Alkenes and cracking Cambridge IGCSE Co-ordinated Sciences (Double Award) revision
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In plain words
Alkenes are hydrocarbons with a double bond between two of the carbon atoms. That double bond is a weak spot that can open up and grab other atoms, which makes alkenes far more reactive than alkanes, and the starting point for plastics. They are made by cracking big alkanes that nobody wants into smaller molecules that everybody wants.
Three things to know
- Alkenes are unsaturated: they contain a C=C double bond. General formula CₙH₂ₙ: ethene C₂H₄, propene C₃H₆.
- Cracking breaks long alkanes into shorter alkanes and alkenes, using a high temperature and a catalyst.
- Alkenes do addition reactions: the double bond opens and atoms add on. Bromine water turns from orange to colourless (the test for an alkene). With hydrogen they give alkanes; with steam they give alcohols.
Worked example
Decane, C₁₀H₂₂, is cracked into octane, C₈H₁₈, and one other product. What is it?
- Atoms can't be lost, so take octane's atoms away from decane's.
- Carbon: 10 − 8 = 2. Hydrogen: 22 − 18 = 4.
- C₂H₄, which is ethene.
Tips and tricks
- The test: bromine water goes from orange to colourless with an alkene, and stays orange with an alkane. Say "colourless", not "clear".
- In an addition reaction two molecules become one. Nothing else is made.
It lands in your notebook with its questions as flashcards.
Alkenes and cracking: 6 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What is the functional group of the alkenes?
C=C is what makes them reactive.
What is seen when an alkene is shaken with bromine water?
The bromine adds across the double bond.
What does cracking a long-chain alkane produce?
A big molecule is broken into smaller, more useful ones.
What is made when ethene reacts with hydrogen?
The hydrogen adds across the double bond.
What type of reaction do alkenes have with bromine?
The two molecules join to make one product.
What is the formula of propene?
Three carbons and twice as many hydrogens.
Quiz
6 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
4 questions, 8 marks. Write your answers on paper, then check them.
Alkenes and cracking
Cambridge IGCSE Co-ordinated Sciences (Double Award) 0654 · 8 marks · papermunch.org
Name ______________________________ Date ______________
Describe a test to tell ethene from ethane, and give the result for each.[2]
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Add bromine water. With ethene it changes from orange to colourless. With ethane it stays orange.
State what cracking is, and why it is carried out.[2]
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Breaking long-chain alkanes into shorter alkanes and alkenes. The shorter molecules are more useful and in greater demand (as fuels, and for making plastics).
Write the equation for the reaction of ethene with bromine, and name the type of reaction.[2]
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C₂H₄ + Br₂ → C₂H₄Br₂. Addition.
Name the product when ethene reacts with (a) hydrogen and (b) steam.[2]
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(a) Ethane. (b) Ethanol.
Answers: Alkenes and cracking
- 1. Add bromine water. With ethene it changes from orange to colourless. With ethane it stays orange.
- 2. Breaking long-chain alkanes into shorter alkanes and alkenes. The shorter molecules are more useful and in greater demand (as fuels, and for making plastics).
- 3. C₂H₄ + Br₂ → C₂H₄Br₂. Addition.
- 4. (a) Ethane. (b) Ethanol.