Alkanes Revision Guide
Quick Revision Summary (Key Takeaway)
Alkanes are saturated hydrocarbons with the general formula CnH2n+2, containing only single C-C and C-H bonds. They are relatively unreactive due to strong C-C and C-H bonds, but undergo combustion and free-radical substitution with halogens in UV light.
Topic Overview
Alkanes are the simplest family of organic compounds, consisting solely of carbon and hydrogen atoms joined by single covalent bonds. They are saturated hydrocarbons, meaning they contain the maximum possible number of hydrogen atoms for a given number of carbon atoms. The general formula for alkanes is CnH2n+2, and they form a homologous series where each successive member differs by a CH2 group. Their structure is tetrahedral around each carbon atom, with bond angles of approximately 109.5°.
Alkanes are relatively unreactive due to the high strength of the C-C and C-H bonds, which are non-polar. However, they undergo two important reactions: combustion, which is highly exothermic and makes alkanes valuable as fuels, and free-radical substitution with halogens in the presence of UV light. This latter reaction is a key mechanism in organic chemistry, illustrating how covalent bonds can break homolytically to form free radicals.
Understanding alkanes is fundamental to organic chemistry, as it introduces concepts such as homologous series, structural isomerism, and reaction mechanisms. Alkanes also provide a basis for understanding more complex organic molecules, such as alkenes and alcohols, and their industrial importance as fuels and feedstock for the petrochemical industry.
Key Concepts
Core ideas you must understand for this topic
- →General formula CnH2n+2 and the concept of saturation (only single bonds).
- →Structural isomerism: compounds with the same molecular formula but different structural arrangements, e.g., butane and 2-methylpropane.
- →Non-polar nature of alkanes, leading to insolubility in water and solubility in non-polar solvents.
- →Combustion reactions: complete combustion produces CO2 and H2O; incomplete combustion produces CO or C and H2O.
- →Free-radical substitution mechanism: initiation (homolytic fission), propagation, and termination steps.
What You Need to Demonstrate
Key skills and knowledge for this topic
- Definition of alkanes as saturated hydrocarbons
- Explanation of fractional distillation of crude oil
- Distinction between thermal and catalytic cracking conditions and products
- Economic reasons for cracking alkanes
- Products of complete and incomplete combustion
- Removal of sulfur dioxide from flue gases using calcium oxide or calcium carbonate
- Stages of free-radical substitution: initiation, propagation, and termination
- Writing balanced equations for free-radical substitution steps
Marking Points
Key points examiners look for in your answers
- Definition of alkanes as saturated hydrocarbons
- Explanation of fractional distillation of crude oil
- Distinction between thermal and catalytic cracking conditions and products
- Economic reasons for cracking alkanes
- Products of complete and incomplete combustion
- Removal of sulfur dioxide from flue gases using calcium oxide or calcium carbonate
- Stages of free-radical substitution: initiation, propagation, and termination
- Writing balanced equations for free-radical substitution steps
Examiner Tips
Expert advice for maximising your marks
- 💡Ensure you can clearly distinguish between the conditions for thermal and catalytic cracking.
- 💡Practice writing the three stages of free-radical substitution for methane and chlorine.
- 💡Be prepared to explain the environmental impact of combustion pollutants and how catalytic converters or flue gas desulfurization mitigate these.
- 💡Remember that curly arrows are NOT required for free-radical mechanisms.
- 💡When drawing mechanisms, always use 'fish-hook' arrows to show the movement of single electrons in homolytic fission. This is a common place to lose marks.
- 💡For combustion equations, always balance them carefully. Remember that the general equation for complete combustion of an alkane is CnH2n+2 + (1.5n+0.5)O2 → nCO2 + (n+1)H2O.
- 💡When asked about the reactivity of alkanes, mention the strength of the C-C and C-H bonds and the non-polar nature of the molecule. Avoid vague statements like 'they are unreactive because they are stable'.
Common Mistakes
Pitfalls to avoid in your exam answers
- Confusing the conditions for thermal versus catalytic cracking
- Failing to identify the specific steps (initiation, propagation, termination) in free-radical mechanisms
- Omitting the dot notation for radicals in mechanisms
- Incorrectly balancing equations for incomplete combustion
- Misunderstanding the economic necessity for cracking long-chain alkanes
- Misconception: Alkanes are reactive because they have many bonds. Correction: Alkanes are relatively unreactive because the C-C and C-H bonds are strong and non-polar, making them less susceptible to attack by polar reagents.
- Misconception: Incomplete combustion only produces carbon monoxide. Correction: Incomplete combustion can also produce carbon (soot) and water, depending on the oxygen supply.
- Misconception: Free-radical substitution always produces a single product. Correction: It often produces a mixture of products, including isomers and multiple substitution products, because radicals can react in various ways.
Revision Plan
How to revise this topic in 1–2 weeks
- 1Day 1-2: Review the structure and naming of alkanes, including the first ten members of the series and their structural isomers.
- 2Day 3-4: Focus on physical properties: boiling points, solubility, and trends down the series. Relate to intermolecular forces.
- 3Day 5-6: Study combustion reactions, including complete and incomplete combustion, and practice balancing equations.
- 4Day 7-8: Learn the free-radical substitution mechanism in detail, including initiation, propagation, and termination steps. Practice drawing the mechanism.
- 5Day 9-10: Attempt past exam questions and mark-scheme style answers. Focus on common pitfalls and examiner tips.
Exam Question Types
How this topic typically appears in the exam
- 📋Naming and drawing isomers: You may be given a molecular formula and asked to draw all possible structural isomers. Practice with C4H10 and C5H12.
- 📋Balancing combustion equations: Questions often ask for balanced equations for complete or incomplete combustion. Ensure you can balance them accurately.
- 📋Free-radical substitution mechanism: You may be asked to write a mechanism for the reaction of methane with chlorine or bromine. Use fish-hook arrows and label each step.
- 📋Multiple-choice questions on properties: These may test understanding of boiling point trends, solubility, and reactivity.
Command Word Expectations (AQA)
What examiners look for when using specific command words in this specification
Give a concise answer without explanation, e.g., 'State the general formula of alkanes' – answer: CnH2n+2.
Provide a reason or mechanism for a phenomenon, e.g., 'Explain why alkanes are insoluble in water' – must include reference to non-polarity and lack of hydrogen bonding.
Work out from given information, e.g., 'Deduce the molecular formula of an alkane with molar mass 72' – show calculations.
How Students Lose Marks (Examiner Pitfalls)
Common mark loss traps and how to write 100% full-mark answers
Step-by-Step Worked Solutions
Detailed solution breakdown for typical exam problems
Question: A sample of an alkane has a molar mass of 72.0 g mol⁻¹. Determine its molecular formula and draw the structures of all possible isomers.
- 1.Step 1: Use the general formula for alkanes: CnH2n+2. The molar mass is 12n + (2n+2) = 14n + 2.
- 2.Step 2: Set 14n + 2 = 72. Solve for n: 14n = 70, so n = 5.
- 3.Step 3: The molecular formula is C5H12. Draw the three structural isomers: pentane, 2-methylbutane, and 2,2-dimethylpropane.
Question: Write the balanced equation for the complete combustion of butane (C4H10) and calculate the volume of oxygen (at room temperature and pressure) needed to completely combust 0.10 mol of butane. (Molar volume at RTP = 24.0 dm³ mol⁻¹)
- 1.Step 1: Write the balanced equation: 2C4H10 + 13O2 → 8CO2 + 10H2O.
- 2.Step 2: Determine the mole ratio: 2 mol butane reacts with 13 mol oxygen, so 0.10 mol butane reacts with (13/2) × 0.10 = 0.65 mol oxygen.
- 3.Step 3: Calculate volume: volume = moles × molar volume = 0.65 × 24.0 = 15.6 dm³.
Active Recall Memory Test
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Frequently Asked Questions
Common questions students ask about this topic
Sample Exam Questions
Worked examples for AQA A-Level Alkanes — try each before revealing the answer