Section 1 of 7Alkanes
1What are alkanes?
Alkanes are a homologous series of saturated hydrocarbons. Every carbon atom forms four single covalent bonds, and there are no C=C double bonds, so no extra atoms can be added across the chain.
2
The general formula is CnH2n+2.
- Methane (n = 1) is CH4, ethane C2H6, propane C3H8, butane C4H10 and pentane C5H12.
- Names use prefixes meth-, eth-, prop-, but-, pent- plus the ending -ane.
3
As a homologous series, alkanes show a gradual trend in physical properties: longer chains have stronger intermolecular forces, so boiling point and viscosity increase, and volatility and flammability decrease.
4
From butane onwards alkanes show structural isomerism. C4H10 can be a straight chain (butane) or branched (2-methylpropane); C5H12 has three isomers.
5
Alkanes are generally unreactive because C–C and C–H bonds are strong and non-polar, but they burn well, which is why they are used as fuels.
6
In plenty of oxygen, complete combustion gives carbon dioxide and water. In limited oxygen, incomplete combustion gives toxic carbon monoxide and/or soot (carbon), releasing less energy.
7Halogens and alkanes
In ultraviolet light, alkanes react with halogens such as bromine or chlorine in a substitution reaction: a hydrogen atom is replaced by a halogen atom and a hydrogen halide is also formed, e.g. CH4 + Br2 → CH3Br + HBr.
8
Because substitution is slow and needs UV light, alkanes do not quickly decolourise bromine water at room temperature in the dark, unlike alkenes. Further substitution can produce a mixture of products such as dibromomethane.
9Methane and chlorine
Chlorine reacts with methane in UV light in the same way as bromine. One hydrogen atom is swapped for a chlorine atom: CH4 + Cl2 → CH3Cl + HCl, giving chloromethane and hydrogen chloride.
10Uses of halogenoalkanes
Halogenoalkanes are useful as solvents, refrigerants, aerosol propellants and in making medicines. CFCs (chlorofluorocarbons) were once common in fridges but are being phased out because they damage the ozone layer.
Section 2 of 7Process & sequence
1
Mix the alkane (e.g. methane) with a halogen such as bromine vapour.
2
Shine ultraviolet light on the mixture; nothing happens in the dark.
3
UV energy breaks the Br–Br bond to form reactive bromine atoms.
4
A bromine atom removes a hydrogen atom from the alkane, forming HBr.
5
The remaining alkyl group bonds to another bromine atom, giving a bromoalkane.
6
The orange-brown colour of bromine fades slowly as it is used up.
7
If bromine is in excess, further H atoms can be substituted (CH2Br2, CHBr3, CBr4).
Substitution of methane with brominewrite these steps in this order; each step is a marking point.
1
Methane + bromine mixed
2
UV light supplied
3
Br–Br bond breaks
4
H replaced by Br
5
Bromomethane + HBr formed
Section 3 of 7Compare
| Name | Molecular formula | Structure outline | State at room temp. |
|---|---|---|---|
| Methane | CH4 | one C, four C–H | gas |
| Ethane | C2H6 | CH3CH3 | gas |
| Propane | C3H8 | CH3CH2CH3 | gas |
| Butane | C4H10 | CH3CH2CH2CH3 | gas |
| Pentane | C5H12 | CH3(CH2)3CH3 | liquid |
Each member differs from the next by CH2. Boiling point rises as chain length increases.
| Reaction | Conditions | Products | Example |
|---|---|---|---|
| Complete combustion | excess oxygen | CO2 + H2O | CH4 + 2O2 → CO2 + 2H2O |
| Incomplete combustion | limited oxygen | CO and/or C + H2O | 2CH4 + 3O2 → 2CO + 4H2O |
| Substitution with bromine | UV light | bromoalkane + HBr | CH4 + Br2 → CH3Br + HBr |
Section 4 of 7Worked examples
1Write a balanced equation for the complete combustion of propane.
- Propane is C3H8.
- Products are CO2 and H2O.
- Balance C3CO2.
- Balance H8 H gives 4H2O.
- Count O on the right6 + 4 = 10 atoms, so 5O2. C3H8 + 5O2 → 3CO2 + 4H2O.
2Substitution
Ethane reacts with chlorine in UV light. Name the type of reaction and write the equation for monosubstitution.
- It is a substitution reaction (one H replaced by Cl).
- C2H6 + Cl2 → C2H5Cl + HCl.
- The organic product is chloroethane; UV light supplies energy to split the Cl–Cl bond.
Section 5 of 7Key terms
Section 6 of 7Command terms
Command terms
Give / State / Name
Recall a fact; one idea per mark.
Define
Give the exact meaning using the specification keywords.
Describe
Give an account of what happens / the pattern in data, quoting values; no reasons needed.
Explain
Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.
Suggest
Use knowledge in a new context; more than one answer may be accepted.
Compare
Similarities and differences; each point must refer to both items.
Calculate
Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.
Discuss
Explore the issue from different angles and link the points.
Evaluate / Assess
Weigh strengths and weaknesses and reach a supported judgement.
Deduce
Draw a conclusion from the information given.
Justify
Support a statement/choice with evidence.
Section 7 of 7Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Hydrocarbon · Alkane · Saturated · Homologous series · Complete combustion · Incomplete combustion · Substitution reaction · Free radical · Halogenoalkane · Structural isomers
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.
How to get full marks (examiner guidance)
How to get full marks (examiner guidance)
Use specification keywords exactly (e.g. electronegativity, nucleophile, dynamic equilibrium, activation energy).
Link ideas
Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.
Use the information in the question
“discuss/evaluate” answers must use the data provided and match the number of marks.
Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.
Avoid vague words
“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.
Do not extrapolate a graph line beyond the data or to zero unless asked.
Six-mark extended responses are marked on content AND a logical, linked line of reasoning.