Year 9 Science · Unit 2 · Lesson 11
Challenge Worksheet
Learning Goals
1. State the general formula of the alkanes, and use it to show that C₅H₁₀ cannot be an alkane.
Find the mistakes: a student’s homework
(a) An alkane with 7 carbon atoms: H = 2 × 7 + 2 = 16, so it is C₇H₁₆.
(b) An alkane with 9 carbon atoms: H = 2 × 9 = 18, so it is C₉H₁₈.
(c) An alkane with 26 hydrogen atoms: n = (26 − 2) ÷ 2 = 12, so it is C₁₂H₂₆.
(d) Ethane is one CH₂ unit bigger than methane (CH₄), so ethane is C₂H₅.
2. Which two lines are wrong? For each, write the correct formula and name the mistake the student made.
Scenario: a candle maker in Darwin
A candle maker wants a wax that stays solid on a shop shelf that can reach 40 °C in the build-up season. Table 1 gives data for five straight-chain alkanes. A substance is solid below its melting point, liquid between its melting and boiling points, and a gas above its boiling point.
| Carbon atoms | Melting point (°C) | Boiling point (°C) |
|---|---|---|
| 5 | −130 | 36 |
| 10 | −30 | 174 |
| 15 | 10 | 271 |
| 20 | 37 | 343 |
| 25 | 54 | 402 |
Source: CRC Handbook of Chemistry and Physics, rounded.
3. Give the formula of the 20-carbon member. Then say whether each row of Table 1 is solid, liquid or gas at 25 °C and at 40 °C.
4. Calculate the average rise in boiling point per extra carbon atom from 5 to 10 carbons, and from 20 to 25 carbons. What do your two answers show about the trend?
5 to 10: °C per C
20 to 25: °C per C
5. The candle maker is offered a cheap wax made only of the 20-carbon alkane. Advise her, using the data. Which alkane in Table 1 would you choose instead, and what is its safety margin in °C?
6. Evaluate the claim: “All alkanes have the same general formula and the same kind of bonds, so they should all have similar properties.” Use Table 1 and ideas from the lesson.