Alkanes Cambridge IGCSE Combined Science revision
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In plain words
The alkanes are the simplest hydrocarbons: chains of carbon atoms with as many hydrogens attached as will fit. They don't do much chemistry, but they do one thing extremely well, which is burn. Methane heats homes; petrol and diesel are mixtures of alkanes.
Three things to know
- Alkanes are saturated hydrocarbons (single bonds only), with the general formula CₙH₂ₙ₊₂: methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀.
- With plenty of oxygen they burn completely to carbon dioxide and water. With too little oxygen, combustion is incomplete and gives poisonous carbon monoxide and soot.
- In ultraviolet light an alkane reacts with chlorine: a chlorine atom takes the place of a hydrogen atom. CH₄ + Cl₂ → CH₃Cl + HCl. This is substitution.
Worked example
Balance the equation for the complete combustion of propane: C₃H₈ + O₂ → CO₂ + H₂O.
- Carbon first: 3 carbons, so 3CO₂. Hydrogen next: 8 hydrogens, so 4H₂O.
- Count the oxygen atoms on the right: (3 × 2) + 4 = 10, so 5O₂.
- C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.
Tips and tricks
- Balance combustion equations in the order C, then H, then O last.
- Complete combustion: carbon dioxide and water. Incomplete: carbon monoxide (toxic) or carbon (soot), and water.
It lands in your notebook with its questions as flashcards.
Alkanes: 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 kind of bonds join the carbon atoms in alkanes?
That is what saturated means.
What are the products when an alkane burns completely?
There is enough oxygen to turn all the carbon into carbon dioxide.
What is needed for methane to react with chlorine?
The light provides the energy to start the reaction.
What type of reaction happens between an alkane and chlorine?
A chlorine atom replaces a hydrogen atom.
What is the formula of ethane?
Two carbons, and (2 × 2) + 2 = 6 hydrogens.
Which poisonous gas forms when an alkane burns in too little oxygen?
It is a product of incomplete combustion.
Quiz
6 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
4 questions, 7 marks. Write your answers on paper, then check them.
Alkanes
Cambridge IGCSE Combined Science 0653 · 7 marks · papermunch.org
Name ______________________________ Date ______________
Write the balanced equation for the complete combustion of methane.[2]
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CH₄ + 2O₂ → CO₂ + 2H₂O.
Name a product of the incomplete combustion of methane, and state why it is dangerous.[2]
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Carbon monoxide: it is toxic. (Or carbon/soot: it harms the lungs.)
Methane reacts with chlorine. State the condition needed and name the two products.[2]
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Ultraviolet light. Chloromethane and hydrogen chloride.
Explain why alkanes are described as saturated.[1]
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All the bonds between their carbon atoms are single bonds.
Answers: Alkanes
- 1. CH₄ + 2O₂ → CO₂ + 2H₂O.
- 2. Carbon monoxide: it is toxic. (Or carbon/soot: it harms the lungs.)
- 3. Ultraviolet light. Chloromethane and hydrogen chloride.
- 4. All the bonds between their carbon atoms are single bonds.