CCEA A Level Chemistry
CCEA A Level Chemistry 2.3 Alkanes Exam Questions
Free CCEA A Level Chemistry revision for 2.3 Alkanes: what you need to know, common mistakes and exam-style questions with answers. Covers saturated hydrocarbons, boiling point trends, combustion, pollutants, catalytic converters, radical substitution with chlorine and bromine, homolytic and heterolytic fission.
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About CCEA A Level Chemistry 2.3 Alkanes
Alkanes are section 2.3 of Unit AS 2. You need to explain boiling point trends with chain length and branching using van der Waals forces, write equations for complete and incomplete combustion, and link the type of flame to carbon content. CCEA asks about pollutants (carbon monoxide, carbon particles, oxides of nitrogen and sulfur, unburned hydrocarbons), the rise in carbon dioxide and global warming, and how catalytic converters work. The key mechanism is radical substitution with chlorine or bromine in ultraviolet light, with initiation, propagation and termination steps. You also need to define radical and distinguish homolytic and heterolytic fission. This content links to energetics and to halogenoalkanes.
What you need to know
- Physical properties Alkanes are saturated hydrocarbons, CnH2n+2. Boiling point increases with chain length (more electrons, stronger van der Waals forces) and decreases with branching (less surface contact).
- Combustion Complete combustion gives CO2 and H2O; incomplete combustion gives CO and/or carbon (soot). Alkanes with a higher carbon content burn with a smokier, more luminous flame.
- Pollutants CO is toxic; carbon particles cause respiratory problems; NOx and SOx cause acid rain; unburned hydrocarbons contribute to smog. Rising CO2 (from about 0.03% to 0.04%) contributes to global warming.
- Catalytic converters Platinum, palladium and rhodium on a ceramic honeycomb convert CO, NOx and unburned hydrocarbons to CO2, N2 and H2O, for example 2CO + 2NO gives 2CO2 + N2.
- Fission Homolytic fission splits a bond so each atom takes one electron, forming radicals. Heterolytic fission gives both electrons to one atom, forming ions. A radical is a species with an unpaired electron.
- Radical substitution Initiation: Cl2 gives 2Cl. in UV light. Propagation: Cl. + CH4 gives CH3. + HCl, then CH3. + Cl2 gives CH3Cl + Cl. Termination: two radicals join, for example CH3. + Cl. gives CH3Cl. Further substitution gives a mixture of products.
Common mistakes in 2.3 Alkanes
- Saying branched alkanes have higher boiling points. Branching reduces surface contact, so van der Waals forces are weaker.
- Writing H. + Cl2 in propagation. The first propagation step forms HCl and a methyl radical, not a hydrogen radical.
- Forgetting the conditions for radical substitution. Ultraviolet light is needed to break the Cl-Cl bond homolytically.
- Saying catalytic converters remove CO2. They convert pollutants into CO2, N2 and H2O.
- Forgetting the unpaired electron dot on radicals. Always show it, for example Cl. and CH3.
Try three 2.3 Alkanes exam-style questions
1. Write the propagation steps for the reaction of bromine with ethane in UV light. (2 marks)
Show the mark scheme answer
Br. + C2H6 gives C2H5. + HBr; C2H5. + Br2 gives C2H5Br + Br.
2. Explain why 2-methylpropane has a lower boiling point than butane. (2 marks)
Show the mark scheme answer
Branched molecule has a smaller surface area of contact; weaker van der Waals forces so less energy needed to separate molecules.
3. Write an equation for the incomplete combustion of propane to form carbon monoxide and water. (1 mark)
Show the mark scheme answer
2C3H8 + 7O2 gives 6CO + 8H2O.
2.3 Alkanes FAQs
What does CCEA A Level Chemistry 2.3 Alkanes cover?
It covers boiling point trends, combustion, pollutants and catalytic converters, homolytic and heterolytic fission, and radical substitution with chlorine and bromine.
Which unit is 2.3 Alkanes examined in?
It is examined in Unit AS 2: Further Physical and Inorganic Chemistry and an Introduction to Organic Chemistry, a 1 hour 30 minute paper with a multiple-choice section and structured questions. Related practical skills can also be assessed in Unit AS 3.
What mechanism do I need for alkanes?
Radical substitution, with initiation, two propagation steps and termination, showing radicals with a single dot and stating ultraviolet light as the condition.
