Crude Oil and Hydrocarbons
Build confidence in Crude Oil and Hydrocarbons for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Crude oil is a mixture of hydrocarbons, mostly alkanes with the general formula CnH2n+2.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Crude oil is a finite resource found in rocks. It is the remains of an ancient biomass, consisting mainly of plankton that was buried in mud millions of years ago.
It is a mixture of a very large number of compounds, most of which are hydrocarbons. A hydrocarbon is a compound made from hydrogen and carbon atoms only — that definition is precise and is examined regularly.
Most of the hydrocarbons in crude oil are alkanes. Alkanes have the general formula CₙH₂ₙ₊₂, so methane is CH₄, ethane is C₂H₆, propane is C₃H₈ and butane is C₄H₁₀. Learning the first four names in order is essential.
Revision notes
What crude oil is
Crude oil is a finite resource found in rocks, formed from the remains of an ancient biomass consisting mainly of plankton buried in mud millions of years ago.
It is a mixture of a very large number of compounds, most of which are hydrocarbons.
Hydrocarbons
A hydrocarbon is a compound made from hydrogen and carbon atoms only.
If a compound contains oxygen, nitrogen or any other element, it is not a hydrocarbon. This precise definition matters in exams.
Alkanes
Most hydrocarbons in crude oil are alkanes, with the general formula CₙH₂ₙ₊₂.
The first four are methane CH₄, ethane C₂H₆, propane C₃H₈ and butane C₄H₁₀. They form a homologous series — a family of compounds with the same general formula and similar properties.
Key points
- Crude oil is a finite resource found in rocks.
- It formed from an ancient biomass, mainly plankton.
- It is a mixture of many compounds.
- A hydrocarbon contains hydrogen and carbon only.
- Most hydrocarbons in crude oil are alkanes.
- Alkanes have the formula CₙH₂ₙ₊₂.
Worked examples
Example 1
Define a hydrocarbon. [1 mark]
Model answer
A hydrocarbon is a compound made from hydrogen and carbon atoms only (1 mark).
Example 2
Give the molecular formula of propane, using the general formula for alkanes. [2 marks]
Model answer
Propane has three carbon atoms, so n = 3 (1 mark). Using CₙH₂ₙ₊₂ gives C₃H₈ (1 mark).
Example 3
Explain why crude oil is described as a finite resource. [2 marks]
Model answer
It formed over millions of years from the remains of ancient biomass (1 mark), so it is being used up far faster than it can be replaced (1 mark).
Common mistakes
Saying a hydrocarbon can contain oxygen.
It contains hydrogen and carbon ONLY.
Getting the alkane general formula wrong.
It is CₙH₂ₙ₊₂. The +2 is essential.
Confusing the order of the first four alkanes.
Methane, ethane, propane, butane — with 1, 2, 3 and 4 carbons.
Saying crude oil is a compound.
It is a mixture of many different compounds.
Exam tips
- Say hydrogen and carbon only in your definition.
- Learn CₙH₂ₙ₊₂ and the first four alkanes.
- Use the general formula to derive any alkane formula.
- Remember crude oil is a mixture, not a compound.
Key terms
- Crude oil
- A finite mixture of hydrocarbons found in rocks.
- Hydrocarbon
- A compound of hydrogen and carbon only.
- Alkane
- A saturated hydrocarbon with formula CₙH₂ₙ₊₂.
- Homologous series
- A family of compounds with the same general formula.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.