine water test for unsaturation, and cracking.
A CCEA GCSE Chemistry answer on alkanes and alkenes, covering saturated and unsaturated hydrocarbons and their general formulae, complete and incomplete combustion, the bromine water test for the carbon-carbon double bond, and the cracking of long-chain hydrocarbons.
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What this dot point is asking
CCEA wants you to describe alkanes and alkenes as saturated and unsaturated hydrocarbons, give their general formulae, describe complete and incomplete combustion, use the bromine water test for a double bond, and explain cracking.
Alkanes and alkenes
The double bond makes alkenes more reactive than alkanes, because it can open up to add other atoms across it. This is the basis of both the bromine water test and addition polymerisation.
Combustion
A typical complete combustion: . Incomplete combustion is dangerous because carbon monoxide is poisonous and odourless.
The bromine water test
Cracking
Cracking is done because there is more demand for short-chain fuels such as petrol than there is supply from crude oil, while the long-chain fractions are in surplus. A typical example: (octane plus ethene).
Worked example
Examples in context
Example 1. Meeting demand for petrol. Refineries crack surplus long-chain fractions to make extra petrol, matching supply to the high demand for car fuel. Cracking is therefore an economic as well as a chemical process, turning low-value oil into high-value fuel.
Example 2. Carbon monoxide danger. Faulty gas heaters that burn fuel with too little oxygen produce carbon monoxide, which is why homes have carbon monoxide alarms. The incomplete combustion studied here has direct safety consequences.
Try this
Q1. State the general formula of an alkene. [1 mark]
- Cue. .
Q2. State why long-chain hydrocarbons are cracked. [1 mark]
- Cue. To make more of the short-chain fuels (and alkenes) that are in higher demand.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20184 marksDescribe the test that distinguishes an alkane from an alkene, including the reagent and the results for each.Show worked answer →
Markers want the reagent and both results.
The test uses bromine water (orange).
Add bromine water to each hydrocarbon and shake. With an alkene (which has a carbon-carbon double bond), the bromine water is decolourised (turns from orange to colourless), because the bromine adds across the double bond.
With an alkane (which is saturated, no double bond), there is no reaction and the bromine water stays orange.
So decolourising bromine water shows an alkene; no change shows an alkane.
Markers reward bromine water as the reagent, decolourised by an alkene, and unchanged by an alkane.
CCEA 20214 marksExplain why long-chain hydrocarbons are cracked, name the conditions, and state the type of products formed.Show worked answer →
The marks are for the reason, the conditions and the products.
Long-chain hydrocarbons are less useful and in less demand than short-chain ones, so they are cracked to make more of the short-chain fractions (such as petrol) that are in high demand, and to make alkenes for plastics.
The conditions are a high temperature and a catalyst (catalytic cracking), or high temperature and steam.
Cracking breaks a long alkane into a shorter alkane and an alkene (such as ethene).
Markers reward the demand reason, high temperature and catalyst, and the products of a shorter alkane plus an alkene.
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Sources & how we know this
- CCEA GCSE Chemistry specification (1110) — CCEA (2017)