🌡 Boiling point
Gets higher. Small molecules boil at low temperatures. Big molecules need much more heat.
Crude Oil and Fuels » Properties of Hydrocarbons
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.7.1.3
- How boiling point, viscosity and flammability change as hydrocarbon molecules get bigger
- Why these properties decide how a hydrocarbon is used as a fuel
- What happens to a hydrocarbon in complete combustion
- How to write balanced equations for complete combustion
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Hydrocarbons are not all the same. A methane molecule has one carbon atom, but some molecules in crude oil have dozens. Three properties change in a clear pattern as the molecules get bigger: boiling point, viscosity and flammability. These properties decide how each hydrocarbon is used as a fuel.
Key terms:
- Boiling point: the temperature at which a liquid turns into a gas.
- Viscosity: how thick and sticky a liquid is, and how slowly it flows.
- Flammability: how easily a substance catches fire.
- Complete combustion: burning in plenty of oxygen so that all the fuel is turned into carbon dioxide and water.
The three trends
See how slowly that thick oil pours? Bigger hydrocarbon molecules mean higher viscosity and higher boiling points
You must be able to recall what happens to each property as the molecules get bigger (more carbon atoms).
💧 Viscosity
Gets higher. Small molecules form runny liquids. Big molecules form thick, sticky liquids.
🔥 Flammability
Gets lower. Small molecules catch fire easily. Big molecules are harder to set alight.
So the pattern is: bigger molecules mean a higher boiling point, a higher viscosity and a lower flammability.
Remember, the bonds inside a molecule do not break when a liquid boils. Only the weak forces between molecules are overcome. Bigger molecules have stronger forces between them, so more energy is needed.
How the trends affect uses
A fuel has to suit the job. Small molecules are easy to ignite and are runny or gaseous, so they work as fuels that need to light quickly. Big molecules are thick and hard to ignite, so they are used where that does not matter.
- Small molecules (such as those in LPG and petrol) have low boiling points, flow easily and ignite easily. They are good for fuels that must light and flow quickly.
- Big molecules (such as those in heavy fuel oil) have high boiling points, are thick and are hard to ignite. They must be heated before they flow well and burn.
A runny, easily lit fuel is needed in a car engine. A thick, slow fuel would not flow through the engine properly.
Combustion of hydrocarbons
A clean blue flame means complete combustion - the fuel's carbon and hydrogen are oxidised to carbon dioxide and water
When a hydrocarbon fuel burns, it reacts with oxygen and releases energy. In combustion, the carbon and hydrogen in the fuel are oxidised: they gain oxygen.
In complete combustion there is plenty of oxygen. The products are always the same two substances:
- the carbon becomes carbon dioxide, CO2
- the hydrogen becomes water, H2O
The word equation for any complete combustion is:
hydrocarbon + oxygen → carbon dioxide + water
Writing balanced combustion equations
You may be given the formula of a hydrocarbon and asked to write the balanced equation. Follow these steps.
- Write the formula of the hydrocarbon, then + O2 → CO2 + H2O.
- Balance the carbon first, using the number in front of CO2.
- Balance the hydrogen next, using the number in front of H2O.
- Balance the oxygen last. Count the oxygen atoms on the right and work out how many O2 molecules are needed.
- Check every atom is the same on both sides. If you get a half, double the whole equation.
Worked example 1: heptane, C7H16
Carbon: 7 carbon, so 7 CO2. Hydrogen: 16 hydrogen, so 8 H2O. Oxygen on the right: 14 + 8 = 22 atoms, so 11 O2.
C7H16 + 11O2 → 7CO2 + 8H2O
Worked example 2: butane, C4H10
Carbon: 4 CO2. Hydrogen: 10 hydrogen, so 5 H2O. Oxygen on the right: 8 + 5 = 13 atoms, which needs 6½ O2. You cannot have half a molecule in the final answer, so double everything.
2C4H10 + 13O2 → 8CO2 + 10H2O
Worked example 3: octane, C8H18
Carbon: 8 CO2. Hydrogen: 18 hydrogen, so 9 H2O. Oxygen on the right: 16 + 9 = 25 atoms, which needs 12½ O2. Double everything.
2C8H18 + 25O2 → 16CO2 + 18H2O
Common mistakes
Changing the small numbers in a formula (for example writing CO3) instead of the big numbers in front. Balancing the oxygen first, which makes it much harder. Forgetting that oxygen is O2, not O. Forgetting to count the oxygen in both the CO2 and the H2O.
Exam-style question
Pentane has the formula C5H12. Write the balanced equation for the complete combustion of pentane. [3 marks]
Model answer
Carbon dioxide and water are the products: C5H12 + O2 → CO2 + H2O. Five carbon atoms give 5CO2. Twelve hydrogen atoms give 6H2O. Oxygen on the right: 10 + 6 = 16 atoms, so 8O2.
C5H12 + 8O2 → 5CO2 + 6H2O
Exam tip
Balance in the order carbon, hydrogen, oxygen. Always finish by counting each atom on both sides.