2022 Oct/Nov · Paper 23 · Q2 (c, d)3 marksHalide ions: tests & reactions with H2SO4 · Halogen trends (volatility, bond strength, oxidising power)
2022 Oct/Nov · Paper 23 · Q3 (b)1 markChlorine with water / NaOH (disproportionation)
2022 May/June · Paper 11 · Q171 markHalide ions: tests & reactions with H2SO4
2022 May/June · Paper 12 · Q221 markHalogen trends (volatility, bond strength, oxidising power)
2022 May/June · Paper 12 · Q231 markChlorine with water / NaOH (disproportionation)
2022 May/June · Paper 13 · Q221 markHalogen displacement reactions
2022 May/June · Paper 13 · Q231 markHalogen trends (volatility, bond strength, oxidising power)
2022 May/June · Paper 22 · Q2 (d)3 marksHalide ions: tests & reactions with H2SO4
2022 May/June · Paper 22 · Q3 (d(i), d(ii), d(iii))3 marksHalide ions: tests & reactions with H2SO4
2022 May/June · Paper 23 · Q2 (b(i), b(ii))4 marksHalogen trends (volatility, bond strength, oxidising power)
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Practice questions
Original questions written by HomiLabs in the style of the exam, with answers and mark schemes. They are not
Cambridge past paper questions and have not been reviewed by Cambridge. Try each one before opening the answer.
Q1Halide ions: tests & reactions with H2SO4 · Core · Paper 2 style · 6 marks
(a)Describe what is seen when aqueous silver nitrate is added to separate acidified solutions of potassium chloride, potassium bromide and potassium iodide.[3]
(b)Describe how aqueous ammonia can be used to confirm the identity of each precipitate.[2]
(c)Write an ionic equation, with state symbols, for the formation of the precipitate with bromide ions.[1]
Show answer
(a) Chloride: white precipitate; bromide: cream precipitate; iodide: yellow precipitate.
1 mark for each correct colour
(b) AgCl dissolves in dilute NH₃; AgBr dissolves only in concentrated NH₃; AgI is insoluble even in concentrated NH₃.
B1 AgCl soluble in dilute; B1 AgBr soluble in conc only and AgI insoluble in both
(c) Ag⁺(aq) + Br⁻(aq) → AgBr(s)
B1 correct with state symbols
Q2Halide ions: tests & reactions with H2SO4 · Core · Paper 1 style · 1 mark
(a)An acidified solution of a sodium halide gives a cream precipitate with aqueous silver nitrate. The precipitate does not dissolve in dilute aqueous ammonia but dissolves in concentrated aqueous ammonia. Which ion is present?[1]
Show answer
(a) C Br⁻
Cream AgBr dissolves in concentrated but not dilute NH₃. Cl⁻ gives white AgCl, soluble in dilute NH₃; I⁻ gives yellow AgI, insoluble in both; AgF is soluble so F⁻ gives no precipitate.
Q3Halide ions: tests & reactions with H2SO4 · Core · Paper 2 style · 4 marks
(a)A student tests tap water for chloride ions. State the reagents she should add, in order, and the result for chloride ions.[2]
(b)The white precipitate darkens when left on a sunny window-sill. Suggest what is happening.[1]
(c)Name the complex ion formed when the precipitate dissolves in dilute aqueous ammonia.[1]
Show answer
(a) Dilute nitric acid, then aqueous silver nitrate; a white precipitate forms.
B1 HNO₃ then AgNO₃; B1 white precipitate
(b) Silver chloride decomposes in light to silver (and chlorine).
B1 light decomposes AgCl to Ag
(c) Diamminesilver(I) ion, [Ag(NH₃)₂]⁺
B1
Q4Halide ions: tests & reactions with H2SO4 · Core · Paper 2 style · 4 marks
A student collects seawater at Clifton beach, Karachi, and tests it with dilute nitric acid and aqueous silver nitrate.
(a)State why a halide test on seawater gives a white precipitate even though seawater contains small amounts of bromide.[1]
(b)Write the equation for the dissolving of silver chloride in aqueous ammonia.[2]
(c)Suggest why adding ammonia allows the bromide to be detected.[1]
Show answer
(a) Chloride ions are in large excess, so white AgCl dominates and hides the cream AgBr.
Q6Halide ions: tests & reactions with H2SO4 · Standard · Paper 2 style · 5 marks
Concentrated sulfuric acid is added to solid potassium bromide. First, steamy fumes of hydrogen bromide form. Some of the hydrogen bromide then reacts further with the sulfuric acid.
(a)Name the two gases (other than water vapour) formed in the redox step, and describe one observation.[2]
(b)Write an equation for the reaction in which hydrogen bromide reduces sulfuric acid.[2]
(c)State the change in oxidation number of sulfur in (b).[1]
Show answer
(a) Bromine and sulfur dioxide; orange/brown fumes (of bromine) seen.
B1 Br₂ and SO₂; B1 orange-brown/red-brown fumes
(b) 2HBr + H₂SO₄ → Br₂ + SO₂ + 2H₂O
B1 correct species; B1 balanced
(c) +6 to +4
B1
Q7Halide ions: tests & reactions with H2SO4 · Standard · Paper 1 style · 1 mark
(a)Solid potassium bromide is warmed with concentrated sulfuric acid. Which set of products forms?[1]
Show answer
(a) B HBr, Br₂ and SO₂
Br⁻ reduces H₂SO₄ only to SO₂ (S +6 → +4), forming Br₂ as well as HBr. H₂S and S need the stronger reducing agent I⁻; HBr only is the chloride pattern.
Q8Halide ions: tests & reactions with H2SO4 · Standard · Paper 3 style · 6 marks
Three white solids P, Q and R are each sodium halides. They are tested in a school laboratory in Peshawar.Test 1 (solution + dilute HNO₃ + AgNO₃(aq)): P yellow precipitate; Q white precipitate; R cream precipitate.Test 2 (precipitate + dilute NH₃(aq)): P no change; Q dissolves; R no change.Test 3 (solid + concentrated H₂SO₄): P purple fumes and smell of bad eggs; Q steamy fumes only; R steamy fumes and orange-brown fumes.
(a)Identify the anion in each of P, Q and R.[3]
(b)Explain why the solutions are acidified with dilute nitric acid, and not dilute hydrochloric acid, before silver nitrate is added.[2]
(c)State one safety precaution needed when testing with concentrated sulfuric acid.[1]
(b) Nitric acid removes carbonate (or hydroxide) ions that would also give a precipitate with Ag⁺; HCl would add Cl⁻ ions, giving a white precipitate in every test.
B1 removes CO₃²⁻/OH⁻ that interfere; B1 HCl adds chloride ions
(c) Work in a fume cupboard (toxic fumes) and wear goggles/gloves.
B1
Q9Halide ions: tests & reactions with H2SO4 · Standard · Paper 2 style · 4 marks
(a)Explain why hydrogen iodide cannot be prepared by adding concentrated sulfuric acid to sodium iodide.[2]
(b)Concentrated phosphoric(V) acid, H₃PO₄, can be used instead. Suggest why this works.[1]
(c)Write an equation for the reaction of sodium iodide with phosphoric(V) acid to form hydrogen iodide.[1]
Show answer
(a) The HI formed is a strong reducing agent and is oxidised by the sulfuric acid to iodine, so little HI is collected.
(b) Phosphoric acid is not an oxidising agent, so it does not oxidise HI.
B1 not an oxidising agent / non-oxidising acid
(c) NaI + H₃PO₄ → NaH₂PO₄ + HI
B1
Q10Halide ions: tests & reactions with H2SO4 · Challenge · Paper 2 style · 8 marks
(a)Concentrated sulfuric acid is added to solid sodium iodide. Name three sulfur-containing products that can form, giving the oxidation number of sulfur in each.[3]
(b)Write an equation for the reaction that forms hydrogen sulfide.[2]
(c)Explain why iodide ions reduce sulfuric acid further than bromide ions do.[2]
(d)Give one observation that shows iodine has been formed.[1]
Show answer
(a) SO₂ (+4), S (0), H₂S (−2).
1 mark each, name with oxidation number
(b) 8HI + H₂SO₄ → 4I₂ + H₂S + 4H₂O
B1 species; B1 balanced
(c) I⁻ is larger than Br⁻ so its outer electron is further from the nucleus and more shielded; it is lost more easily, so iodide is the stronger reducing agent.
B1 larger ion / more shielding / weaker attraction to outer electrons; B1 loses electrons more easily, stronger reducing agent
(d) Purple fumes or a black/grey solid.
B1 purple fumes / black (grey-black) solid (not a yellow solid, which is sulfur)
Q11Halide ions: tests & reactions with H2SO4 · Challenge · Paper 2 style · 6 marks
1.000 g of a mixture of sodium chloride and sodium bromide is dissolved in water and acidified. Excess aqueous silver nitrate is added. The dried precipitate of silver chloride and silver bromide has a mass of 2.000 g.
(a)Calculate the percentage by mass of sodium chloride in the mixture, to 3 significant figures. (Mr: NaCl = 58.5, NaBr = 102.9, AgCl = 143.4, AgBr = 187.8)[4]
(b)The precipitate is shaken with an excess of dilute aqueous ammonia and the solid left is filtered off and dried. Calculate the mass of solid left, in g, to 3 significant figures.[2]
Show answer
(a) 27.9%
M1 let mass NaCl = x g, NaBr = (1 − x) g; M1 mass AgCl = 143.4x/58.5 = 2.4513x; mass AgBr = 1.8251(1 − x); M1 2.4513x + 1.8251(1 − x) = 2.000; A1 x = 0.2793 g so 27.9%
(b) 1.32 g (AgBr)
M1 AgCl dissolves, AgBr remains; A1 mass AgBr = 1.8251 × 0.7207 = 1.315 g ≈ 1.32 g
Q12Halide ions: tests & reactions with H2SO4 · Challenge · Paper 2 style · 4 marks
0.050 mol of potassium bromide reacts with excess concentrated sulfuric acid. All the bromide is first converted to HBr. 40% of this HBr then reacts: 2HBr + H₂SO₄ → Br₂ + SO₂ + 2H₂O.
(a)Calculate the mass of bromine, in g, formed. Give your answer to 3 significant figures. (Mr Br₂ = 159.8)[2]
(b)Calculate the volume of sulfur dioxide, in cm³, formed at room temperature and pressure. (1 mol of gas occupies 24.0 dm³)[1]
(c)Suggest why the remaining hydrogen bromide is not oxidised.[1]
(b) Volatility decreases because molecules have more electrons, so instantaneous dipole–induced dipole (London) forces are stronger and need more energy to overcome.
B1 more electrons / larger molecules; B1 stronger id–id forces, more energy to separate molecules
Q14Halogen trends (volatility, bond strength, oxidising power) · Core · Paper 1 style · 1 mark
(a)[1]
Show answer
(a) C HI
The H–I bond is the weakest (longest), so HI decomposes most easily to H₂ and violet iodine vapour. HCl has the strongest bond and does not decompose at this temperature.
Q15Halogen trends (volatility, bond strength, oxidising power) · Standard · Paper 2 style · 4 marks
(a)Describe and explain the trend in the H–X bond energies.[2]
(b)Use these values to explain the trend in thermal stability of the hydrogen halides.[1]
(c)Explain why the Cl–Cl bond is stronger than the I–I bond.[1]
Show answer
(a) Bond energy decreases from H–Cl to H–I because the halogen atom is larger, so the bond is longer and the shared electrons are further from the halogen nucleus (weaker attraction).
B1 decreases; B1 larger atom / longer bond / weaker attraction of bonding pair
(b) Thermal stability decreases from HCl to HI because less energy is needed to break the weaker bond.
B1
(c) Cl atoms are smaller; the bonding pair is closer to both nuclei, so it is attracted more strongly.
B1
Q16Halogen trends (volatility, bond strength, oxidising power) · Standard · Paper 2 style · 5 marks
(a)State the trend in oxidising power of the halogens from chlorine to iodine.[1]
(b)Explain this trend in terms of atomic structure.[3]
(c)State the trend in electronegativity down the group.[1]
Show answer
(a) It decreases.
B1
(b) Atomic radius increases down the group and there is more shielding by inner shells; the attraction of the nucleus for an incoming electron is weaker, so the atom gains an electron less readily.
B1 larger atom / more shells; B1 more shielding; B1 weaker attraction for electron to be gained, so weaker oxidising agent
(c) It decreases.
B1
Q17Halogen trends (volatility, bond strength, oxidising power) · Standard · Paper 1 style · 1 mark
(a)Which property increases from chlorine to iodine?[1]
Show answer
(a) C strength of instantaneous dipole–induced dipole forces
Iodine molecules have the most electrons, so their London forces are strongest. Ionisation energy, electronegativity and oxidising power all decrease down the group.
Q18Halogen trends (volatility, bond strength, oxidising power) · Challenge · Paper 2 style · 5 marks
Boiling points: Cl₂ −34 °C; Br₂ 59 °C; I₂ 184 °C. Astatine, At, is below iodine in Group 17.
(a)Predict the state of astatine at room temperature and explain your prediction using the data.[2]
(b)Predict how astatine would react with hydrogen and how stable hydrogen astatide, HAt, would be to heat. Explain.[2]
(c)Would astatine displace iodine from potassium iodide solution? Explain.[1]
Show answer
(a) Solid; its boiling point should be well above that of iodine because At₂ has even more electrons and stronger London forces.
B1 solid (black); B1 stronger id–id forces, b.p. higher than 184 °C
(b) Very slow, incomplete (reversible) reaction even on heating; HAt very unstable, decomposing easily, because the H–At bond is very weak (large atom, long bond).
B1 slow/reversible reaction; B1 HAt decomposes easily, weak bond
(c) No; astatine is a weaker oxidising agent than iodine.
B1
Q19Halogen trends (volatility, bond strength, oxidising power) · Challenge · Paper 2 style · 5 marks
Chlorine and hydrogen explode in sunlight. Bromine and hydrogen react only on heating with a catalyst. Iodine and hydrogen react slowly and reversibly when heated.Bond energies / kJ/mol: H–H 436; Cl–Cl 242; Br–Br 193; I–I 151; H–Cl 431; H–Br 366; H–I 299.
(a)Use the bond energies to calculate ΔH for H₂(g) + X₂(g) → 2HX(g) for X = Cl, Br and I. Assume all species are gases.[3]
(b)Use your answers to explain the observations.[2]
(b) Reactions become much less exothermic down the group (the H–X bond formed gets weaker faster than X–X); the reaction with iodine releases little energy, so it is slow and reversible, while chlorine's large release makes it explosive.
B1 less exothermic down group; B1 links to reactivity: iodine reaction barely favourable / reversible
Q20Chlorine with water / NaOH (disproportionation) · Core · Paper 2 style · 4 marks
(a)Write an equation for the reaction of chlorine with water.[1]
(b)Give the oxidation number of chlorine in each chlorine-containing species in your equation.[2]
(c)Use your answer to (b) to explain why this is a disproportionation reaction.[1]
Show answer
(a) Cl₂ + H₂O ⇌ HCl + HClO
B1 (accept → or HOCl)
(b) Cl₂ 0; HCl −1; HClO +1
B1 HCl −1 and Cl₂ 0; B1 HClO +1
(c) Chlorine is simultaneously oxidised (0 → +1) and reduced (0 → −1).
B1 same element both oxidised and reduced
Q21Chlorine with water / NaOH (disproportionation) · Standard · Paper 2 style · 5 marks
(a)Write an equation for the reaction of chlorine with cold dilute aqueous sodium hydroxide.[1]
(b)Write an equation for the reaction of chlorine with hot concentrated aqueous sodium hydroxide.[2]
(c)Name the chlorine-containing product of (b) in which chlorine has a positive oxidation number, and give that oxidation number.[2]
Show answer
(a) Cl₂ + 2NaOH → NaCl + NaClO + H₂O
B1
(b) 3Cl₂ + 6NaOH → 5NaCl + NaClO₃ + 3H₂O
B1 products NaCl and NaClO₃; B1 balanced
(c) Sodium chlorate(V), +5.
B1 name; B1 +5
Q22Chlorine with water / NaOH (disproportionation) · Standard · Paper 1 style · 1 mark
(a)Chlorine is bubbled into hot concentrated aqueous potassium hydroxide. What are the oxidation numbers of chlorine in the two chlorine-containing products?[1]
Show answer
(a) B −1 and +5
Hot concentrated alkali gives KCl (−1) and KClO₃ (+5). A (−1 and +1) is the result with cold dilute alkali.
Q23Chlorine with water / NaOH (disproportionation) · Challenge · Paper 2 style · 5 marks
A factory near Kala Shah Kaku makes household bleach from chlorine and sodium hydroxide.
(a)Calculate the mass of sodium chlorate(I), NaClO, in kg, that could be made from 10.0 kg of chlorine with cold dilute NaOH. Give your answer to 3 significant figures. (Mr: Cl₂ = 71.0, NaClO = 74.5)[2]
(b)Calculate the volume, in dm³, of 2.00 mol/dm³ NaOH needed for (a). Give your answer to 3 significant figures.[1]
(c)If the NaOH were hot and concentrated, what mass of sodium chlorate(V), NaClO₃, in kg, would form from the same mass of chlorine? (Mr NaClO₃ = 106.5)[2]
Q24Chlorine with water / NaOH (disproportionation) · Challenge · Paper 2 style · 6 marks
(a)Write the two ionic half-equations for chlorine in hot alkali, one forming Cl⁻ and one forming ClO₃⁻.[2]
(b)Combine them to give the overall ionic equation.[1]
(c)A solution of chlorate(I) ions, ClO⁻, is warmed. Write an ionic equation for the disproportionation that occurs and state the oxidation number changes.[2]
(d)Suggest why household bleach should be stored in a cool place.[1]
(d) Warming makes the chlorate(I) disproportionate, so the bleach loses its active ingredient.
B1
Q25Halogen displacement reactions · Core · Paper 2 style · 5 marks
Chlorine water is added to three tubes containing aqueous potassium bromide (A), potassium iodide (B) and potassium chloride (C). A little cyclohexane is added to each tube and the tubes are shaken.
(a)Describe the colour of the aqueous layer and of the cyclohexane layer in tubes A and B after shaking.[2]
(b)Write ionic equations for the reactions in tubes A and B.[2]
(c) No change (colourless/pale); chlorine cannot oxidise chloride, and no reaction occurs.
B1
Q26Halogen displacement reactions · Standard · Paper 1 style · 1 mark
(a)Which mixture produces a reaction?[1]
Show answer
(a) C bromine water + potassium iodide solution
Bromine is a stronger oxidising agent than iodine, so it oxidises I⁻ to I₂. In A, B and D the halogen is weaker than the halide's own element, so there is no reaction.
Q27Halogen displacement reactions · Challenge · Paper 4 style · 6 marks
25.0 cm³ of a chlorine water sample is added to an excess of aqueous potassium iodide. The iodine formed is titrated with 0.100 mol/dm³ sodium thiosulfate; 18.40 cm³ is needed.
(a)Write an ionic equation for the reaction of chlorine with iodide ions.[1]
(b)The equation for the titration is I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻. Calculate the concentration of chlorine in the sample in g/dm³. Give your answer to 3 significant figures.[3]
(c)Name a suitable indicator and give the colour change at the end-point.[2]
(c) Starch, added near the end-point; blue-black to colourless.
B1 starch; B1 blue-black → colourless
Q28Chlorine in water treatment · Core · Paper 2 style · 3 marks
(a)Explain why chlorine is added to drinking water.[1]
(b)Name the species formed when chlorine reacts with water that acts as the main disinfectant.[1]
(c)Give one risk of using chlorine in water treatment.[1]
Show answer
(a) To kill bacteria (and other microorganisms) that cause disease such as cholera and typhoid.
B1
(b) Chloric(I) acid, HClO (and ClO⁻ ions).
B1
(c) Chlorine is toxic; it can react with organic matter to form harmful chlorinated compounds.
B1 any one
Q29Chlorine in water treatment · Standard · Paper 2 style · 5 marks
A water treatment plant serving part of Lahore treats 2.4 × 10⁷ dm³ of water per day, adding chlorine at 1.5 mg per dm³.
(a)Calculate the mass of chlorine, in kg, used each day.[2]
(b)Chlorine arrives in cylinders each containing 68 kg. How many full cylinders are needed for one week?[1]
(c)The chlorine reacts with water: Cl₂ + H₂O ⇌ HCl + HClO. Explain why this is described as a disproportionation reaction.[2]
Show answer
(a) 36 kg
M1 1.5 mg × 2.4 × 10⁷ = 3.6 × 10⁷ mg; A1 = 36 kg
(b) 4 cylinders
B1 7 × 36 = 252 kg; ÷ 68 = 3.71, so 4
(c) Chlorine (0) is both oxidised to +1 in HClO and reduced to −1 in HCl.
B1 oxidation numbers 0 → +1 and 0 → −1; B1 same element oxidised and reduced
Q30Halogen compounds & oxidation states · Challenge · Paper 2 style · 6 marks
(a)Give the oxidation number of chlorine in each of: Cl₂O₇, ClO₂, NaClO₄, HClO, Cl₂O.[3]
(b)Name NaClO₃ and NaClO using oxidation numbers.[1]
(c)Chlorine dioxide, ClO₂, is used to bleach paper pulp. It can be made by reducing sodium chlorate(V) with sulfur dioxide in acid: 2ClO₃⁻ + SO₂ → 2ClO₂ + SO₄²⁻. Show, using oxidation numbers, that this is a redox reaction.[2]
