AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Crude oil, hydrocarbons and alkanes: 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.
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
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.
What earns the marks
- 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.
Common mistakes and how to put them right
Writing the general formula as CₙH₂ₙ.
Put it right: 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.
Put it right: Calculate 2n + 2.
Describing crude oil as a single compound rather than a mixture of many hydrocarbons separated by fractional distillation.
Exam tips
- 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.
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The start of the Crude oil, hydrocarbons and alkanes: Most of the hydrocarbons in crude oil are hydrocarbons called alkanes. The general formula for the homologous series of alkanes is CₙH₂ₙ₊₂ lesson, made from checked teaching notes for this specification point.
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Which general formula represents the homologous series of alkanes found in crude oil?
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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. A strong answer earns marks for these points. 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. Exam tip: When asked to deduce a formula, identify n from the number of carbon atoms, then compute 2n + 2 carefully.
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Key words
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Crude oil
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Homologous series
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Saturated
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See every definition
- Crude oil
- A mixture of many compounds, most of which contain only carbon and hydrogen, so they are hydrocarbons.
- Homologous series
- A family with the same general formula, similar reactions and a gradual change in physical properties as chain length increases.
- Saturated
- Every carbon forms four single covalent bonds, so no more atoms can add without substitution.
Label the diagram
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