Chemistry • Year 12 • Module 7 • Lesson 3
Hydrocarbons: Structure, Homologous Series & Physical Properties
Synthesise data, evaluate competing claims, and critique scientific arguments about hydrocarbon properties at Band 5–6 level.
1. Data–driven evaluation — which hydrocarbon suits the job?
Read the scenario and data, then answer the extended-response question below.
Scenario
A chemical engineer at an Australian oil and gas company is selecting a hydrocarbon for two different industrial purposes: (A) a solvent that must remain liquid between 15°C and 80°C for an outdoor process in northern Queensland, and (B) a gaseous fuel to be piped from a central storage unit and used at room temperature (25°C) without pressurised storage cylinders.
The engineer has access to the five straight-chain alkanes summarised below.
| Alkane | Formula | Boiling point (°C) | State at 25°C, 1 atm | Miscible with water? |
|---|---|---|---|---|
| Methane | CH4 | −162 | Gas | No |
| Propane | C3H8 | −42 | Gas | No |
| Butane | C4H10 | −1 | Gas | No |
| Hexane | C6H14 | 69 | Liquid | No |
| Octane | C8H18 | 126 | Liquid | No |
In your extended response, address all of the following: 8 marks
- Identify and justify the best alkane for purpose A (liquid solvent, 15–80°C). Use boiling point data and LDF reasoning; compare at least two candidates.
- Identify and justify the best alkane for purpose B (gaseous fuel at 25°C without pressurised storage). Explain why other candidates are unsuitable.
- Explain, using intermolecular force theory, why all compounds in the table are immiscible with water.
- Compare the expected boiling points of hexane and octane using London dispersion force theory. Explain which structural feature of octane accounts for its higher boiling point, using the terms “surface area” and “electron cloud size.”
- Reach an evidence-based judgement naming your final selection for each purpose and one key limitation of your choice.
2. Source critique — evaluate the scientific claim
Read the following excerpt from a student’s assignment, then answer the questions below. 7 marks
Student excerpt (Year 12 Chemistry assignment, submitted 2025)
“Hexane (C6H14) has a higher boiling point than hex-1-yne (C6H10) because hexane contains stronger covalent bonds. Hexane has only single C–C bonds, but hex-1-yne contains a triple bond which is actually weaker overall and releases energy more easily, so hex-1-yne requires less energy to boil. This also explains why alkynes are more reactive than alkanes — their bonds break more easily during boiling. Furthermore, hexane is slightly soluble in water because small hydrocarbon molecules can fit between water molecules, disrupting fewer hydrogen bonds.”
(a) Identify three distinct scientific errors in the student’s excerpt. For each error, state the incorrect claim and explain the correct chemistry. 6 marks (2 per error)
(b) Describe a simple experimental observation that would directly disprove the student’s claim that hexane is “slightly soluble in water.” 1 mark
Q1 — Model answer (8 marks)
Purpose A — liquid solvent, 15–80°C (2 marks): Hexane (BP 69°C) is liquid from below 15°C to 69°C, so it evaporates before 80°C and is unsuitable at high temperatures. Octane (BP 126°C) is liquid across the entire required range (15–80°C) and its boiling point is well above 80°C, making it the best candidate. Correct identification of octane [1]; comparison of at least two candidates using BP data [1].
LDF reasoning for purpose A (1 mark): Octane has a longer chain (C8 vs C6), more electrons, greater surface area, and stronger London dispersion forces than hexane → higher boiling point → remains liquid at 80°C.
Purpose B — gaseous fuel at 25°C, no pressurised storage (1 mark): Methane (BP −162°C), propane (BP −42°C), and butane (BP −1°C) are all gases at 25°C. However, methane (BP −162°C) is so far below room temperature that it cannot be liquefied without cryogenic cooling — used as LNG. Propane (BP −42°C) can be stored as a liquid under moderate pressure; without pressurised storage it is a gas at 25°C. Butane (BP −1°C) is borderline — just a gas at 25°C but easily liquefied at slight pressure. Methane is the best choice if “no pressurised storage” means truly atmospheric pressure, as it is already gaseous and its weak LDF mean it cannot be liquefied easily without special equipment. [Accept propane or methane with clear reasoning.] [1 mark]
Immiscibility with water (1 mark): All alkanes are non-polar hydrocarbons. Dissolving any of them in water would require disrupting water’s strong hydrogen-bond network. Alkanes can only form weak dispersion forces with water molecules, which cannot adequately replace the broken H-bonds. The energy trade-off is unfavourable, so all alkanes are immiscible with water regardless of chain length.
Hexane vs octane LDF comparison (1 mark): Octane (C8) has a longer carbon chain than hexane (C6). Longer chains have a larger electron cloud (more electrons) and greater molecular surface area, which allows more simultaneous instantaneous dipole–induced dipole interactions (London dispersion forces) between neighbouring molecules. This increase in dispersion force strength raises the boiling point from 69°C (hexane) to 126°C (octane), which is why octane — not hexane — remains liquid across the full 15–80°C range required for purpose A.
Evidence-based judgement (1 mark): Final selections: Purpose A = octane (limitation: flammable, relatively high cost); Purpose B = methane or propane (limitation: flammable gas, explosion risk; methane requires cryogenic storage as liquid). [1 mark for naming both with a valid limitation for each.]
Q2(a) — Three errors in the student excerpt
Error 1 — Boiling point depends on covalent bonds: Incorrect claim: “hexane has a higher boiling point because it has stronger covalent bonds.” Correct chemistry: boiling point depends on intermolecular force strength, not intramolecular covalent bond strength. To boil a liquid, molecules must be separated from each other (intermolecular forces overcome); the covalent bonds within each molecule remain intact. Hexane (C6) has a higher boiling point than hex-1-yne (also C6) primarily because hexane’s longer saturated chain gives slightly greater surface area and stronger LDF. [1 mark for identifying the error; 1 mark for correct explanation.]
Error 2 — Alkynes are more reactive because bonds break more easily during boiling: Incorrect claim: reactivity of alkynes is explained by bonds breaking during boiling. Correct chemistry: chemical reactivity and physical boiling are completely different processes. Boiling involves overcoming intermolecular forces (between molecules) — the covalent bonds within each molecule remain intact. Alkynes are more reactive than alkanes because of the availability of π electrons at the triple bond that are accessible to reagents in addition reactions, not because their bonds are weaker or break during boiling. [1 mark each.]
Error 3 — Hexane is slightly soluble in water: Incorrect claim: hexane is slightly soluble because small molecules fit between water molecules. Correct chemistry: hexane is non-polar and is insoluble in water (not “slightly soluble”). Dissolving hexane would require disrupting water’s hydrogen-bond network, which hexane cannot replace with comparable interactions. Molecular size is irrelevant to this argument; polarity is the determining factor. [1 mark each.]
Q2(b) — Experimental disproof of hexane solubility
Adding hexane to water and shaking the mixture: a soluble substance would produce a clear, single-phase solution. Hexane forms two distinct layers (hexane floats on water) even after vigorous shaking, demonstrating it is insoluble, not “slightly soluble.” The two-layer observation directly contradicts the claim. [1 mark for describing a two-phase / layer observation that constitutes clear evidence of insolubility.]