MASTER CHEMISTRY

Model Answer (3 marks):
• In gases, particles are far apart / have large spaces between them [1]
• Applying pressure forces particles closer together / reduces the space [1]
• In solids, particles are already very close together / touching / no space between them so they cannot be pushed closer [1]

Model Answer (4 marks):
• Brownian motion is the random, erratic movement of small visible particles (e.g. pollen, smoke) suspended in a fluid [1]
• The particles change direction unpredictably / zig-zag randomly [1]
• This is caused by unequal/random collisions of invisible gas or liquid molecules hitting the visible particles [1]
• This is evidence that molecules/particles are in constant random motion as predicted by kinetic theory [1]

Model Answer (3 marks):
• The fastest-moving particles escape from the liquid surface [1]
• This reduces the average kinetic energy of the particles remaining in the liquid [1]
• Lower average kinetic energy = lower temperature; this is the cooling effect of evaporation [1]

Model Answer (4 marks):
• Ionic compounds have a giant lattice structure of many oppositely charged ions [1]
• High melting point: large amount of energy needed to overcome many strong electrostatic attractions [1]
• In solid state, ions are in fixed positions and cannot move → no electrical conduction [1]
• When molten, ions are free to move and carry charge → conducts electricity [1]

Model Answer (2 marks):
• Ar = (35 × 75 + 37 × 25) / 100 [1]
• = (2625 + 925) / 100 = 3550 / 100 = 35.5 [1]

Model Answer (5 marks):
• Diamond: each C atom bonded to 4 others in a tetrahedral 3D giant covalent lattice [1]
• Diamond is very hard because all bonds are strong covalent bonds in 3D → used in cutting tools [1]
• Graphite: each C atom bonded to 3 others; forms flat hexagonal layers [1]
• Layers held by weak forces → can slide over each other → lubricant [1]
• One electron per carbon is delocalised → graphite conducts electricity [1]

Answer:
n = m / Mr = 44 / 44 = 1.0 mol

Model Answer (3 marks):
• Divide by Ar: C = 40/12 = 3.33; H = 6.7/1 = 6.7; O = 53.3/16 = 3.33 [1]
• Divide by smallest (3.33): C = 1; H = 2.01 ≈ 2; O = 1 [1]
• Empirical formula: CH₂O [1]

Model Answer (3 marks):
• Moles CaCO₃ = 10 / 100 = 0.1 mol [1]
• Moles CO₂ = 0.1 mol (1:1 ratio) [1]
• Volume = 0.1 × 24 = 2.4 dm³ [1]

Model Answer (3 marks):
• Moles NaOH = 0.200 × 25.0/1000 = 0.00500 mol [1]
• Moles HCl = 0.00500 mol (1:1 ratio from NaOH + HCl → NaCl + H₂O) [1]
• Concentration HCl = 0.00500 / (20.0/1000) = 0.250 mol/dm³ [1]

Model Answer (3 marks):
• Mass of pure Fe = 80/100 × 250 = 200 g [1]
• Moles Fe = 200 / 56 = 3.57 mol (3 s.f.) [2]

Model Answer (4 marks):
• Moles H₂SO₄ = 0.100 × 18.0/1000 = 0.00180 mol [1]
• Moles NaOH = 2 × 0.00180 = 0.00360 mol [1]
• Concentration NaOH = 0.00360 / (30.0/1000) = 0.120 mol/dm³ [1]
• Award mark for correct unit: mol/dm³ [1]

Model Answer (4 marks):
• Cathode: the spoon (object to be plated) [1]
• Anode: a piece of pure silver metal [1]
• Electrolyte: silver nitrate solution (aqueous silver salt) [1]
• Silver ions from solution are reduced at the cathode → silver deposited on spoon; anode dissolves to replenish Ag⁺ ions [1]

Model Answer (4 marks):
• (a) Cathode: Cu²⁺ + 2e⁻ → Cu (reduction) [2]
• (b) Anode (copper): Cu → Cu²⁺ + 2e⁻ (oxidation) [2]
(1 mark each for correct species, 1 mark each for correct charges/electrons)

Model Answer (4 marks):
• Cathode: Lead (Pb) is deposited [1] — Pb²⁺ ions gain electrons / are reduced: Pb²⁺ + 2e⁻ → Pb [1]
• Anode: Bromine (Br₂) is produced [1] — Br⁻ ions lose electrons / are oxidised: 2Br⁻ → Br₂ + 2e⁻ [1]

Model Answer (3 marks):
• Energy to break bonds: 436 + 242 = 678 kJ [1]
• Energy released forming bonds: 2 × 431 = 862 kJ [1]
• ΔH = 678 − 862 = −184 kJ/mol (exothermic) [1]

Model Answer (3 marks):
• Activation energy is the minimum energy required for a reaction to occur / for bonds to be broken [1]
• A catalyst provides an alternative reaction pathway [1]
• This alternative pathway has a lower activation energy → more particles have enough energy → faster reaction [1]

Model Answer (3 marks):
• Mass of solution ≈ 100 g; ΔT = 36 − 20 = 16°C [1]
• Q = mcΔT = 100 × 4.18 × 16 = 6688 J [1]
• = 6.69 kJ (released, exothermic) [1]

Model Answer (5 marks):
• Examples: Combustion (e.g. burning wood/gas), Neutralisation (acid + alkali), Respiration, Rusting of iron, Hand warmers [1 each, max 3] [3]
• Explanation using combustion: bonds broken in CH₄ and O₂ require energy (endothermic) [1]
• More energy is released when new C=O and O–H bonds form in CO₂ and H₂O → overall exothermic [1]

Model Answer (3 marks):
• Increasing temperature increases the kinetic energy of particles [1]
• Particles move faster → collide more frequently [1]
• More collisions have energy ≥ activation energy → more successful collisions → faster rate [1]

Model Answer (4 marks):
• Sulfur precipitate forms as the reaction proceeds, making the solution increasingly cloudy/opaque [1]
• When the cross disappears beneath the flask, the reaction has reached a fixed "extent" of reaction; time is recorded [1]
• Rate ∝ 1/time; faster rate = shorter time [1]
• Control variables (any 2): volume of solutions, temperature, distance of flask from eye/cross size, same flask [1]

Model Answer (3 marks):
• Both curves level off at the SAME final volume (horizontal plateau) [1]
• Fast reaction (a): steep initial gradient, reaches plateau quickly [1]
• Slow reaction (b): gentle initial gradient, takes longer to reach the same plateau [1]

Model Answer (3 marks):
• Increasing pressure decreases the volume / compresses the gas [1]
• Gas particles are closer together / higher concentration of gas particles [1]
• More frequent collisions per unit time → more successful collisions → faster rate [1]

Model Answer (5 marks):
• Add excess copper(II) oxide to warm dilute sulfuric acid and stir [1]
• Filter to remove excess unreacted copper(II) oxide [1]
• Heat the blue filtrate (copper sulfate solution) to evaporate until saturated [1]
• Leave to cool and crystallise; filter to collect crystals [1]
• Pat dry with filter paper / leave in warm oven to dry (do not overheat — may lose water of crystallisation) [1]

Model Answer (3 marks):
• Full equation: 2HCl(aq) + Na₂CO₃(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g) [1]
• Ionic: 2H⁺(aq) + CO₃²⁻(aq) → H₂O(l) + CO₂(g) [2] (−1 for wrong/missing state symbols)

Model Answer (4 marks):
• Strong acid = fully ionised in solution (all molecules produce H⁺) [1]
• Weak acid = partially/incompletely ionised in solution (only some molecules produce H⁺) [1]
• HCl is strong: HCl → H⁺ + Cl⁻ (100%) → lower pH at same concentration [1]
• CH₃COOH is weak: CH₃COOH ⇌ CH₃COO⁻ + H⁺ (partial) → higher pH at same concentration [1]

Model Answer (3 marks):
• NH₃ + HNO₃ → NH₄NO₃ [1]
• Ammonium nitrate contains nitrogen (from both NH₄⁺ and NO₃⁻) [1]
• Nitrogen is needed for plant growth / protein synthesis / promotes leaf growth [1]

Model Answer (4 marks):
• Observation: solution turns orange/brown [1]
• Cl₂ + 2KBr → 2KCl + Br₂ / Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂ [1]
• Chlorine is reduced (gains electrons) from 0 to −1 state [1]
• Bromide is oxidised (loses electrons) from −1 to 0; Cl₂ is a stronger oxidising agent than Br₂ [1]

Model Answer (4 marks):
• Pure iron has layers of atoms that slide over each other easily [1]
• In steel, carbon atoms (different size) disrupt the regular layers, making it harder for layers to slide → steel is harder [1]
• Uses of stainless steel: cutlery / surgical instruments / kitchen sinks / food processing equipment (any 2) [2]

Model Answer (5 marks):
• Conditions: 450°C temperature, 200 atm pressure, iron catalyst, N₂:H₂ ratio = 1:3 [2]
• N₂ + 3H₂ ⇌ 2NH₃ (forward reaction is exothermic) [1]
• Very low temperature: yield of NH₃ would be high but rate too slow → not economically viable [1]
• Very high temperature: rate is fast but equilibrium shifts to left (less NH₃) → lower yield [1]
• 450°C = compromise: acceptable rate AND acceptable yield

Model Answer (6 marks):
• Fermentation: raw material = glucose (from renewable crops); hydration: ethene (from crude oil — non-renewable) [1]
• Fermentation: ~30°C, yeast, anaerobic (slow batch process); Hydration: 300°C, H₃PO₄ catalyst, 65 atm (continuous) [1]
• Fermentation is much slower; hydration gives faster continuous production [1]
• Fermentation produces dilute impure ethanol needing distillation; hydration gives purer ethanol directly [1]
• Fermentation is renewable/sustainable; hydration depletes finite fossil fuel resources [1]
• Fermentation is cheaper in less-developed countries; hydration needs expensive industrial infrastructure [1]

Model Answer (4 marks):
• (a) A diamine and a dicarboxylic acid (or diacid chloride) [1]
• (b) Water (H₂O) is eliminated when the monomers join [1]
• (c) Property: strong, tough, resistant to abrasion, low friction, can be drawn into fibres [1]
• Use: ropes, clothing (stockings), toothbrush bristles, gears/bearings [1]

Answer:
Rf = distance moved by spot / distance moved by solvent = 6.0 / 8.0 = 0.75

Model Answer (6 marks):
• Reagent: add aqueous NaOH to separate samples [1]
• Fe²⁺: green precipitate → Fe²⁺ + 2OH⁻ → Fe(OH)₂ (green ppt) [1]
• Green ppt slowly turns brown/rust on standing (oxidised by atmospheric O₂) [1]
• Fe³⁺: red-brown precipitate immediately → Fe³⁺ + 3OH⁻ → Fe(OH)₃ (red-brown ppt) [1]
• Both precipitates insoluble in excess NaOH [1]
• Precipitate not formed if NaOH added to solid — must be in solution; mark for clear procedure [1]

Model Answer (6 marks):
• Add water: BaSO₄ is insoluble → white residue confirms BaSO₄; filter [1]
• Flame test on filtrate: yellow flame → Na⁺ present → Na₂CO₃ may be present [1]
• Add dilute HCl to filtrate: effervescence/CO₂ → limewater turns milky → CO₃²⁻ → confirms Na₂CO₃ [1]
• Add NaOH to filtrate and warm: pungent NH₃ gas; turns damp red litmus blue → NH₄⁺ → confirms NH₄Cl [1]
• To confirm BaSO₄: acidify with HNO₃, add Ba(NO₃)₂ solution — no extra precipitate [1]
• Award mark for logical order avoiding interference between tests [1]

Model Answer (4 marks):
• Mr Fe₂O₃ = (2×56) + (3×16) = 160 [1]
• Moles Fe₂O₃ = 320/160 = 2.0 mol [1]
• Moles Fe = 2 × 2.0 = 4.0 mol (from 1:2 ratio) [1]
• Mass Fe = 4.0 × 56 = 224 g [1]

Model Answer (5 marks):
• Sun's radiation (short wavelength) passes through atmosphere and warms Earth's surface [1]
• Earth radiates heat (longer wavelength/infrared); CO₂ and other greenhouse gases absorb and re-emit this radiation back to Earth [1]
• Increasing CO₂ traps more heat → global temperature rise (enhanced greenhouse effect) [1]
• Consequence 1: Melting of polar ice caps → rising sea levels → flooding of low-lying areas [1]
• Consequence 2: More extreme weather events (droughts, floods, storms) / disruption of ecosystems / loss of biodiversity [1]

Model Answer (4 marks):
• C₃H₈ + 5O₂ → 3CO₂ + 4H₂O [1]
• Exothermic reaction [1]
• Incomplete combustion produces CO (carbon monoxide) [1]
• CO is toxic / colourless and odourless / binds to haemoglobin preventing O₂ transport → can cause death [1]

Model Answer (4 marks):
• Divide by Ar: C = 85.7/12 = 7.14; H = 14.3/1 = 14.3 [1]
• Divide by smallest (7.14): C = 1; H = 2.0 → Empirical formula = CH₂ [1]
• Mr of CH₂ = 14; n = 42/14 = 3 [1]
• Molecular formula = C₃H₆ (propene) [1]

Model Answer (5 marks):
• Test 1 rules out CO₃²⁻ (would effervesce with HCl) [1]
• Test 2: white precipitate with acidified BaCl₂ = SO₄²⁻ (sulfate) present [1]
• Ionic equation: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) [1]
• Precipitate = barium sulfate (BaSO₄) [1]
• Flame test (lilac/violet) → K⁺ (potassium ion) present [1]
• Solution X contains K₂SO₄ / potassium sulfate