Learning Goals
- Identify and name the first five alkanes and write their molecular formulas.
- Use the general formula CₙH₂ₙ₊₂ to find the number of hydrogen atoms in an alkane.
- Describe how boiling point changes as alkane chain length increases.
Warm Up
Label the diagram
The diagram below shows the structural formulae of the first four alkanes. Use the lesson to fill in the table, name each alkane and complete all columns.
| Label |
Alkane name |
Molecular formula |
Number of C atoms |
Number of H atoms |
| A |
|
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| B |
|
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|
|
| C |
|
|
|
|
| D |
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|
|
Your Turn
Fill the gap
Choose the correct word from the word bank to complete the passage below. Each word is used once only. Two words in the bank are NOT needed.
methane
saturated
formula
chain
boiling
CH₂
propane
double
ionic
Alkanes are the simplest hydrocarbons. The first and smallest alkane is , which has the molecular formula CH₄. Alkanes belong to a homologous seriesa family of compounds that share the same general (CₙH₂ₙ₊₂) and differ from each other by one unit. Because each consecutive member has one extra carbon atom, the length increases by one carbon each time. Alkanes are described as because all carbon–carbon bonds are single bonds, there are no bonds. As the chain grows longer, the point increases because longer molecules attract each other more strongly. A real-life example of a three-carbon alkane is , which is used as camping gas in remote areas of Australia.
Show What You Know
1. List the first five alkanes in order from smallest to largest, giving the name and molecular formula of each one.
Recall
2 marks
2. Pentane has 5 carbon atoms. Use the general formula CₙH₂ₙ₊₂ to show your working and write the molecular formula of pentane.
Recall
2 marks
Wrap Up
In one sentence, what was the main idea of this lesson?