Alkanes: single bonds, combustion and reaction limits
Connect saturated hydrocarbon bonding with general low reactivity, while recognising combustion and chlorine substitution as exceptions.
Section C12.4 · Alkanes
Cross-referenced from the related Cambridge IGCSE Chemistry 0620 or Physics 0625 teaching to the 0652 syllabus. Exact 0652 Core/Supplement objective coverage and this final-series course’s examiner fit still need specialist review. Written practice is self-checked and does not predict a Cambridge grade or demonstrate supervised laboratory competence.
Before you begin
- Read the alkane general formula and distinguish C–C from C=C.
- Balance ordinary chemical equations and distinguish complete from incomplete combustion.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does hydrocarbon mean?
What you will learn
- Describe alkanes as saturated hydrocarbons with single covalent bonding.
- Explain why generally unreactive does not mean unable to combust or react with chlorine.
Start with both parts of the classification
Alkanes are hydrocarbons: their molecules contain carbon and hydrogen only. Their bonding is single covalent, and they are saturated because their carbon–carbon bonds are single. Ethane CH₃CH₃ and propane CH₃CH₂CH₃ illustrate this pattern. A compound containing oxygen is not a hydrocarbon merely because its carbon chain has single bonds.
Alkanes are generally unreactive in the reactions considered here, but the syllabus explicitly includes combustion and substitution by chlorine as exceptions. Do not turn a general description into a claim that they never react. In the usual aqueous-bromine test without ultraviolet irradiation, an alkane retains the bromine colour; the detailed alkene comparison follows separately.
Combustion depends on the oxygen supply
Complete combustion with sufficient oxygen produces carbon dioxide and water. For methane, CH₄+2O₂→CO₂+2H₂O; for propane, C₃H₈+5O₂→3CO₂+4H₂O. A balanced equation preserves each element: it does not mean carbon and hydrogen remain joined in their original molecule. Combustion releases energy, which helps explain alkane fuels.
Limited oxygen can lead to incomplete combustion, with carbon monoxide and/or carbon as carbon-containing products as well as water. Carbon monoxide is toxic. A combustion question that specifies complete combustion calls for CO₂ and H₂O; a question stating limited oxygen cannot be answered by assuming only CO₂. Chlorine substitution is a different reaction family; the Extended lesson explains ultraviolet light and the replacement products.
Sources for this lesson
Pause and explain
Which claim about alkanes is correct?
Worked example
Explain why propane is an alkane that can still burn. Balance its complete combustion.
Show the worked solution
- CH₃CH₂CH₃ contains only C and H and has single covalent bonds; its carbon–carbon bonds are single.
- Complete combustion places three C atoms in 3CO₂ and eight H atoms in 4H₂O.
- Those products contain ten O atoms, so use 5O₂: C₃H₈+5O₂→3CO₂+4H₂O.
Answer Propane is a saturated hydrocarbon; C₃H₈+5O₂→3CO₂+4H₂O is its complete combustion.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Classify ethane CH₃CH₃ and state the two required exceptions to general alkane unreactivity.
Give me a hint
Check composition and carbon–carbon bonding separately.
Compare my reasoning
- Ethane contains only C and H, so it is a hydrocarbon.
- Its covalent bonds are single and its carbon–carbon bond is single, so it is saturated.
- Combustion and substitution by chlorine are the two stated exceptions.
Ethane is a saturated, single-bonded hydrocarbon; it can combust and undergo chlorine substitution.
Look for these in your work
- Both composition and bonding support the classification.
- The two reaction exceptions are named.
Balance complete combustion of ethane C₂H₆ using whole-number coefficients.
Give me a hint
Balance C, then H, then O; multiply the whole equation if needed.
Compare my reasoning
- One C₂H₆ produces 2CO₂ and 3H₂O.
- The products need seven O atoms, equivalent to 3.5O₂.
- Multiply every coefficient by two: 2C₂H₆+7O₂→4CO₂+6H₂O.
2C₂H₆+7O₂→4CO₂+6H₂O.
Look for these in your work
- Carbon, hydrogen and oxygen totals agree.
- Subscripts stay unchanged and all coefficients are whole numbers.
A report says an alkane fuel cannot burn because it is saturated, then claims limited oxygen guarantees only carbon dioxide. Correct both statements.
Give me a hint
Saturation is a bond description, while the oxygen supply affects combustion products.
Compare my reasoning
- Saturated means its carbon–carbon bonds are single; combustion is still a stated alkane reaction.
- Sufficient oxygen gives complete combustion with carbon dioxide and water.
- Limited oxygen can produce carbon monoxide and/or carbon as well as water, so only carbon dioxide is not guaranteed.
Saturated alkanes can burn; limited oxygen can give carbon monoxide and/or carbon rather than only carbon dioxide, alongside water.
Look for these in your work
- Bond classification and reactivity are separated.
- The oxygen condition is used to assess the product claim.
Common mistakes
- Interpreting generally unreactive as never reacting.
- Calling any molecule with single carbon–carbon bonds a hydrocarbon despite other elements.
- Changing atom subscripts to balance combustion.
- Assuming complete combustion when the question states an insufficient oxygen supply.
What can you explain now?
Which two reaction exceptions are required for generally unreactive alkanes?
Compare with the explanation
Combustion and substitution by chlorine.
Their low general reactivity does not exclude either specified reaction.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain alkane composition, bonding and the two reaction exceptions. Balance a new complete-combustion equation without altering a subscript.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.