Year 9 Science · Unit 2 · Lesson 11
Hydrocarbons and Simple Alkanes
Foundation Worksheet
Learning Goals
- Name the first five alkanes and write their molecular formulas.
- Use the general formula CₙH₂ₙ₊₂ to decide whether a formula belongs to an alkane.
- Use boiling point data to predict whether an alkane is a gas or a liquid.
Warm Up
Fill the gap
Choose the correct word from the word bank. Each word is used once. Two words will not be used.
Alkanes are the simplest hydrocarbons. The smallest alkane is , CH₄. Alkanes form a homologous series: a family of compounds with the same general , CₙH₂ₙ₊₂, where each member differs from the next by one unit. Each new member makes the carbon one atom longer. Alkanes are called because every carbon–carbon bond is a single bond: there are no bonds. As the chain grows, the point rises, because longer molecules attract each other more strongly. The three-carbon alkane, , is sold in LPG bottles for barbecues.
Name the diagram
The diagram shows the displayed formulas of four alkanes. Name each one and write its molecular formula.
| Label | Alkane name | Molecular formula |
|---|---|---|
| A | ||
| B | ||
| C | ||
| D |
Your Turn
Sort it: alkane or not?
Test each formula against CₙH₂ₙ₊₂, then write it in the correct box.
Fits CₙH₂ₙ₊₂: an alkane
Does not fit: not an alkane
1. Pentane has 5 carbons. Use CₙH₂ₙ₊₂ to find how many hydrogens it has, then give its formula. Show your working.
Molecular formula
Go Deeper
Read the data
| Alkane | Carbon atoms | Boiling point (°C) |
|---|---|---|
| Methane | 1 | −162 |
| Ethane | 2 | −89 |
| Propane | 3 | −42 |
| Butane | 4 | −1 |
| Pentane | 5 | 36 |
Source: the values used in the lesson.
2. Anything that boils below 25 °C has already boiled on a 25 °C day. Which of the five are gases on such a day, and which one is not?
3. In Table 1, each extra carbon raises the boiling point by a little less than the step before. Use this pattern to estimate the boiling point of hexane (6 carbons). Is hexane a gas or a liquid at 25 °C?
Scenario: a cold morning in the Snowy Mountains
A camper’s stove burns butane, stored as a liquid in a canister; it only works if the butane boils to a gas. At dawn (−5 °C) it will not light, but a friend’s propane stove lights straight away.
4. Use Table 1 to explain why the butane stove fails at −5 °C but the propane stove works.
Show What You Know
5. A friend says: “Methane (CH₄) is a gas, pentane (C₅H₁₂) is a liquid and candle wax (about C₂₅H₅₂) is a solid, so they cannot all belong to the same family.” Explain why all three are alkanes, and why their states at room temperature are different.