AQA GCSE Combined Science (8464), Higher tier · Chemistry › Organic chemistry › Carbon compounds as fuels and feedstock
Practise Crude oil, hydrocarbons and alkanes. 12 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
What crude oil is, what hydrocarbons and alkanes are, and how to name, recognise and draw the first four alkanes. Expect short recall questions, formula questions using the general formula CnH2n+2, and displayed formulae to draw, usually on Paper 2.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
State what crude oil isA finite resource found in rocks: the remains of an ancient biomass, mainly plankton, that was buried in mud.
3
Define a hydrocarbonA compound whose molecules are made of hydrogen and carbon atoms only.
4
Name the first four alkanes and their formulaeMethane CH4, ethane C2H6, propane C3H8 and butane C4H10.
5
Use the general formula of the alkanesRecognise an alkane from its formula, and write the formula for any number of carbon atoms: 10 carbon atoms gives C10H22.
5
Draw the displayed formula of an alkaneShow every atom and every bond: each carbon atom forms four single bonds and each hydrogen atom forms one.
6
Describe the features of a homologous seriesSame general formula, each member differs from the next by CH2, similar chemical reactions and a gradual trend in physical properties.
7
Work out an alkane's formula from its MrMr = 14n + 2, so solve for n: an Mr of 72 gives n = 5, which is C5H12.
Notes
Crude oil
Crude oil is a finite resource found in rocks. It is being used far faster than it forms, so it will run out.
It is the remains of an ancient biomass, consisting mainly of plankton, that was buried in mud and changed over millions of years.
Crude oil is a mixture of a very large number of compounds. They are not chemically joined to each other, so they can be separated by a physical method (fractional distillation).
Most of the compounds in crude oil are hydrocarbons: molecules made up of hydrogen and carbon atoms only.
Most of these hydrocarbons are alkanes.
The alkanes
The alkanes are a homologous series: a family of compounds with the same general formula, in which each member differs from the next by CH2.
General formula: CnH2n+2, where n is the number of carbon atoms.
The first four are methane CH4, ethane C2H6, propane C3H8 and butane C4H10. These are the only alkane names you must know.
Every carbon atom in an alkane forms four single covalent bonds. There are only single bonds, so alkanes are saturated.
Members of a homologous series have similar chemical reactions and show a gradual trend in physical properties, such as boiling point.
Showing alkane molecules
A molecular formula shows the number of atoms of each element in one molecule, e.g. C3H8.
A displayed formula shows every atom and every bond as a line. Ethane, C2H6:
H H
| |
H - C - C - H
| |
H H
Check a displayed formula by counting: 4 bonds on every C, 1 bond on every H.
You may also see formulae such as CH3CH2CH3 (propane), which show how the atoms are grouped along the chain.
Relative formula mass of an alkane: Mr = 12n + (2n + 2) = 14n + 2. grade 7+
Cheatsheet
Crude oil: finite resource found in rocks; remains of ancient biomass (mainly plankton) buried in mud
Crude oil = a mixture of a very large number of compounds, mostly hydrocarbons
Hydrocarbon = compound made of hydrogen and carbon atoms only
Homologous series: same general formula, differ by CH2, similar reactions, trend in physical properties
Each C atom forms 4 bonds; each H atom forms 1 bond
Alkane Mr = 14n + 2 grade 7+
How to answer each type of question
Describe what crude oil is
1 to 3 marks3
Use the key words: finite, mixture, hydrocarbons, ancient biomass, plankton, buried in mud.
For 'hydrocarbon', say hydrogen and carbon atoms only: 'only' is often a separate mark.
Example. Describe what crude oil is and how it was formed.
Show the model answer
Crude oil is a mixture of a very large number of compounds, most of them hydrocarbons (1). It formed from the remains of an ancient biomass, mainly plankton (1), that was buried in mud (1).
Give the formula or name of an alkane
1 mark each4
Use CnH2n+2: double the number of carbon atoms and add 2 to get the hydrogens.
To recognise an alkane, check that the formula fits: H = 2 × C + 2.
Write formulae with capital letters and subscripts, e.g. C3H8, not C3h8.
Example. (a) Give the molecular formula of the alkane with 9 carbon atoms. (b) Which of these is an alkane: C6H12, C6H14 or C6H6? (c) Name the alkane C2H6.
Draw the carbon atoms in a line, joined by single bonds.
Add hydrogen atoms until every carbon atom has four bonds.
Count the hydrogens: there should be 2n + 2.
Example. Draw the displayed formula of propane, C3H8.
Show the model answer
H H H
| | |
H - C - C - C - H
| | |
H H H
Three carbon atoms joined by single bonds (1). Eight hydrogen atoms, with every bond drawn (1).
Calculate: find an alkane from its relative formula mass
2 to 3 marks7
Write Mr in terms of n: 12n + (2n + 2) = 14n + 2.
Set this equal to the Mr you are given and solve for n.
Write the formula, and name the alkane if it is one of the first four.
Example. An alkane has a relative formula mass (Mr) of 58. Relative atomic masses (Ar): H = 1, C = 12 Determine the molecular formula of the alkane and name it.
Show the model answer
14n + 2 = 58 (1) n = 4, so the formula is C4H10 (1) Butane (1) (Finding C4H10 by checking, 48 + 10 = 58, also scores the marks.)
Shortcuts and memory tricks
First four alkanes in order: 'My Elephant Plays Bass' = methane (1 C), ethane (2 C), propane (3 C), butane (4 C).
Alkane hydrogens: double the carbons and add two. C5 gives H12; C8 gives H18.
Hydro-carbon means hydrogen + carbon and nothing else. If there is O, N or Cl in the formula, it is not a hydrocarbon.
The ending tells you the family: '-ane' means an alkane, with single bonds only.
Quick check on any displayed formula: 4 lines on every C, 1 line on every H.
Where marks are lost
Writing that a hydrocarbon is 'a mixture of hydrogen and carbon'. It is a compound, and the key word is 'only'.
Saying crude oil formed from dinosaurs or trees. The answer examiners want is plankton buried in mud.
Calling crude oil a compound. It is a mixture of many compounds.
Drawing a carbon atom with only three bonds (a missing hydrogen), or a bond line with no atom at the end.
Using superscripts or lower-case letters in formulae, e.g. C4H10 or c4h10.
Exam technique
'Displayed formula' means every bond drawn. CH3CH3 will not score the mark.
If you are asked for a 'molecular formula', give the numbers of each atom (e.g. C3H8), not a drawing.
In 'which of these is an alkane' questions, test every option with H = 2C + 2 before you choose.
Show the 14n + 2 step in Mr questions so that you can pick up method marks even if you slip later.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Crude oil is a finite resource found in rocks. What is meant by a finite resource?
A resource that will run out because it is not being replaced.
The general formula of the alkanes is CnH2n+2. Give the formula of the alkane with 7 carbon atoms.
C7H16
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
Crude oil is a finite resource found in rocks.
(a) What is meant by a finite resource?[1]
(b) Crude oil is the remains of an ancient biomass. What did this biomass mainly consist of? Tick (✓) one box.[1]
Plankton
Dinosaurs
Trees
Volcanic ash
(c) Most of the compounds in crude oil are hydrocarbons. What is a hydrocarbon?[2]
(d) Name the alkane with the formula C3H8.[1]
Show the answer and mark scheme
(a)Answer: A resource that will run out because it is not being replaced.
it will run out / it is not being made (or replaced) as fast as it is used
(b)Answer: Plankton
(c)Answer: A compound made of hydrogen and carbon atoms only.
a compound / molecule containing hydrogen and carbon (atoms)
only
(d)Answer: Propane
propane
Question 2Medium3 marks
A technician tests an unknown colourless liquid taken from crude oil processing.
A few drops of the liquid are added to bromine water: the bromine water stays orange.
A small sample is burned: it burns with a clean, non-smoky blue flame.
(a) What does the bromine water result show about the liquid?[1]
(b) Suggest what type of hydrocarbon this liquid is, and suggest whether it has a small or large number of carbon atoms. Give a reason for your suggestion about chain length.[2]
Show the answer and mark scheme
(a)Answer: It is saturated (it does not contain a carbon–carbon double bond).
the liquid is saturated / does not contain a C=C double bond (it is not an alkene)
(b)Answer: An alkane, probably with a small number of carbon atoms: a clean, non-smoky flame suggests complete combustion, which is typical of shorter-chain hydrocarbons.
an alkane (a saturated hydrocarbon)
a small number of carbon atoms (short chain), because it burns with a clean, non-smoky flame (long-chain hydrocarbons burn with smokier flames)
Question 3Hard6 marks
The alkanes are a homologous series with the general formula CnH2n+2. Relative atomic masses (Ar): H = 1 C = 12
(a) An alkane has a relative formula mass (Mr) of 114. Determine the molecular formula of the alkane.[3]
(b) Calculate the percentage by mass of carbon in this alkane. Give your answer to 3 significant figures.[2]
(c) How many hydrogen atoms are there in one molecule of the alkane with 20 carbon atoms?[1]