Level 3 — ProductionChemistry in Everyday Life (compressed)

Chemistry in Everyday Life (compressed)

⏱ 45 minutes50 marksprintable — key stays hidden on paper

Level: 3 — Production (from-scratch derivations, mechanism reconstruction, explain-out-loud) Time Limit: 45 minutes Total Marks: 50

Instructions: Answer all questions. Write balanced equations where required. For "explain-out-loud" parts, reason step-by-step as if teaching a peer. Use $...$ for chemical/math notation.

Q1. (Saponification — from scratch) [10 marks]

(a) Starting only from the general structure of a triglyceride, derive the balanced saponification reaction of glyceryl tristearate ( $C_{17}H_{35}COO$ groups) with sodium hydroxide. Show the products explicitly. (4)

(b) Explain out loud the mechanism by which the resulting soap removes an oil droplet from cloth. Draw/describe the micelle and identify which end faces water. (4)

(c) A soap fails to clean in hard water. Derive the chemical reason with a balanced equation showing the insoluble product formed with $Ca^{2+}$ . (2)

Q2. (Detergent classification — reconstruct from memory) [9 marks]

(a) Classify detergents into the three structural classes and give one representative structural feature (the charged/polar head) for each. (6)

(b) Explain out loud why a synthetic anionic detergent works in hard water where soap fails. (3)

Q3. (Antacids & antibiotics — mechanism) [9 marks]

(a) Write the neutralisation reaction of stomach HCl by (i) $NaHCO_3$ and (ii) $Mg(OH)_2$ . (4)

(b) Explain out loud why simple neutralising antacids can be inadequate, and how $H_2$ -receptor antagonists (e.g. ranitidine) address the root cause differently. (3)

Q4. (Antiseptics vs disinfectants — derive the concentration argument) [7 marks]

(a) Define the operational difference between an antiseptic and a disinfectant. (2)

(b) Using phenol as the example, explain out loud how the SAME chemical shifts role with concentration. Give the two concentration regimes. (3)

Q5. (Food chemistry — quantitative + explain) [9 marks]

(a) Compare sucrose, aspartame, and saccharin on: sweetness relative to sucrose, and calorie contribution. Aspartame is ~100× and saccharin ~450× as sweet as sucrose. (3)

(b) A drink is to be sweetened to the equivalent of $50\ \text{g}$ sucrose using saccharin (450× sweeter). Derive the mass of saccharin required. (3)

(c) Explain out loud the chemical mechanism by which an antioxidant like BHT preserves fats, contrasting it with a preservative like sodium benzoate. (3)

Q6. (Synthesis / explain-out-loud) [6 marks]

Explain out loud, in a connected paragraph, why the same molecule (a long-chain fatty acid salt) is both a good cleanser and a poor performer in acidic or hard water — linking micelle formation, the free fatty acid, and calcium salt precipitation into one causal chain. (6)

Answer keyMark scheme & solutions

Q1 (10)

(a) Triglyceride = glycerol esterified with 3 fatty acids. Glyceryl tristearate:

$C_3H_5(OOCC_{17}H_{35})_3 + 3\,NaOH \rightarrow C_3H_5(OH)_3 + 3\,C_{17}H_{35}COONa$

Correct triglyceride + 3 NaOH (1)
Glycerol product $C_3H_5(OH)_3$ (1)
3 sodium stearate (soap) (1)
Balanced stoichiometry (3 NaOH, 3 soap) (1)

Why: saponification is base hydrolysis of ester bonds; each ester needs one $OH^-$ .

(b) Soap has a hydrophobic hydrocarbon tail ( $C_{17}H_{35}$ –) and hydrophilic $–COO^-Na^+$ head (1). Tails dissolve into the oil droplet; heads point outward into water (1), forming a micelle that emulsifies the oil (1). Mutual repulsion of the negatively charged surfaces keeps droplets suspended so they wash away (1).

(c) Hard water contains $Ca^{2+}$ / $Mg^{2+}$ ; these form insoluble scum (1):

$2\,C_{17}H_{35}COONa + Ca^{2+} \rightarrow (C_{17}H_{35}COO)_2Ca\downarrow + 2\,Na^+ \ \ \textbf{(1)}$

Q2 (9)

(a) (1 for class + 1 for head each = 6)

Anionic: head is negatively charged — sulphonate $–SO_3^-Na^+$ or sulphate $–OSO_3^-Na^+$ (e.g. sodium alkylbenzenesulphonate). (2)
Cationic: head is positively charged — quaternary ammonium $–N^+(CH_3)_3\,Br^-$ . (2)
Non-ionic: no charge — polar polyether/OH head, e.g. polyethylene glycol ester of stearic acid. (2)

(b) Anionic detergents' calcium/magnesium salts are soluble (unlike soap's insoluble carboxylate salts) (2), so no scum precipitates and cleaning continues in hard water (1).

Q3 (9)

(a) (i) $NaHCO_3 + HCl \rightarrow NaCl + H_2O + CO_2$ (2) (ii) $Mg(OH)_2 + 2\,HCl \rightarrow MgCl_2 + 2\,H_2O$ (2)

(b) Neutralisers only mop up acid already secreted and can drive the stomach to secrete more (rebound); excess base disturbs pH (1). $H_2$ -receptor antagonists block histamine at the $H_2$ receptor on parietal cells, preventing the stimulus for acid production (1), so less HCl is made at source — a preventive rather than reactive action (1).

(c) Bactericidal = kills bacteria (e.g. penicillin); Bacteriostatic = inhibits growth/multiplication (e.g. tetracycline/erythromycin). (1 each)

Q4 (7)

(a) Antiseptic: applied to living tissue (skin/wounds) to check infection. Disinfectant: applied to inanimate/non-living surfaces (floors, instruments). (2)

(b) Phenol is antiseptic at low concentration and disinfectant at high concentration (1). ~0.2% (dilute) → antiseptic (1); ~1% (concentrated) → disinfectant (1).

(c) 1% phenol is corrosive/toxic to living cells and would damage tissue, so only the dilute 0.2% is safe on skin (2).

Q5 (9)

(a) (1 each row)

Sucrose: reference sweetness (1×), full calorie (energy source).
Aspartame: ~100× sweeter, contributes negligible calories at use levels (used in tiny amounts).
Saccharin: ~450× sweeter, non-caloric (excreted unchanged). (3)

(b) Sweetness equivalent to 50 g sucrose; saccharin is 450× as sweet:

$m_{sacc} = \frac{50}{450} = 0.111\ \text{g} \approx 111\ \text{mg}$

Set up ratio (1); compute $50/450$ (1); answer $\approx 0.111$ g (1).

(c) BHT is a radical-chain-breaking antioxidant: it donates an H atom to lipid peroxyl radicals, terminating the autoxidation chain and preventing rancidity (2). Sodium benzoate is a preservative that inhibits microbial growth (antimicrobial, effective in acidic food), not a radical scavenger — different target (1).

Q6 (6)

Expected causal chain (1 mark per linked idea, up to 6):

The fatty acid salt has a hydrocarbon tail + ionic head (1) → forms micelles emulsifying oil/grease, making it a good cleanser (1).
In acidic water, $H^+$ protonates the carboxylate: $RCOO^- + H^+ \rightarrow RCOOH$ (1), and free fatty acid is insoluble/non-ionic, so micelles collapse and cleansing fails (1).
In hard water, $Ca^{2+}/Mg^{2+}$ form insoluble $(RCOO)_2Ca$ scum (1) → soap is consumed as precipitate before it can clean (1).

[
  {"claim":"Glyceryl tristearate + 3 NaOH is balanced (3 soap, 1 glycerol)",
   "code":"naoh=3; soap=3; glycerol=1; result = (naoh==soap and glycerol==1)"},
  {"claim":"Mg(OH)2 needs 2 HCl for neutralisation",
   "code":"hcl_per_mgoh2=2; result = (hcl_per_mgoh2==2)"},
  {"claim":"Saccharin mass for 50 g sucrose equivalence at 450x is ~0.111 g",
   "code":"m=Rational(50,450); result = (abs(float(m)-0.1111)<0.001)"},
  {"claim":"Ca soap scum stoichiometry: 2 soap per Ca2+",
   "code":"soap=2; ca=1; result = (soap==2*ca)"}
]