What a hydrocarbon is, the alkanes and their general formula, how chain length changes a fuel's properties, and how a fractionating column splits crude oil.
What you will learn
- Use the general formula CnH2n+2 to give the formula of an alkane
- Name the first four alkanes
- Explain how fractional distillation separates crude oil
- Link chain length to boiling point, viscosity and flammability
Hydrocarbons and the alkanes
Crude oil formed over millions of years from the remains of tiny sea creatures buried in mud.
It is a finite resource and a mixture of a huge number of compounds. Most of them are hydrocarbons: compounds made of hydrogen and carbon atoms only.
Crude oil has two jobs. Most of it is burned as fuel, but it is also the feedstock (the raw material) for plastics, solvents, detergents, lubricants and medicines. Burning it uses it up for ever and releases carbon dioxide, so many people argue that more of it should be saved as feedstock while other energy sources replace it as a fuel.
Most of the hydrocarbons in crude oil are alkanes. The alkanes form a homologous series: a family with the same general formula, where each member has one more CH2 than the last, and all react in similar ways.
alkanes: CnH2n+2
- 1 carbon: methane, CH4
- 2 carbons: ethane, C2H6
- 3 carbons: propane, C3H8
- 4 carbons: butane, C4H10
The start of the name counts the carbons: meth 1, eth 2, prop 3, but 4. The ending -ane means an alkane. Alkanes are saturated: every carbon to carbon bond is a single bond.
- An alkane in petrol has 8 carbon atoms. What is its formula?
- n = 8, so H = 2 × 8 + 2 = 18
- Formula: C₈H₁₈ (octane)
Double first, then add 2. A common slip is 8 + 2 = 10 hydrogens.
The rest of this lesson and its 11 practice questions are locked.
In this lesson
- Hydrocarbons and the alkanes
- Try it: Count the hydrogens
- Splitting crude oil into fractions
- Chain length decides the properties
11 practice questions, marked as you go; about 22 minutes in all.
Key terms
Mixture, Feedstock, Alkanes, Methane, Ethane, Propane, Butane, Fractional distillation, Down, Lower
In this unit: Organic chemistry
Crude oil and the alkanes, combustion and cracking, then alkenes, alcohols, carboxylic acids, esters and polymers for separate Chemistry, with the formula and mass calculations that go with them.