Alkanes (AQA GCSE Chemistry): Revision Notes
Alkanes
What are alkanes?
Alkanes are a type of hydrocarbon. A hydrocarbon is a compound that contains only hydrogen atoms and carbon atoms.
Most of the hydrocarbon molecules found in crude oil are alkanes. Crude oil is a mixture containing a very large number of different compounds.
Crude oil is one of the most important sources of alkanes in the real world. The separation and processing of these different alkane compounds from crude oil forms the basis of the petroleum industry.
The general formula for alkanes
All alkanes follow the same pattern. They have the general formula .
This means:
- n = the number of carbon atoms
- For every n carbon atoms, there are (2n+2) hydrogen atoms
When writing alkane formulae, be careful not to write C₂H₁₀ or C₄H₁₀ - these would be wrong! Always use the general formula to check your answer.
Worked Example: Using the General Formula
Let's find the molecular formula for an alkane with 5 carbon atoms:
Step 1: Identify the value of n n = 5 (number of carbon atoms)
Step 2: Apply the general formula Number of hydrogen atoms = 2n + 2 = 2(5) + 2 = 10 + 2 = 12
Step 3: Write the molecular formula (pentane)
The first three alkanes
Here are the first three alkanes you need to know:
Methane ()
- Contains 1 carbon atom
- Contains 4 hydrogen atoms
- Simplest alkane
Ethane ()
- Contains 2 carbon atoms
- Contains 6 hydrogen atoms
Propane ()
- Contains 3 carbon atoms
- Contains 8 hydrogen atoms
Memory tip: All alkane names end in '-ane'. This makes them easy to identify: methane, ethane, propane, butane, pentane, and so on.
How alkanes are bonded
In alkanes, atoms are held together by covalent bonds. This means atoms share electrons.
Key bonding facts:
- Each hydrogen atom forms one bond
- Each carbon atom forms four bonds
- Carbon atoms are joined together by single covalent bonds only
The fact that carbon always forms four bonds is crucial for understanding alkane structures. This is why carbon can form long chains and complex structures while still following predictable patterns.
Displayed structures
You can show alkane structures in two ways:
- Molecular formula (like )
- Displayed structure (showing all the bonds as lines)
In displayed structures:
- Each line represents one covalent bond
- You can see exactly how all the atoms connect together
Displayed structures are particularly helpful for visualising larger alkane molecules where the molecular formula alone doesn't show you the arrangement of atoms. They help you understand the three-dimensional shape of molecules.
Saturated hydrocarbons
Alkanes are called saturated hydrocarbons. This means:
- Their carbon atoms are joined together by single covalent bonds only
- They contain the maximum possible number of hydrogen atoms
- They cannot take on any more hydrogen atoms
The term "saturated" comes from the idea that the molecule is "saturated" with hydrogen - it has as many hydrogen atoms as it can possibly hold given its carbon skeleton.
Key Points to Remember:
- Alkanes are hydrocarbons - they contain only carbon and hydrogen atoms
- General formula is - use this to work out any alkane formula
- All alkane names end in '-ane' - methane, ethane, propane, butane
- Alkanes are saturated - they have single bonds only between carbon atoms
- Each carbon makes four bonds, each hydrogen makes one bond