Alkanes (AQA GCSE Chemistry Combined Science): Revision Notes
Alkanes
What are hydrocarbons?
Hydrocarbons are chemical compounds that contain only two types of atoms: hydrogen and carbon. These are some of the most important compounds you'll study in organic chemistry.
Crude oil contains a mixture of many different compounds. Most of these compounds are hydrocarbons, with alkanes being the main type found in crude oil.
What are alkanes?
Alkanes are a specific family of hydrocarbons. They have some key features:
- They follow a general formula: CH (where n = number of carbon atoms)
- All alkane names end with "-ane"
- They are made up of only carbon and hydrogen atoms
When writing alkane formulas, be careful with the order. Write carbon first, then hydrogen. For example, methane is CH₄, not H₄C.
The first three alkanes
Let's look at the simplest alkanes you need to know:
Methane (CH₄)
- Contains 1 carbon atom and 4 hydrogen atoms
- Simplest alkane
Ethane (C₂H₆)
- Contains 2 carbon atoms and 6 hydrogen atoms
- Second simplest alkane
Propane (C₃H₈)
- Contains 3 carbon atoms and 8 hydrogen atoms
- Commonly used in camping gas
Pattern Recognition: Alkane Formula
Let's see how the CH formula works:
- Methane: n = 1, so H = 2(1) + 2 = 4 → CH₄ ✓
- Ethane: n = 2, so H = 2(2) + 2 = 6 → C₂H₆ ✓
- Propane: n = 3, so H = 2(3) + 2 = 8 → C₃H₈ ✓
You can see the pattern: as we add more carbon atoms, the number of hydrogen atoms increases following the formula!
Structure and bonding
Alkanes are held together by covalent bonds. This means atoms share electrons to stay connected.
Key bonding rules to remember:
- Each carbon atom forms 4 bonds
- Each hydrogen atom forms 1 bond
- All bonds in alkanes are single bonds
When you draw the structure of an alkane, each line represents one covalent bond. The atoms share electrons to form these bonds.
Saturated hydrocarbons
Alkanes are called saturated hydrocarbons because:
- All carbon-carbon bonds are single bonds
- No double or triple bonds exist
- The carbon atoms are "saturated" with hydrogen atoms
This is different from other hydrocarbons like alkenes, which have double bonds and are called unsaturated.
Why alkanes matter
Alkanes are incredibly important because:
- They're the main compounds in crude oil
- They're used as fuels (like methane in natural gas)
- They're used to make other chemicals
Key Points to Remember:
- Hydrocarbons contain only hydrogen and carbon atoms
- Alkanes follow the formula CH
- All alkane names end in "-ane"
- Alkanes are saturated (single bonds only)
- Carbon always makes 4 bonds, hydrogen always makes 1 bond