Level 4 — ApplicationChemistry in Everyday Life (compressed)

Chemistry in Everyday Life (compressed)

50 marksprintable — key stays hidden on paper

Time: 60 minutes Total Marks: 50 Instructions: Attempt all questions. All problems are application-based; reason from mechanisms and principles.

Q1. (10 marks) A patient suffering from hyperacidity is prescribed a combination antacid tablet containing aluminium hydroxide and magnesium hydroxide, together with a small dose of an H₂-receptor blocker.

(a) Write balanced neutralisation equations for both hydroxides reacting with stomach HCl. (3) (b) Explain why the two hydroxides are combined rather than using either alone (consider bowel side-effects). (2) (c) The H₂-blocker acts by a fundamentally different mechanism from the hydroxides. Explain both mechanisms and state why the combination is more effective long-term. (3) (d) If the stomach contains $0.050\ \text{mol}$ of excess HCl, calculate the mass of $\text{Mg(OH)}_2$ (M = 58 g/mol) alone required to neutralise it completely. (2)

Q2. (10 marks) Sodium benzoate is used as a preservative but is only effective in acidic foods (pH < 4), whereas it is nearly useless in a food of pH 6.

(a) Explain, using the acid–base behaviour of benzoic acid ( $\text{p}K_a = 4.20$ ), why activity depends on pH. (3) (b) For a soft drink buffered at pH 3.0, calculate the ratio of undissociated benzoic acid to benzoate ion. (3) (c) A manufacturer wants a "sugar-free" version and replaces sugar with aspartame. Give one chemical limitation of aspartame that restricts its use in baked goods. (2) (d) Explain how an antioxidant such as BHT preserves fats, in terms of radical chemistry. (2)

Q3. (12 marks) A student saponifies glyceryl tristearate (a fat) with NaOH.

(a) Write the saponification equation and name the two products. (3) (b) $445\ \text{g}$ of glyceryl tristearate (M = 890 g/mol) is completely saponified. Calculate the mass of sodium stearate soap produced (M of sodium stearate = 306 g/mol). (4) (c) The student notices the soap does not lather well in the local hard water. Explain chemically what happens and identify the precipitate. (3) (d) Suggest a class of cleansing agent that would work in this hard water and justify your choice. (2)

Q4. (10 marks) Three surfactants are supplied: (i) sodium lauryl sulphate, (ii) cetyltrimethylammonium bromide, (iii) a polyethylene-glycol type molecule.

(a) Classify each as anionic, cationic or non-ionic. (3) (b) One of them is used in hair conditioners rather than as a general cleanser. Identify it and explain why, relating its charge to hair surface chemistry. (3) (c) Draw a labelled diagram of a micelle removing an oily stain, indicating hydrophilic and hydrophobic ends. (2) (d) Explain why detergents are preferred over soaps for washing in hard water, at a molecular level. (2)

Q5. (8 marks) An antiseptic solution of iodine (tincture of iodine) and an antibiotic (a penicillin) are both used to prevent infection but must NOT be interchanged.

(a) Distinguish clearly between an antiseptic and an antibiotic by site of action and target range. (3) (b) Dettol contains chloroxylenol and terpineol. Explain why it can be diluted for skin but bleach (a disinfectant) cannot. (2) (c) Penicillin is described as bactericidal and narrow-spectrum. Explain both terms and give the structural feature responsible for its activity. (3)

Answer keyMark scheme & solutions

Q1 (10)

(a) (3, 1.5 each) $\text{Al(OH)}_3 + 3\,\text{HCl} \rightarrow \text{AlCl}_3 + 3\,\text{H}_2\text{O}$ $\text{Mg(OH)}_2 + 2\,\text{HCl} \rightarrow \text{MgCl}_2 + 2\,\text{H}_2\text{O}$ Why: both are basic hydroxides neutralising excess gastric HCl.

(b) (2) Mg(OH)₂ alone is laxative (causes diarrhoea); Al(OH)₃ alone is constipating. Combining them cancels the opposing bowel effects (1 mark each effect).

(c) (3) Hydroxides act chemically, neutralising acid already secreted (symptomatic). The H₂-blocker binds histamine H₂ receptors on parietal cells, preventing acid secretion at source (1+1). Long-term this stops overproduction rather than mopping up acid, so lower acid load and lasting relief (1).

(d) (2) $\text{Mg(OH)}_2 + 2\text{HCl}$ : needs $0.050/2 = 0.025$ mol Mg(OH)₂. Mass $= 0.025 \times 58 = 1.45\ \text{g}$ . (1 mol ratio, 1 mass)

Q2 (10)

(a) (3) Only the undissociated (neutral) benzoic acid can cross microbial cell membranes and lower internal pH; the benzoate anion cannot penetrate. At low pH the equilibrium lies toward neutral HA, giving high active fraction; at pH 6 it is almost fully ionised → inactive.

(b) (3) Henderson–Hasselbalch: $\text{pH} = \text{p}K_a + \log\frac{[\text{A}^-]}{[\text{HA}]}$ $3.0 = 4.20 + \log\frac{[\text{A}^-]}{[\text{HA}]} \Rightarrow \log\frac{[\text{A}^-]}{[\text{HA}]} = -1.20$ $\frac{[\text{HA}]}{[\text{A}^-]} = 10^{1.20} \approx 15.8$ So ~15.8 : 1 undissociated acid to anion. (setup 1, arithmetic 2)

(c) (2) Aspartame is unstable to heat (decomposes / loses sweetness on cooking), so unsuitable for baked goods.

(d) (2) BHT is a chain-breaking antioxidant: it donates an H atom to lipid peroxyl/free radicals, forming a stable resonance-stabilised radical, interrupting the autoxidation chain and preventing rancidity.

Q3 (12)

(a) (3) $(\text{C}_{17}\text{H}_{35}\text{COO})_3\text{C}_3\text{H}_5 + 3\,\text{NaOH} \rightarrow 3\,\text{C}_{17}\text{H}_{35}\text{COONa} + \text{C}_3\text{H}_5(\text{OH})_3$ Products: sodium stearate (soap) and glycerol.

(b) (4) Moles fat $=445/890 = 0.500$ mol. Soap moles $=3 \times 0.500 = 1.50$ mol. Mass $=1.50 \times 306 = 459\ \text{g}$ . (moles 1, ratio 1, mass 2)

(c) (3) Hard water contains $\text{Ca}^{2+}/\text{Mg}^{2+}$ . These react with stearate ions forming insoluble calcium/magnesium stearate (scum), removing soap from solution so lather fails. $2\,\text{C}_{17}\text{H}_{35}\text{COO}^- + \text{Ca}^{2+} \rightarrow (\text{C}_{17}\text{H}_{35}\text{COO})_2\text{Ca}\downarrow$

(d) (2) Synthetic detergents (e.g. sodium alkylbenzenesulphonate): their Ca/Mg salts are water-soluble, so they do not form scum and clean effectively in hard water.

Q4 (10)

(a) (3, 1 each) (i) Sodium lauryl sulphate — anionic. (ii) Cetyltrimethylammonium bromide — cationic. (iii) PEG type — non-ionic.

(b) (3) The cationic one (cetyltrimethylammonium bromide). Hair/protein surfaces carry a net negative charge; the positively charged head adsorbs onto hair, coating it, giving smoothness/conditioning (charge attraction 2, use 1).

(c) (2) Diagram: oil droplet at centre; surfactant molecules radiating outward with hydrophobic tails buried in oil and hydrophilic heads facing water. (labels 1, structure 1)

(d) (2) Detergent Ca/Mg salts remain soluble (sulphonate/sulphate heads do not precipitate), so unlike soaps they don't form insoluble scum in hard water and cleaning efficiency is retained.

Q5 (8)

(a) (3) Antiseptic: applied externally on living tissue (skin/wounds) to kill/inhibit microbes without harming tissue. Antibiotic: substance (produced by/derived from micro-organisms) taken internally/systemically to kill or inhibit specific bacteria. Antiseptics are broad topical; antibiotics target particular organisms.

(b) (2) Dettol is safe/non-corrosive to skin at low concentration, so dilution retains antiseptic action without tissue damage; bleach (disinfectant) is corrosive/toxic to living tissue and used only on inanimate objects.

(c) (3) Bactericidal = kills bacteria (vs bacteriostatic which only inhibits). Narrow-spectrum = effective against a limited range of organisms. Active structural feature: the strained β-lactam ring, which inhibits bacterial cell-wall (peptidoglycan) synthesis.

[
  {"claim":"Mg(OH)2 mass for 0.050 mol HCl = 1.45 g","code":"n=Rational(50,1000)/2; result=(n*58==Rational(145,100))"},
  {"claim":"Benzoic acid HA:A- ratio at pH 3 with pKa 4.20 ~ 15.8","code":"import sympy as sp; r=sp.Float(10)**sp.Float('1.20'); result=abs(r-15.8)<0.1"},
  {"claim":"Sodium stearate mass from 445 g tristearate = 459 g","code":"n=Rational(445,890); soap=3*n; result=(soap*306==459)"},
  {"claim":"Moles of fat = 0.5","code":"result=(Rational(445,890)==Rational(1,2))"}
]