AQA GCSE Chemistry Foundation (8462), Foundation tier · Organic chemistry › Carbon compounds as fuels and feedstock
Practise Properties of hydrocarbons. 6 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How the size of a hydrocarbon molecule affects its boiling point, viscosity and flammability, why this decides how it is used as a fuel, and how to write balanced equations for complete combustion. Expect data-table questions, 'compare' questions and 'write a balanced equation' questions.
Key facts
Bigger molecules → higher boiling point
Bigger molecules → more viscous (flow less easily)
Bigger molecules → less flammable
Viscosity: how thick a liquid is (high viscosity = flows slowly)
Complete combustion: hydrocarbon + oxygen → carbon dioxide + water
Combustion releases energy; the C and H in the fuel are oxidised
CH4 + 2O2 → CO2 + 2H2O
C3H8 + 5O2 → 3CO2 + 4H2O
Notes
Properties depend on molecule size
diagram
Some properties of hydrocarbons depend on the size of their molecules (the number of carbon atoms).
As the molecules get bigger: the boiling point increases, the viscosity increases (the liquid is thicker and flows less easily) and the flammability decreases (they are harder to set alight).
Boiling points of the alkanes C1 to C12: they rise as the molecules get bigger.
So small hydrocarbons are gases or runny liquids that ignite easily, and large ones are thick liquids or solids that are hard to ignite.
Using the properties
These properties decide how a hydrocarbon is used as a fuel.
Hydrocarbons with small molecules make good fuels: they are very flammable and flow easily. Petrol is an example.
Hydrocarbons with large molecules are less useful as fuels, because they are viscous and hard to ignite.
Complete combustion
Burning (combustion) of hydrocarbon fuels releases energy.
During combustion, the carbon and hydrogen in the fuel are oxidised: they combine with oxygen.
Complete combustion (in plenty of oxygen): hydrocarbon + oxygen → carbon dioxide + water.
Example: CH4 + 2O2 → CO2 + 2H2O
To balance: (1) put the number of C atoms in front of CO2; (2) put half the number of H atoms in front of H2O; (3) count the O atoms on the right and halve that number for O2; (4) if you get a half, double everything.
Example: C2H6 gives 2CO2 + 3H2O, which has 7 O atoms, so 3½O2. Doubling: 2C2H6 + 7O2 → 4CO2 + 6H2O.
How to answer each type of question
Compare the properties of two hydrocarbons
1 to 3 marksGrade 4
Decide which hydrocarbon has the bigger molecules.
State how each property compares, using 'higher', 'more viscous' and 'less flammable'.
Example. Hydrocarbon A has 6 carbon atoms in each molecule. Hydrocarbon B has 20 carbon atoms in each molecule. Compare the boiling point, viscosity and flammability of A and B.
Show the model answerHide the model answer
B has a higher boiling point than A (1). B is more viscous than A (1). B is less flammable than A (1).
Use data to choose a fuel
2 to 3 marksGrade 5
Compare each boiling point with room temperature (about 20 °C) to find the state.
Use molecule size to judge flammability and viscosity.
Quote the data in your answer.
Example. The table gives data for three hydrocarbons.
Hydrocarbon
Formula
Boiling point in °C
P
C3H8
−42
Q
C10H22
174
R
C18H38
317
Which hydrocarbon would be best as the fuel for a gas barbecue? Explain your answer.Show the model answerHide the model answer
P (1). Its boiling point (−42 °C) is below room temperature, so it is a gas (1). It has the smallest molecules, so it is the most flammable and ignites easily (1).
Don’t lose marks
Saying larger molecules are more flammable 'because there is more fuel'. They are less flammable.
Explaining boiling points by breaking covalent bonds. Boiling overcomes the weak forces between molecules.
Giving carbon monoxide or soot as products of complete combustion. They come from incomplete combustion.
Changing a formula to balance an equation, e.g. writing CO3. Only change the numbers in front.
Doubling only some terms when clearing a half, e.g. 2C2H6 + 7O2 → 2CO2 + 3H2O.
More tips
Memory tricks
Think honey and petrol: long chains are like honey (thick, hard to light); short chains are like petrol (runny, catch fire easily).
Two up, one down: boiling point and viscosity go up with size; flammability goes down.
Balancing order: C, then H, then O. Oxygen goes last because O2 is on its own.
Odd number of O atoms on the right? Double every number in the equation.
Alkane check: n carbons need (3n + 1) ÷ 2 molecules of O2. Propane: (9 + 1) ÷ 2 = 5.
Exam technique
'Compare' means say how the two are different, using comparative words: higher, more viscous, less flammable.
When you use data, quote the numbers with units, e.g. 'its boiling point, −42 °C, is below room temperature'.
After balancing an equation, count every atom on both sides. It takes 20 seconds and catches most slips.
Use the words 'viscosity' and 'flammability' rather than 'thickness' and 'burns better'.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Name the products of complete combustionA hydrocarbon burns completely in oxygen to form carbon dioxide and water.
Grade 4
Recall how properties change with molecule sizeBigger molecules: higher boiling point, higher viscosity and lower flammability.
Grade 5
Choose a suitable fuel using property dataFor example, a fuel for a gas stove must have a boiling point below room temperature and be very flammable.
Grade 5
State what happens to the fuel in combustionThe carbon and hydrogen in the fuel are oxidised, and energy is released.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Name the two products of the complete combustion of a hydrocarbon.
Carbon dioxide and water
What is meant by the viscosity of a liquid?
How easily it flows (how thick it is).
Write a balanced symbol equation for the complete combustion of hexane.
2C6H14 + 19O2 → 12CO2 + 14H2O
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy2 marks
(a) Which statement about alkanes is correct? Tick (✓) one box.[1]
As the chain length increases, the boiling point increases
As the chain length increases, the flammability increases
As the chain length increases, the viscosity decreases
Short-chain alkanes are more viscous than long-chain alkanes
(b) What is meant by the viscosity of a liquid?[1]
Show the answer and mark scheme
(a)Answer: As the chain length increases, the boiling point increases
(b)Answer: How easily it flows (how thick it is).
how easily the liquid flows / its resistance to flowing / how thick it is
Question 2Medium5 marks
Alkanes are used as fuels.
(a) Balance the equation for the complete combustion of propane. C3H8 + .....O2 → .....CO2 + .....H2O[1]
(b) Write a balanced equation for the complete combustion of butane, C4H10.[2]
(c) During combustion, the carbon and hydrogen in the fuel are oxidised. What does ‘oxidised’ mean here?[1]
(d) Explain why the combustion of hydrocarbons is useful.[1]
Show the answer and mark scheme
(a)Answer: C3H8 + 5O2 → 3CO2 + 4H2O
C3H8 + 5O2 → 3CO2 + 4H2O
(b)Answer: 2C4H10 + 13O2 → 8CO2 + 10H2O
correct formulae: C4H10 + O2 → CO2 + H2O
balanced: 2C4H10 + 13O2 → 8CO2 + 10H2O
(c)Answer: They gain oxygen.
they gain oxygen / combine with oxygen
(d)Answer: It releases a lot of energy, so hydrocarbons can be used as fuels.
it releases (a lot of) energy / is exothermic, so hydrocarbons are useful fuels