Year 9 Science · Unit 2 · Lesson 13
Challenge Worksheet
Learning Goals
1. In a straight-chain alkane, each end carbon carries 3 hydrogen atoms and each middle carbon carries 2. Use this to explain where the “+2” in CₙH₂ₙ₊₂ comes from.
2. A technician labels a gas cylinder “octane, C₈H₁₈”. Straight-chain pentane, hexane and heptane boil at 36 °C, 69 °C and 98 °C. The petrol fraction boils from 40 to 175 °C. Estimate octane’s boiling point, predict its state at 25 °C and name its crude oil fraction. What is wrong with the label?
Find the mistakes: a student’s naming homework
(a) CH₃CH₂CH₂CH₂CH₂CH₂CH₃ is heptane, C₇H₁₄.
(b) Straight-chain octane has formula C₈H₁₈, and its condensed formula is CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH₃.
(c) A chain of three carbons with a CH₃ branch on the middle carbon is propane, because the main chain has three carbons.
(d) C₅H₁₂ can only be drawn one way.
3. Three of the four statements are wrong. Identify each one, explain the error, and write a correct version.
Data: octane ratings of isomers
The octane rating (RON) measures how smoothly a fuel burns in an engine without knocking. Heptane is set at 0 and isooctane at 100. A fuel rated 95 behaves like a mixture of 95% isooctane and 5% heptane. You do not need to learn the branched names.
| Alkane (shape) | Carbon atoms | Octane rating (RON) |
|---|---|---|
| pentane (straight) | 5 | 62 |
| 2-methylbutane (branched) | 5 | 92 |
| hexane (straight) | 6 | 25 |
| 2-methylpentane (branched) | 6 | 73 |
| heptane (straight) | 7 | 0 |
| 2-methylhexane (branched) | 7 | 42 |
| isooctane (highly branched) | 8 | 100 |
Source: published research octane numbers, rounded to whole numbers.
4. A student concludes: “The longer the chain, the higher the octane rating.” Use the three straight-chain alkanes to test this conclusion.
5. For 5, 6 and 7 carbons, find the gap in rating between the two shapes. What do the three gaps show?
5 C 6 C 7 C
6. Premium petrol is rated 98. What mixture of isooctane and heptane does it behave like? Which single alkane in Table 1 would come closest to it?
7. Student A says: “A molecular formula like C₆H₁₄ tells you everything about a molecule.” Student B says: “No, the condensed formula CH₃CH₂CH₂CH₂CH₂CH₃ tells you more.” Evaluate both positions. Use hexane and its isomer 2-methylpentane, which boils at 60 °C (hexane boils at 69 °C), and Table 1.