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This lesson introduces hydrocarbons, crude oil and the alkane homologous series as required by the Edexcel GCSE Chemistry specification (1CH0), Topic 9: Separate Chemistry 2. You need to know what hydrocarbons are, how crude oil formed, and how to name, draw and recognise the first four alkanes. This is the foundation for all of the organic chemistry and earth science content that follows.
A hydrocarbon is a compound that contains only hydrogen and carbon atoms. No other elements are present. Hydrocarbons are the simplest organic compounds.
| Feature | Detail |
|---|---|
| Elements present | Carbon (C) and hydrogen (H) only |
| Type of bonding | Covalent bonds (atoms share pairs of electrons) |
| Examples | Methane (CH₄), ethane (C₂H₆), propene (C₃H₆) |
Exam Tip: If a molecule contains any element other than carbon and hydrogen — for example oxygen or nitrogen — it is not a hydrocarbon. This is a common one-mark definition question on the exam.
Crude oil is a fossil fuel. It is a complex mixture of many different hydrocarbons.
Crude oil is a finite (non-renewable) resource — once it is used up, it cannot be replaced within a human timescale.
| Property | Detail |
|---|---|
| Origin | Ancient marine organisms buried under sediment |
| Timescale | Formed over millions of years |
| Composition | Mixture of many hydrocarbons |
| Resource type | Finite (non-renewable) |
Exam Tip: Always describe crude oil as a mixture of hydrocarbons, not a single compound. Because it is a mixture, it can be separated by physical methods (fractional distillation).
Alkanes are a family (homologous series) of hydrocarbons that contain only single covalent bonds between carbon atoms. Because every carbon atom forms the maximum number of bonds to hydrogen, alkanes are described as saturated hydrocarbons.
The general formula for alkanes is:
CₙH₂ₙ₊₂
where n is the number of carbon atoms. For example, if n = 3 then the formula is C₃H₈ (propane).
| Name | Molecular formula | Number of C atoms | Number of H atoms | State at room temperature |
|---|---|---|---|---|
| Methane | CH₄ | 1 | 4 | Gas |
| Ethane | C₂H₆ | 2 | 6 | Gas |
| Propane | C₃H₈ | 3 | 8 | Gas |
| Butane | C₄H₁₀ | 4 | 10 | Gas |
Exam Tip: Learn the names and formulae of the first four alkanes. A helpful mnemonic is Monkeys Eat Peanut Butter (methane, ethane, propane, butane).
A displayed formula shows every atom and every bond in the molecule. In a displayed formula:
Methane (CH₄): Each of the four hydrogen atoms is bonded to the single carbon atom: H–C(–H)(–H)–H arranged tetrahedrally (drawn flat on paper as a cross shape).
Ethane (C₂H₆): Two carbon atoms are bonded together (C–C), with three hydrogen atoms bonded to each carbon.
Propane (C₃H₈): A chain of three carbon atoms (C–C–C), with hydrogen atoms filling the remaining bonds on each carbon (3 H on each end carbon, 2 H on the middle carbon).
Butane (C₄H₁₀): A chain of four carbon atoms (C–C–C–C), with hydrogen atoms filling the remaining bonds (3 H on each end carbon, 2 H on each middle carbon).
Exam Tip: When drawing displayed formulae, count the bonds on each carbon — it must always be exactly four. If a carbon has fewer than four bonds, you have missed a hydrogen.
A homologous series is a family of compounds that:
| Member | Formula | Differs from previous by |
|---|---|---|
| Methane | CH₄ | — |
| Ethane | C₂H₆ | +CH₂ |
| Propane | C₃H₈ | +CH₂ |
| Butane | C₄H₁₀ | +CH₂ |
| Pentane | C₅H₁₂ | +CH₂ |
The concept of a homologous series applies to other families too — alkenes (CₙH₂ₙ), alcohols (CₙH₂ₙ₊₁OH), and carboxylic acids all form their own homologous series.
Question: An alkane has 6 carbon atoms. What is its molecular formula? [1 mark]
Answer: Using the general formula CₙH₂ₙ₊₂ with n = 6:
Question: A hydrocarbon has the molecular formula C₅H₁₂. Show that it is an alkane. [2 marks]
Answer:
| Term | Definition |
|---|---|
| Hydrocarbon | A compound containing only hydrogen and carbon |
| Crude oil | A finite resource; a mixture of hydrocarbons formed from ancient marine organisms |
| Alkane | A saturated hydrocarbon with the general formula CₙH₂ₙ₊₂ |
| Saturated | Contains only single C–C bonds |
| Homologous series | A family of compounds with the same general formula and similar chemical properties, each member differing by CH₂ |
| Displayed formula | A diagram showing every atom and every covalent bond in a molecule |
| Fossil fuel | A fuel formed from the remains of dead organisms over millions of years (e.g. crude oil, coal, natural gas) |
The general formula is the single most powerful tool in this lesson. Practise using it in both directions: formula to name, and name to formula.
Worked example 1 — pentane (n = 5)
Substitute n = 5 into CₙH₂ₙ₊₂:
Worked example 2 — decane (n = 10)
Substitute n = 10 into CₙH₂ₙ₊₂:
Worked example 3 — identifying a mystery alkane
A hydrocarbon has the molecular formula C₇H₁₆. Is it an alkane?
Exam Tip: In exam questions you are often asked to "deduce" whether a hydrocarbon is saturated. Substitute n into the general formula and compare. If the hydrogen count matches CₙH₂ₙ₊₂ it is a saturated alkane; if it matches CₙH₂ₙ it is an unsaturated alkene.
Every member of the alkane homologous series is made by inserting one extra –CH₂– unit into the chain. This is why ethane (C₂H₆) and propane (C₃H₈) differ by exactly one carbon and two hydrogens. The repeating unit explains why:
graph LR
H1[H] --- C1[C]
H2[H] --- C1
H3[H] --- C1
C1 --- C2[C]
H4[H] --- C2
H5[H] --- C2
C2 --- C3[C]
H6[H] --- C3
H7[H] --- C3
C3 --- C4[C]
H8[H] --- C4
H9[H] --- C4
H10[H] --- C4
Count: 4 carbons, 10 hydrogens — matches C₄H₁₀.
| Feature | Alkanes | Alkenes |
|---|---|---|
| General formula | CₙH₂ₙ₊₂ | CₙH₂ₙ |
| Bonding between C atoms | All single (C–C) | At least one C=C double bond |
| Saturation | Saturated | Unsaturated |
| Reaction with bromine water | No reaction (stays orange) | Decolourises (turns colourless) |
| Main use | Fuels | Making polymers, alcohols, margarine |
Common mistake: Writing butane as C₄H₈. Students confuse alkanes with alkenes. Alkanes have the maximum number of hydrogens (CₙH₂ₙ₊₂); alkenes have two fewer (CₙH₂ₙ).
Common mistake: Drawing a carbon with three or five bonds. Every carbon must form exactly four covalent bonds. Always count.
Common mistake: Confusing "homologous series" with "homogeneous mixture". A homologous series is a family of compounds (like alkanes). A homogeneous mixture is a uniform mixture (like salt water).
| Grade band | What a typical answer looks like |
|---|---|
| Grades 1–3 | "Hydrocarbons are made of hydrogen and carbon. Methane is CH₄." Names alkanes but cannot use the general formula. |
| Grades 4–5 | States that alkanes are saturated hydrocarbons with only single bonds. Uses the general formula CₙH₂ₙ₊₂ to work out a molecular formula. Correctly names and draws the first four alkanes. |
| Grades 6–7 | Uses precise terms homologous series and functional group (C–H/C–C). Explains that members differ by a CH₂ unit and share similar chemical properties because they have the same functional group. Links saturation to reactivity with bromine water. |
| Grades 8–9 | Evaluates why alkanes are unreactive: strong C–C and C–H bonds and the absence of a C=C functional group limits addition reactions. Predicts molecular formulae for any alkane using CₙH₂ₙ₊₂ and justifies trends in boiling point using intermolecular forces. Compares alkanes to alkenes precisely using the terms saturated/unsaturated. |
Quick recall: First four alkanes are methane, ethane, propane, butane (CH₄, C₂H₆, C₃H₈, C₄H₁₀). The general formula CₙH₂ₙ₊₂ lets you derive any alkane from its carbon count in seconds — practise this until it is automatic.
Common pitfall: Students often forget that "saturated" is a chemical statement about bonding, not about how full a fuel tank is. Saturated means no double bonds: every carbon already holds its maximum four hydrogens, so alkanes do not decolourise bromine water.
Edexcel alignment: This content is aligned with Edexcel GCSE Chemistry (1CH0) specification Topic 8 Fuels and Earth science / Topic 9 Separate chemistry — specifically 8.1 Fuels, 9.1 Hydrocarbons. Assessed on Paper 2.