A-LevelChemistryGroup 179701/23 · May/June 2017 · Q215 Marks

The halogens, chlorine, bromine and iodine, and their compounds, show a variety of similarities and trends in their physical and chemical properties.

(a)(i)Give the colours and states of chlorine, bromine and iodine at room temperature and pressure. [Table with headers: halogen, colour, state and rows for chlorine, bromine, iodine][2]
(a)(ii)The halogens become less volatile down the group. Explain this trend in volatility.[2]
(b)The halogens are oxidising agents. State and explain the trend in oxidising power of the halogens.[3]
(c)(i)State any observations that would be made on addition of concentrated sulfuric acid to • solid sodium chloride, • solid sodium iodide.[2]
(c)(ii)Give reasons for the difference in the observations in (i).[2]
(c)(iii)The addition of concentrated sulfuric acid to solid sodium bromide, NaBr, produces brown fumes and an acidic gas that decolourises acidified potassium manganate(VII) solution. This acidic gas is a significant contributor to acid rain. Write the equation for the reaction of concentrated sulfuric acid with sodium bromide.[2]
(d)(i)An aqueous solution, Z, contains a mixture of sodium chloride and sodium iodide. Excess aqueous silver nitrate is added to Z in a test-tube. A yellow precipitate forms. Explain the colour of this precipitate.[1]
(d)(ii)Aqueous ammonia is then added to the test-tube in (i). The mass of precipitate decreases. Explain this observation.[1]

Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

[1] chlorine yellow/green gas
[1] bromine red / brown / orange liquid
[1] iodine grey/black solid
[1] increasing number of electrons
[1] (gives) increasing strength of van der Waals' / id-id forces / London / dispersion forces
[1] oxidising power decreases down the group. ora
[1] ability to accept electrons decreases (down the group) ora
[1] because (outer shell experiences) more shielding OR increased distance from nucleus (to outer shell) (outweighs the increasing nuclear charge down the group) ora
[1] solid sodium chloride: steamy / misty / white fumes
[1] solid sodium iodide: purple fumes
[1] (conc sulfuric) not powerful enough oxidising agent (to oxidise chloride) OR chloride not powerful enough reducing agent (to reduce sulfuric acid)
[1] iodide reduces sulfuric acid OR iodide / I⁻ is oxidised OR sulfuric acid oxidises iodide
[2] 2NaBr + 2H2SO4 → Br2 + SO2 + Na2SO4 + 2H2O OR NaBr + H2SO4 → NaHSO4 + HBr AND 2HBr + H2SO4 → Br2 + SO2 + 2H2O OR 2NaBr + H2SO4 → Na2SO4 + 2HBr AND 2HBr + H2SO4 → Br2 + SO2 + 2H2O
[1] AgI (and AgCl solid) / silver ions reacting with iodide ions
[1] AgCl (precipitate) dissolves (in ammonia) owtte

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