HSC Exam Practice

Sample response. A homologous series is a family of organic compounds that share the same functional group (or general formula) and differ from one successive member to the next by a single –CH₂– unit, showing gradually changing physical properties but similar chemical properties.

Marking notes. 1 mark for identifying a family with the same functional group / general formula; 1 mark for –CH₂– difference and/or gradually changing physical properties.

Sample response. Alkane: C_nH_2n+2; C–C single bond; sp³ hybridisation; 109.5° bond angle (tetrahedral). Alkene: C_nH_2n; C=C double bond; sp² at the double-bond carbons; ~120° (trigonal planar). Alkyne: C_nH_2n−2; C≡C triple bond; sp at the triple-bond carbons; 180° (linear).

Marking notes. 1 mark per correctly completed row (formula + bond type + hybridisation + angle for each series). Partial credit accepted if 3 of 4 columns correct per row.

Sample response. Both are non-polar hydrocarbons, so the only intermolecular forces present are London dispersion forces [1]. Octane (C8) has a much longer carbon chain than propane (C3): it has more electrons, a more polarisable electron cloud, and a greater surface area for intermolecular contact, resulting in stronger LDF [1]. To separate octane molecules into the gas phase (boiling) requires more energy to overcome these stronger forces, so octane has a much higher boiling point (126 vs −42°C) [1].

Marking notes. 1 mark for identifying LDF as the only IMF; 1 mark for linking longer chain to greater surface area / more electrons / stronger LDF; 1 mark for concluding that stronger LDF require more energy → higher boiling point. Do not award the mark for “stronger covalent bonds.”

Sample response. Pentane (BP 36°C) has a higher boiling point than 2,2-dimethylpropane (BP 10°C) [1]. Both share the same molecular formula (C₅H₁₂) and molecular mass, so the difference is not due to mass [1]. Pentane is elongated (straight chain), giving greater surface area for intermolecular contact and stronger LDF; 2,2-dimethylpropane is compact and nearly spherical, reducing the contact area and LDF strength, and therefore requiring less energy to boil [1].

Marking notes. 1 mark for correctly stating which is higher; 1 mark for noting same formula/mass; 1 mark for surface area / shape reasoning.

Sample response. In water: hexane is non-polar and forms only London dispersion forces; water molecules are held by a strong hydrogen-bond network. Dissolving hexane in water would require disrupting those hydrogen bonds, but hexane cannot replace them with interactions of comparable strength, so dissolution is energetically unfavourable and hexane is insoluble [1 for correct IMF identification in water + 1 for explanation]. In heptane: both hexane and heptane are non-polar hydrocarbons, so only London dispersion forces are involved on both sides. Disrupting hexane–hexane and heptane–heptane interactions and replacing them with hexane–heptane dispersion forces involves little energetic penalty, making mixing favourable [1].

Marking notes. 1 mark for identifying H-bonds in water being disrupted without adequate replacement; 1 mark for explaining why water insolubility occurs energetically; 1 mark for dispersion-only mixing explanation for heptane.

Sample response. Straight-chain alkanes are non-polar and have only London dispersion forces between molecules. Short-chain alkanes (C1–C4: methane, ethane, propane, butane) have few electrons and small surface area, so their LDF are very weak and insufficient to hold molecules together at room temperature — they are gases at 25°C [1 for any named gas + LDF reasoning]. Longer chains (C5+: pentane, hexane, heptane) have more electrons and greater surface area, producing LDF strong enough to maintain the liquid phase at 25°C [1 for any named liquid + LDF reasoning]. The physical state of an alkane at room temperature thus depends on whether its LDF are strong enough to overcome thermal motion [1 for general principle].

(a) Sample response. As the number of carbon atoms increases from C1 to C10, the boiling point increases from −162°C (methane, C1) to +174°C (decane, C10). The increase is approximately monotonic; the gap between successive members is largest at low carbon numbers and decreases slightly toward C10. [1 mark for direction of trend; 1 mark for quoting two specific data values correctly from the graph.]

(b) Sample response. Longer alkane chains have more electrons and a larger electron cloud. This makes the cloud more easily polarised, producing stronger instantaneous dipole–induced dipole (London dispersion) interactions with neighbouring molecules. Greater surface area also means more simultaneous interaction sites. Stronger LDF require more energy (heat) to overcome during boiling, raising the boiling point. [1 mark each for: polarisability/electrons increases with chain length; stronger LDF; more energy needed to boil → higher BP. Max 3 marks.]

(c) Sample response. Propane (C3, BP −42°C) and butane (C4, BP −1°C) have very short chains and weak LDF; at typical refinery operating temperatures they are already in the gas phase and rise to the cooler top of the distillation column. Kerosene fractions (C10+) have BP above 174°C (from the graph, C10 = 174°C) due to their much longer chains and much stronger LDF; they remain liquid at higher temperatures and are drawn off lower in the column where temperatures are higher. The gradual differences in boiling point caused by chain length–dependent LDF make physical separation by fractional distillation possible. [1 mark for C3/C4 BP values from graph; 1 mark for LDF/chain length reasoning for LPG; 1 mark for kerosene comparison to C10 data; 1 mark for explicitly linking to column position.]

Sample response. The physical properties of hydrocarbons — boiling point, physical state, and solubility — are governed almost entirely by the strength of London dispersion forces (LDF), which in turn depend on molecular size and shape. LDF are weak intermolecular attractions arising from instantaneous dipoles; their strength increases with the number of electrons and the surface area available for contact between neighbouring molecules. As chain length increases along a homologous alkane series, both factors increase, so boiling point rises steadily (e.g., methane BP −162°C; octane BP 126°C). Branching in an isomer reduces surface area even at constant molecular formula: 2,2-dimethylpropane (BP 10°C) boils much lower than pentane (BP 36°C) despite having the same molecular formula C₅H₁₂, because its compact shape reduces the number of simultaneous LDF interactions. All hydrocarbons are non-polar, so they are insoluble in water (disrupting H-bonds without adequate replacement) but miscible with non-polar solvents like hexane or heptane (replacing dispersion forces with similar dispersion forces). For industrial selection: a liquid solvent required between 15–80°C must have a boiling point above 80°C; from the alkane series, octane (BP 126°C) is the best match — its longer C8 chain produces stronger LDF and a boiling point well above the required upper limit. A fuel that must be gaseous at 25°C requires a boiling point below 25°C; propane (BP −42°C) and butane (BP −1°C) are both gases at room temperature and atmospheric pressure, making them suitable without cryogenic storage.

Marking notes. 1 mark — defines LDF and their origin. 1 mark — links chain length to electron count/surface area → stronger LDF → higher BP (with named example from the alkane series). 1 mark — correctly explains branching reduces BP via surface area (with a named isomer pair example). 1 mark — explains water insolubility using H-bond disruption argument. 1 mark — explains non-polar solvent miscibility. 1 mark — applies boiling point reasoning to identify a suitable liquid solvent candidate (octane or similar) by comparing BP to the required operating range. 1 mark — applies boiling point reasoning to identify a suitable gaseous fuel candidate (propane/butane) by comparing BP to room temperature. 1 mark — reaches an explicit evaluative conclusion linking structural feature (chain length or branching) to the suitability or unsuitability of a specific hydrocarbon for a named purpose.

Sample response. The article contains a scientific error: it attributes storage difficulty to bond strength inside the molecule (“weaker bonds”), but storage conditions depend on boiling point (i.e. intermolecular force strength), not on intramolecular covalent bond strength. LNG (mainly methane, CH₄, BP −162°C) has extremely weak LDF because it is the smallest alkane with only 1 carbon atom. Its boiling point is far below room temperature, meaning it must be cooled to cryogenic temperatures (−162°C or below) to remain a liquid — this is more demanding storage, not easier. LPG (propane BP −42°C; butane BP −1°C) has longer chains, stronger LDF, and higher boiling points; it can be stored as a liquid under moderate pressure at room temperature without cryogenic cooling. The article’s claim is also confused about what “safety” means: LNG’s lower boiling point actually makes it harder and more expensive to store safely. Both gases are flammable; flammability depends on fuel properties, not on LDF-related bond discussion.

Marking notes. 1 mark — identifies the error (bond strength vs intermolecular forces). 1 mark — correctly states that storage conditions depend on boiling point / IMF. 1 mark — explains methane LDF strength and boiling point correctly (BP −162°C = cryogenic requirement). 1 mark — explains LPG (propane/butane) LDF and boiling points and moderate pressure storage. 1 mark — reaches an explicit evaluative judgement that LNG requires more demanding storage, not less.