Crude oil, hydrocarbons and alkanes — AQA GCSE Combined Science
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Crude oil, hydrocarbons and alkanes explained
Crude oil is a dark, smelly liquid trapped in porous rocks such as sandstone beneath impermeable cap rock.
Read the full explanation
It formed over millions of years: plankton and other tiny marine organisms died, sank into mud and were buried. Without oxygen, decay was incomplete, so the remains were compressed and heated, slowly converting the ancient biomass into crude oil. Because this process takes millions of years and we burn oil far faster than it forms, crude oil is finite: once used, it cannot be replaced within a human timescale. A useful check is to compare the rate of formation with the rate of extraction; the huge mismatch explains why chemists seek renewable alternatives and why oil is described as a non-renewable resource.
Crude oil is a mixture of a very large number of compounds. Most of the compounds in crude oil are hydrocarbons, which are molecules made up of hydrogen and carbon atoms only.
Crude oil is not a single substance but a mixture of a very large number of compounds, so it has no sharp boiling point and its composition varies between sources. Most of these compounds are hydrocarbons: molecules containing only carbon and hydrogen atoms. In a hydrocarbon such as methane, CH₄, or octane, C₈H₁₈, every atom is either carbon or hydrogen; there is no oxygen, nitrogen or sulfur in the molecule itself. Because the compounds have different chain lengths and boiling points, they can be separated by fractional distillation. A quick test of understanding is to examine a formula: if it contains only C and H, it is a hydrocarbon; if another element appears, it is not.
Most of the hydrocarbons in crude oil are hydrocarbons called alkanes. The general formula for the homologous series of alkanes is CₙH₂ₙ₊₂
Crude oil is a mixture of many compounds, most of which contain only carbon and hydrogen, so they are hydrocarbons. The majority of these belong to the alkane family. Alkanes form a homologous series: a family with the same general formula, similar reactions and a gradual change in physical properties as chain length increases. Their general formula is CₙH₂ₙ₊₂, where n is the number of carbon atoms. For example, when n = 1 the formula is CH₄ (methane); when n = 2 it is C₂H₆ (ethane); when n = 3 it is C₃H₈ (propane). Substituting n into 2n + 2 gives the hydrogen count, so each alkane has two more hydrogen atoms than twice its carbon atoms. Alkanes are saturated: every carbon forms four single covalent bonds, so no more atoms can add without substitution.
The first four members of the alkanes are methane, ethane, propane and butane.
The alkane homologous series begins with the simplest member, methane, which has one carbon atom and the formula CH₄. The next three members are ethane (two carbon atoms, C₂H₆), propane (three carbon atoms, C₃H₈) and butane (four carbon atoms, C₄H₁₀). Their names use stems that indicate the number of carbon atoms: meth- for one, eth- for two, prop- for three and but- for four, with the ending -ane showing membership of the alkane family. Each successive member has one more carbon atom and two more hydrogen atoms than the previous one, so the formulas follow CₙH₂ₙ₊₂. These four are important fuels: methane is natural gas, propane and butane are used in bottled gas, and all burn in plentiful oxygen to produce carbon dioxide and water.
Alkane molecules can be represented in the following forms:
Alkanes are saturated hydrocarbons whose molecules can be shown in several complementary ways. A molecular formula such as C₂H₆ states only the number of each atom. A displayed formula draws every atom and every covalent bond, so it reveals the single C–C and C–H bonds that make the molecule saturated. A structural formula such as CH₃CH₃ condenses the same information onto one line, grouping atoms bonded to each carbon. A skeletal formula omits C and H labels, showing a zig-zag carbon chain. Whichever representation is used, the molecule is unchanged: the same atoms are joined in the same order. Choosing a form depends on the purpose, for example counting atoms, checking bonding or naming the alkane.
C₂H₆ or
C₂H₆ is the molecular formula of ethane, the two-carbon alkane. It tells you there are two carbon atoms and six hydrogen atoms, but not how they are joined. Ethane is saturated: the two carbons share a single covalent bond, and each carbon also bonds to three hydrogen atoms, giving each carbon four single bonds. The same molecule can be shown as a displayed formula with all atoms and bonds drawn, or as the condensed structural formula CH₃CH₃. Checking against the general formula CₙH₂ₙ₊₂ with n = 2 gives C₂H₆, confirming ethane is an alkane. Recognising C₂H₆ as ethane, and linking the formula to its structure, is the key skill.
Students should be able to recognise substances as alkanes given their formulae in these forms.
Alkanes are saturated hydrocarbons whose molecules contain only single covalent bonds between carbon atoms. Their general formula is CnH2n+2, so for n = 1, 2, 3 and 4 the formulae are CH₄, C₂H₆, C₃H₈ and C₄H₁₀. To recognise an alkane from a displayed, structural or molecular formula, count the carbon atoms and check that the hydrogen count equals twice the carbon count plus two. For example, C₅H₁₂ fits because 2 × 5 + 2 = 12, whereas C₅H₁₀ does not because it has two fewer hydrogen atoms and is unsaturated. Recognising the pattern allows you to classify an unknown hydrocarbon correctly and to predict the formula of the next member of the homologous series.
Your focus
- State that crude oil is a finite resource found in rocks.
- Describe how crude oil formed from ancient plankton buried in mud.
- Explain why crude oil is classified as non-renewable.
Show all 21 objectives
- Describe crude oil as a mixture of a very large number of compounds.
- Define a hydrocarbon as a molecule made of carbon and hydrogen atoms only.
- Identify whether a given formula represents a hydrocarbon.
- State that most hydrocarbons in crude oil are alkanes.
- Apply the general formula CₙH₂ₙ₊₂ to deduce the formula of an alkane from its carbon number.
- Explain that alkanes are a homologous series of saturated hydrocarbons.
- Name the first four alkanes in order.
- Write the molecular formula of methane, ethane, propane and butane.
- Describe how the formulas of successive alkanes differ by CH₂.
- Draw and interpret displayed, structural and molecular formulae for simple alkanes.
- Convert between different representations of the same alkane molecule.
- Explain why different representations can describe the same substance.
- Identify C₂H₆ as ethane and state the number of each type of atom present.
- Describe the single covalent bonding in an ethane molecule.
- Relate the molecular formula of ethane to the alkane general formula.
- Identify a molecular formula as that of an alkane by applying CnH2n+2.
- Describe alkanes as saturated hydrocarbons containing only single covalent bonds.
- Deduce the formula of an alkane from its position in the homologous series.
Crude oil, hydrocarbons and alkanes exam tips
Marking Points
- Crude oil is described as a finite resource because it is being used up much faster than it is formed.
- Crude oil is found in rocks, typically trapped in porous reservoir rock beneath impermeable rock.
- Crude oil originated from the remains of ancient biomass, mainly plankton.
- The plankton were buried in mud, where lack of oxygen prevented complete decay.
- Over millions of years, heat and pressure changed the buried biomass into crude oil.
- A correct comparison of formation time (millions of years) with extraction time (decades) shows why the resource is finite.
- Crude oil is a mixture, not a single compound, so it contains many different substances.
- The compounds in crude oil have different boiling points, which allows separation by fractional distillation.
- Most compounds in crude oil are hydrocarbons.
- A hydrocarbon is defined as a molecule containing only carbon and hydrogen atoms.
- Formulae such as CH₄ and C₈H₁₈ contain only carbon and hydrogen, so they are hydrocarbons.
- A formula containing another element, for example C₂H₅OH, is not a hydrocarbon because it contains oxygen.
- Crude oil is a mixture of hydrocarbons, and most of its hydrocarbons are alkanes.
- Alkanes are a homologous series, meaning they share a general formula and show trends in properties.
- The general formula of alkanes is CₙH₂ₙ₊₂, where n is the number of carbon atoms.
- Substituting n into 2n + 2 gives the number of hydrogen atoms, e.g. n = 4 gives C₄H₁₀.
- Alkanes are saturated hydrocarbons containing only single covalent bonds between carbon atoms.
- The first four alkanes are methane, ethane, propane and butane, in order of increasing carbon chain length.
- Their molecular formulas are CH₄, C₂H₆, C₃H₈ and C₄H₁₀ respectively.
- The prefixes meth-, eth-, prop- and but- indicate one, two, three and four carbon atoms.
- Each successive alkane has one more carbon atom and two more hydrogen atoms than the previous member.
- These alkanes are commonly used as fuels because they burn exothermically in oxygen.
- Recognise that a molecular formula gives the number of each type of atom but not the arrangement or bonding.
- Interpret a displayed formula by counting each drawn line as one covalent bond and each labelled atom as one atom.
- Convert between a displayed formula and a condensed structural formula such as CH₃CH₃ for ethane.
- Explain that all representations of a given alkane describe the same molecule with the same molecular formula.
- Use the general formula CₙH₂ₙ₊₂ to check that a proposed representation is a valid alkane.
- Identify that alkanes are saturated because every carbon is joined by single bonds only.
- State that C₂H₆ is the molecular formula of ethane and contains two carbon atoms and six hydrogen atoms.
- Describe the bonding in ethane as two carbon atoms joined by a single covalent bond, with each carbon also bonded to three hydrogen atoms.
- Explain that ethane is saturated because all its carbon–carbon and carbon–hydrogen bonds are single bonds.
- Check that C₂H₆ fits the alkane general formula CₙH₂ₙ₊₂ when n = 2.
- Write the condensed structural formula CH₃CH₃ or draw the displayed formula for ethane.
- State that alkanes are saturated hydrocarbons containing only single covalent bonds between carbon atoms.
- Apply the general formula CnH2n+2 to a given molecular formula and check that the hydrogen count equals twice the carbon count plus two.
- Recognise the displayed or structural formulae of methane, ethane, propane and butane as alkanes.
- Identify a formula that does not fit CnH2n+2, such as C₃H₆, and explain that it is not an alkane because it is unsaturated.
- Use the pattern of the homologous series to deduce the formula of the next alkane, for example C₅H₁₂ after C₄H₁₀.
Examiner Tips
- 💡Use the word finite and immediately explain it as used up faster than it forms.
- 💡Link each stage of formation in order: plankton, burial in mud, no oxygen, heat and pressure, millions of years.
- 💡When asked why crude oil is non-renewable, compare the timescale of formation with the rate of human use.
- 💡Use the phrase mixture of compounds and support it with the idea of different boiling points.
- 💡When identifying a hydrocarbon, check that the formula contains only C and H, with no other element symbols.
- 💡Contrast a hydrocarbon with a non-hydrocarbon such as ethanol to show the boundary of the definition.
- 💡When asked to deduce a formula, identify n from the number of carbon atoms, then compute 2n + 2 carefully.
- 💡Use the word saturated to show that alkanes contain only single carbon–carbon bonds.
- 💡Link the homologous series idea to trends such as increasing boiling point with increasing chain length.
- 💡Learn the first four names and formulas as a sequence so you can recall them quickly.
- 💡Use the prefix to count carbon atoms and the general formula to check the hydrogen count.
- 💡When naming an alkane from a displayed formula, count the carbon atoms first, then choose the correct prefix and add -ane.
- 💡When asked to draw a displayed formula, show every atom and every bond, and check each carbon has four bonds.
- 💡If a question gives one representation and asks for another, first write the molecular formula to fix the numbers of atoms.
- 💡Use the general formula CₙH₂ₙ₊₂ to test whether a suggested formula is an alkane before commenting on its structure.
- 💡Use the general formula CₙH₂ₙ₊₂ to confirm a molecular formula belongs to an alkane.
- 💡When drawing ethane, count four bonds around each carbon to check the structure is complete.
- 💡Link the name ethane to the two-carbon chain, remembering the prefix eth- means two carbons.
- 💡Write out the calculation 2n + 2 beside the formula so the examiner can see how you checked it.
- 💡When a displayed formula is given, count carbon atoms first, then count hydrogen atoms, and compare the two numbers.
- 💡If asked to explain, link the formula to saturation and single bonds rather than only stating the name of the substance.
Common Mistakes
- Saying crude oil is a fossil fuel made from dead dinosaurs; correction: the main source is plankton, a marine biomass.
- Stating crude oil is found in underground caves or lakes; correction: it is held in pores within rocks such as sandstone.
- Claiming crude oil is renewable because more is always forming; correction: formation takes millions of years, so it is finite on a human timescale.
- Calling crude oil a compound; correction: it is a mixture of many compounds.
- Defining a hydrocarbon as a molecule containing carbon and hydrogen; correction: it contains only carbon and hydrogen.
- Assuming any fuel is a hydrocarbon; correction: fuels such as ethanol contain oxygen and are not hydrocarbons.
- Writing the general formula as CₙH₂ₙ: this is the alkene general formula; alkanes are saturated and use CₙH₂ₙ₊₂.
- Counting hydrogen atoms incorrectly for a given n, for example giving C₃H₆ instead of C₃H₈; always calculate 2n + 2.
- Describing crude oil as a single compound rather than a mixture of many hydrocarbons separated by fractional distillation.
- Mixing up the order or formulas, such as giving propane as C₃H₆; propane is C₃H₈ because it is an alkane.
- Using the wrong prefix, for example writing 'methene' or 'buthene' for an alkane; the -ane ending is essential.
- Assuming methane contains more than one carbon atom because it is a common fuel; methane is CH₄ with a single carbon atom.
- Treating a molecular formula as if it showed how atoms are joined; correction: it shows only the numbers of atoms, so a displayed or structural formula is needed to show bonding.
- Counting the C–C bond twice when working out the number of bonds in a displayed formula; correction: count each drawn line once, as one shared pair of electrons.
- Writing a condensed formula with the wrong number of hydrogen atoms, for example CH₂CH₃ for ethane; correction: check against CₙH₂ₙ₊₂, giving CH₃CH₃ for C₂H₆.
- Reading C₂H₆ as two carbon atoms and six separate hydrogen molecules; correction: the subscript 6 refers to six hydrogen atoms within the molecule.
- Drawing a double bond between the carbons in ethane; correction: ethane is saturated, so the C–C bond is single.
- Confusing C₂H₆ with C₂H₄, which is ethene and has a carbon–carbon double bond; correction: check the hydrogen count against CₙH₂ₙ₊₂.
- Assuming every hydrocarbon formula is an alkane. Correction: test the formula against CnH2n+2 before classifying it.
- Miscounting hydrogen atoms in a displayed formula by ignoring hydrogens shown at the ends of bonds. Correction: count every hydrogen atom attached to every carbon atom.
- Writing C₃H₈ as C₃H₆ or C₄H₁₀ as C₄H₈. Correction: apply the rule 2n + 2 carefully, so propane is C₃H₈ and butane is C₄H₁₀.