HSC Exam Practice

Sample response. Saponification is the base hydrolysis of a triglyceride (fat or oil) with a concentrated alkali (NaOH or KOH), producing glycerol (propane-1,2,3-triol) and the sodium or potassium salt of a long-chain fatty acid (soap). The two organic products are: (1) glycerol (C₃H₅(OH)₃); (2) sodium carboxylate (soap, e.g. sodium stearate, C₁₇H₃₅COONa).

Marking notes. 1 mark: definition includes base hydrolysis of a triglyceride with NaOH/KOH. 1 mark: glycerol (or propane-1,2,3-triol) correctly named. 1 mark: sodium/potassium carboxylate (soap) correctly named.

Sample response. C₃H₅(OOCC₁₇H₃₅)₃ + 3NaOH → C₃H₅(OH)₃ + 3C₁₇H₃₅COONa. Conditions: concentrated NaOH(aq), heat under reflux.

Marking notes. 1 mark: correct balanced equation with single forward arrow (→, NOT ↔). 1 mark: correct coefficient 3 for both NaOH and sodium stearate. 1 mark: conditions stated (conc. NaOH AND heat under reflux; both required for this mark).

Sample response. Saponification produces the carboxylate salt (RCOO⁻Na⁺), not the free carboxylic acid (RCOOH). For the reverse reaction (re-esterification of carboxylate with glycerol) to occur, the carboxylate anion must first be protonated back to the free acid (RCOOH), which requires strongly acidic conditions — the opposite of the basic (alkaline) conditions used in saponification. Under basic conditions, this protonation cannot occur; the reverse reaction is therefore chemically blocked, and the reaction proceeds to completion (~100% yield).

Marking notes. 1 mark: identifies that the product is the carboxylate salt (RCOO⁻), not the free acid. 1 mark: explains that re-esterification requires acidic conditions to protonate COO⁻ back to COOH. 1 mark: concludes that under basic saponification conditions the reverse pathway is blocked — reaction is irreversible / goes to completion.

Sample response. A soap molecule is amphipathic, meaning it contains both a water-compatible and an oil-compatible region in the same molecule. The hydrophobic tail is a long non-polar hydrocarbon chain (C₁₄–C₁₈); it is attracted to non-polar oils and fats via London dispersion forces ("like dissolves like") and is insoluble in water. The hydrophilic head is an ionic carboxylate group (–COO⁻Na⁺); it is strongly attracted to water molecules via ion–dipole interactions and is highly soluble in water.

Marking notes. 1 mark: correctly uses the term amphipathic and explains its meaning (compatible with both polar and non-polar environments). 1 mark: identifies and describes the hydrophobic tail (long non-polar hydrocarbon chain). 1 mark: identifies and describes the hydrophilic head (ionic carboxylate, –COO⁻Na⁺). 1 mark: names the correct IMFs — London dispersion for tail with oils; ion–dipole for head with water.

Sample response. Ionic equation: 2C₁₇H₃₅COO⁻(aq) + Ca²⁺(aq) → (C₁₇H₃₅COO)₂Ca(s)↓. Precipitate: calcium stearate (or calcium carboxylate; colloquially soap scum). Practical consequences: (1) white/grey deposits form on bathroom surfaces, sinks, clothing, and hair; (2) soap molecules are consumed in the scum precipitate and are no longer available for cleaning — more soap must be used per wash, increasing cost and waste.

Marking notes. 1 mark: correct ionic equation with correct coefficients and state symbols (s)↓ for precipitate. 1 mark: precipitate correctly named (calcium stearate / calcium carboxylate). 1 mark each for any two correct practical consequences (max 2 marks; e.g. surface deposits / scum; soap wastage requiring more soap per wash; fabric stiffening/greying; hair deposits).

Sample response. (i) Head group: soap has a carboxylate head (–COO⁻Na⁺); an anionic synthetic detergent has a sulfonate head (–SO₃⁻Na⁺). (ii) Hard water: soap's carboxylate head reacts with Ca²⁺/Mg²⁺ to form an insoluble precipitate (scum), reducing cleaning effectiveness; the detergent's sulfonate head forms soluble Ca²⁺/Mg²⁺ salts, so no scum forms and cleaning is unimpaired. (iii) Feedstock: soap is made from renewable natural fats and oils (plant or animal); synthetic anionic detergents are derived from non-renewable petrochemicals.

Marking notes. 1 mark per criterion correctly distinguished (head group; hard water behaviour; feedstock), max 3.

Part (a) — Theoretical yield calculation (3 marks).
Moles of glyceryl tristearate = 10.0 g ÷ 891.48 g mol⁻¹ = 0.01122 mol [1 mark].
Moles of sodium stearate = 3 × 0.01122 = 0.03366 mol [1 mark].
Theoretical mass = 0.03366 mol × 306.46 g mol⁻¹ = 10.31 g (accept 10.3–10.4 g) [1 mark].
Full working must be shown; award partial marks for correct method with arithmetic error.

Part (b) — Percentage yield for Trial 2 (1 mark).
% yield = (25.6 g ÷ 10.31 g) × 100 = 248%.
Note: A yield above 100% indicates experimental error in the student's collection method (likely incomplete washing/drying of glycerol and salt impurities from the soap), not a chemical anomaly. Award 1 mark for correct calculation using their Part (a) theoretical yield value, regardless of whether the result exceeds 100%.

Part (c) — Account for Trial 1 vs Trial 2 yield difference (3 marks).
Trial 1 used only 1.0 mol NaOH for a triglyceride with 3 ester bonds, so only approximately 1/3 of the ester bonds could be hydrolysed; with insufficient NaOH, two ester bonds remain unreacted and the soap yield is approximately 1/3 of maximum [1 mark]. Trial 2 used 3.0 mol NaOH (stoichiometrically correct: 3 mol per mol triglyceride), providing enough alkali to hydrolyse all three ester bonds [1 mark]. Heating under reflux in Trial 2 provided activation energy to overcome the energy barrier of ester hydrolysis, increasing the reaction rate and ensuring the reaction reached completion within 60 min; without heat (Trial 1), the reaction proceeds very slowly and incompletely [1 mark].

Part (a) — Trend description (2 marks). As water hardness increases, the percentage of households reporting soap scum increases markedly: from 12% (Sydney, ~40 mg/L) to 78% (Murray–Darling, ~380 mg/L) to 91% (Broken Hill, ~520 mg/L) [1 mark]. The relationship is positive and steep: a ~9.5-fold increase in hardness (40 → 380 mg/L) corresponds to a ~6.5-fold increase in scum incidence (12% → 78%) [1 mark for quantitative reference to data].

Part (b) — Account for trend with ionic equation (2 marks). As Ca²⁺ concentration increases in hard water, more soap carboxylate anions react with Ca²⁺: 2RCOO⁻(aq) + Ca²⁺(aq) → (RCOO)₂Ca(s)↓ [1 mark for correct ionic equation]. The product, calcium carboxylate, is insoluble and precipitates as visible white soap scum. At higher Ca²⁺ concentrations, more soap is precipitated per litre of water, resulting in more extensive scum deposits and higher rates of household reporting [1 mark for explanation linking Ca²⁺ concentration to amount of precipitate].

Sample response. Both soap and synthetic detergents clean by the same fundamental mechanism: each is an amphipathic molecule with a long hydrophobic non-polar tail (London dispersion attractions with grease) and a hydrophilic head (ion–dipole or H-bond attractions with water). Above the critical micelle concentration, molecules aggregate into micelles, trapping grease in the non-polar interior and dispersing it in water — emulsification, not dissolution. In this core mechanism, the two are equivalent.

However, the two classes diverge significantly in hard water. Soap's carboxylate head (–COO⁻) reacts with Ca²⁺ and Mg²⁺ ions (common in inland Australian bore water, e.g. Murray–Darling basin, ~350–520 mg/L): 2RCOO⁻(aq) + Ca²⁺(aq) → (RCOO)₂Ca(s)↓. The precipitate (calcium carboxylate, soap scum) is insoluble, removing soap from solution, depositing on surfaces and fabrics, and wasting soap. At 380 mg/L Ca²⁺, bar soap may achieve only ~40–50% grease removal efficiency. Synthetic anionic detergents (e.g. sodium dodecylbenzenesulfonate, head group –SO₃⁻) form soluble calcium salts — no scum precipitates, and cleaning efficiency is maintained at ~87% regardless of hardness. In hard-water regions like inland NSW, this is a genuine and practically significant advantage of synthetic detergents.

However, "always superior" is too absolute. In soft-water regions (e.g. Sydney catchment, ~40 mg/L), soap and detergents have comparable cleaning performance — the hard-water advantage disappears. Furthermore, soap has two genuine environmental advantages: (1) biodegradability — fatty acid carboxylate salts biodegrade readily (>90% in 28 days), whereas early synthetic detergents were poorly biodegradable and caused river foam in Australian waterways through the 1950s–60s; modern formulations are now required to be biodegradable but some still carry a higher aquatic toxicity risk; (2) feedstock — soap is manufactured from renewable animal tallow and plant oils (Australian tallow from the beef industry), while most synthetic detergents are derived from non-renewable petrochemicals with a higher manufacturing carbon footprint. NSW phosphate-free detergent regulations (2014) addressed nutrient runoff to the GBR catchment, but biodegradability and feedstock origin remain relevant considerations.

Conclusion: the claim is correct in hard-water contexts (e.g. inland NSW, Murray–Darling basin) where soap's scum-forming reaction with Ca²⁺ substantially impairs cleaning. It is incorrect as an absolute statement: in soft water, soap and detergents are equivalent in performance; soap has renewable and biodegradable advantages in all water types; and "always superior" ignores application specificity, water chemistry, environmental context, and cost. An evidence-based recommendation would be to use synthetic detergents where water hardness exceeds ~150 mg/L Ca²⁺, and to prefer certified biodegradable formulations to minimise aquatic ecosystem impact.

Marking criteria.

• 1 mark — Correctly describes the shared cleaning mechanism of soap and detergents (amphipathic structure, micelle formation, emulsification — must name all three for this mark).
• 1 mark — Writes the correct ionic equation for soap scum formation in hard water with correct coefficients and state symbols.
• 1 mark — Explains at the molecular level why soap forms scum in hard water but detergents (sulfonate/non-ionic head groups) do not.
• 1 mark — Uses a specific named Australian hard-water context (Murray–Darling, Broken Hill, inland NSW, or any specific region with cited hardness value).
• 1 mark — Identifies that in soft water (Sydney or equivalent) soap and detergents have comparable performance — demonstrates the context-dependence of the claim.
• 1 mark — Addresses biodegradability and/or feedstock as an environmental dimension where soap is advantageous.
• 1 mark — Evaluates the claim as partially correct rather than wholly right or wrong, with justification that references both hard-water advantage of detergent and soft-water equivalence or environmental advantage of soap.
• 1 mark — Precision and integration: consistent use of correct chemical terminology (carboxylate, sulfonate, amphipathic, emulsification, ionic equation, hard water) throughout the response.