HSC Exam Practice

1.1 — Definitions (2 marks)

Primary alcohol (1°): An alcohol in which the carbon bearing the -OH group is bonded to one (or zero, as in methanol) other carbon atoms. General structure R-CH₂OH. [1]

Tertiary alcohol (3°): An alcohol in which the carbon bearing the -OH group is bonded to three other carbon atoms; no hydrogen is bonded to the C-OH carbon. General structure R-C(OH)(R’)(R”). [1]

1.2 — Classification (3 marks; 1 per compound)

(a) CH₃CH₂OH — Primary (1°): C-OH (C1) is bonded to 1 other carbon (C2). [1]

(b) CH₃CH(OH)CH₃ (propan-2-ol) — Secondary (2°): C-OH (C2) is bonded to 2 other carbons (C1 and C3). [1]

(c) (CH₃)₃COH (2-methylpropan-2-ol) — Tertiary (3°): C-OH is bonded to 3 other carbons (three CH₃ groups). [1]

1.3 — Hydrogen bonding in ethanol (3 marks)

In ethanol, hydrogen bonds form between the δ+ hydrogen of the O-H bond of one molecule and the lone pairs on the oxygen of a neighbouring molecule [1]. The O-H hydrogen (donor, carries δ+ partial charge due to high electronegativity of oxygen) donates the bond; the oxygen’s lone pairs (acceptor) receive it [1]. Each hydrogen bond has an energy of approximately 20–25 kJ/mol [1].

1.4 — Propan-1-ol vs propane BP (3 marks)

Propan-1-ol contains a hydroxyl group (-OH). The polar O-H bond makes the hydrogen a hydrogen bond donor (δ+) and the oxygen’s lone pairs hydrogen bond acceptors, enabling hydrogen bonds (~20–25 kJ/mol) between molecules [1]. Propane has only C-H and C-C bonds; only weak London dispersion forces act between propane molecules [1]. Breaking the H-bond network in propan-1-ol requires far more energy than overcoming dispersion forces in propane, resulting in a boiling point 139°C higher [1].

1.5 — Ethanol vs hexan-1-ol solubility (4 marks)

Ethanol is fully miscible with water; hexan-1-ol is practically insoluble. [1]

Both alcohols have an -OH group that can form H-bonds with water (O-H donor; lone-pair acceptor on O). [1]

Ethanol (C2): the small 2-carbon non-polar chain creates minimal disruption to water’s H-bond network when dissolved. The -OH interaction with water compensates for this small disruption — fully miscible. [1]

Hexan-1-ol (C6): the large 6-carbon non-polar chain requires significant disruption of water-water H-bonds. The energy cost of incorporating the 6-carbon chain into water’s H-bond network far exceeds what the single -OH can compensate for — practically insoluble. [1]

1.6 — Methanol classification (2 marks)

Methanol is classified as primary by convention because its C-OH carbon (which has no other carbon neighbours) satisfies the “≤1 other carbon” criterion for primary classification. The primary class includes alcohols where C-OH is bonded to at most one other carbon. [1]

Common student error: classifying methanol as a “0° alcohol” because its C-OH has zero carbon neighbours. No such classification exists; methanol is unambiguously primary. [1]

2.1 — C3 gap and IMF (3 marks)

Difference: 97 − (−42) = 139°C. [1]

Propan-1-ol has an -OH group enabling hydrogen bonds (~20–25 kJ/mol) between molecules via the δ+ O-H hydrogen (donor) and oxygen lone pairs (acceptor). [1]

Propane has only C-H and C-C bonds; only weak London dispersion forces (~1–5 kJ/mol) act between molecules. Overcoming ethanol’s H-bond network requires far more energy than propane’s dispersion forces. [1]

2.2 — Shared trend in both series (3 marks)

London (dispersion) forces increase with chain length in both series. [1]

Longer chains have greater molecular surface area, enabling more contact between neighbouring molecules. [1]

Greater contact area increases the magnitude of instantaneous dipoles and induced dipoles between molecules, strengthening dispersion forces and requiring more energy to separate molecules — higher boiling point. This trend operates on top of H-bonding in alcohols and is the sole intermolecular force in alkanes. [1]

2.3 — Butan-1-ol vs 2-methylpropan-2-ol (3 marks)

Both have molecular formula C₄H₁₀O with one -OH group: equivalent hydrogen bonding (same donor/acceptor capacity). H-bonding alone does not explain the 35°C difference. [1]

Butan-1-ol is a primary alcohol (straight chain, 118°C): the extended chain maximises molecular surface area, enabling stronger London dispersion forces. [1]

2-methylpropan-2-ol is a tertiary alcohol (most branched, 83°C): the compact, spherical molecular shape minimises surface area, weakening dispersion forces. Lower BP results. [1]

3.1 — Source critique: two errors (5 marks)

Error 1 (2 marks): “water is organic too” — Water (H₂O) is an inorganic compound; it contains no carbon atoms. The claim that water is organic is incorrect [1]. Ethanol is miscible with water not because both are “organic”, but because ethanol’s -OH group can form hydrogen bonds with water molecules (O-H donor and lone-pair acceptor), and ethanol’s small 2-carbon non-polar chain causes minimal disruption to water’s H-bond network. The “like dissolves like” principle refers to polar solutes in polar solvents, not organic vs inorganic [1].

Error 2 (2 marks): “This is the same reason ethanol mixes easily with water — because like dissolves like, and water is organic too” — The statement misapplies “like dissolves like” by claiming ethanol mixes with water because both are “organic.” Water is not organic, and that is not why ethanol dissolves in water [1]. The correct explanation (from the lesson’s IMF framework) is that ethanol is miscible with water because the -OH group acts as a hydrogen bond donor (δ+ H on O-H) and acceptor (lone pairs on O), forming H-bonds with water molecules. The small 2-carbon non-polar chain causes minimal disruption to water’s H-bond network. It is the “like dissolves like” principle in the sense that both water and ethanol are polar molecules capable of hydrogen bonding — not because both are “organic” [1].

Overall quality (1 mark): Two fundamental scientific errors in one short passage (misclassification of water as organic; incorrect mechanism of germicidal action) indicate this source is unreliable for HSC Chemistry. Students should verify all claims against NESA-approved resources. [1]

4.1 — Extended response marking criteria (7 marks)

H-bonding baseline (1 mark): All alcohols have one -OH group that acts as a H-bond donor (O-H, δ+) and acceptor (lone pairs on O). H-bonding (~20–25 kJ/mol) is the dominant IMF distinguishing alcohols from alkanes and is essentially equivalent across the C4 isomers.

Effect of chain length on BP (1 mark): Increasing chain length increases molecular surface area → stronger London dispersion forces in addition to H-bonding → higher boiling point. Demonstrated by the primary alcohol series: methanol (65°C) < ethanol (78°C) < propan-1-ol (97°C) < butan-1-ol (118°C).

Effect of branching on BP, with named C4 examples (2 marks): Among constitutional isomers (same molecular formula, same -OH), BP differences arise from branching/shape effects on dispersion forces [1]. Butan-1-ol (1°, straight, 118°C) > 2-methylpropan-1-ol (1°, branched, 108°C) > butan-2-ol (2°, 100°C) > 2-methylpropan-2-ol (3°, most compact/branched, 83°C). More branching → more compact shape → less surface area → weaker dispersion forces → lower BP [1].

Effect of chain length on water solubility (1 mark): Shorter chains (C1–C3 alcohols) are fully miscible with water because -OH H-bonds with water and the small chain causes minimal disruption to water’s H-bond network. Longer chains (C5+) are sparingly soluble or insoluble because the growing non-polar chain disrupts more water H-bonds than the single -OH can compensate for.

Effect of branching on water solubility, with named example (1 mark): Branching increases water solubility for the same carbon count. 2-methylpropan-2-ol (3°, branched, fully water-miscible) vs butan-1-ol (1°, straight, partially miscible): the compact shape of the tertiary alcohol exposes less non-polar surface to water, reducing disruption to water’s H-bond network — the -OH compensates more effectively.

Overall evaluation/conclusion (1 mark): Molecular structure affects both properties: chain length determines the overall level of dispersion forces (longer = higher BP) and the balance between non-polar/polar character (longer = less water-soluble). Degree of branching modulates surface area (more branched = lower BP; more water-soluble) without affecting H-bonding strength. The single -OH group provides a fixed H-bonding contribution that becomes relatively less important as chain length grows.