This question is about iodine and compounds of iodine.
(a) Use the kinetic particle model to describe the separation between the molecules and the type of motion of the molecules in:
- solid iodine
- iodine gas. [4]
(b) The graph shows how the volume of iodine gas changes with pressure. The temperature is kept constant.
Describe how the volume of iodine gas changes with pressure.[1]
(c)
(i) Complete the word equation to show the halogen and halide compound which react to form the products iodine and potassium bromide.
(ii) Explain, in terms of the reactivity of the halogens, why aqueous iodine does not react with aqueous potassium chloride.
(d) Iodine reacts with aqueous sodium thiosulfate, $\mathrm{Na}_2 \mathrm{~S}_2 \mathrm{O}_3$.
(i) Balance the chemical equation for this reaction.
$\mathrm{Na}_2 \mathrm{~S}_2 \mathrm{O}_3+\mathrm{I}_2 \rightarrow \mathrm{Na}_2 \mathrm{~S}_4 \mathrm{O}_6+\ldots . . \mathrm{NaI}$
(ii) The energy level diagram for this reaction is shown.
Explain how this diagram shows that the reaction is exothermic [1]
(e) Describe a test for iodide ions.
test
observations[2]
(f) Molten sodium iodide is electrolysed.
Predict the product at the positive electrode.[1][Total: 14]
▶️ Answer/Explanation
(a) Ans:
Solid iodine: Molecules are closely packed in fixed positions (1), vibrating about fixed points (1).
Iodine gas: Molecules are far apart (1), moving rapidly in random directions (1).
(b) Ans: Volume decreases as pressure increases
This demonstrates Boyle’s Law – inverse relationship between pressure and volume at constant temperature.
(c)(i) Ans: Bromine + Potassium iodide → Iodine + Potassium bromide
This is a halogen displacement reaction where bromine (more reactive) displaces iodine from potassium iodide.
(c)(ii) Ans: Chlorine is more reactive than iodine
Iodine cannot displace chlorine from KCl as it’s less reactive in the halogen group reactivity series.
(d)(i) Ans: $\mathrm{Na}_2 \mathrm{S}_2 \mathrm{O}_3 + \mathrm{I}_2 \rightarrow \mathrm{Na}_2 \mathrm{S}_4 \mathrm{O}_6 + 2\mathrm{NaI}$
The balanced equation shows 1:1 ratio of thiosulfate to iodine, producing sodium tetrathionate and sodium iodide.
(d)(ii) Ans: Products are at lower energy than reactants
The diagram shows energy is released (exothermic) as the products are more stable at a lower energy level.
(e) Ans:
Test: Add silver nitrate solution to iodide solution
Observation: Yellow precipitate forms (silver iodide)
(f) Ans: Iodine
During electrolysis of molten NaI, iodide ions are oxidized to iodine at the positive electrode (anode).
This question is about chlorine and compounds of chlorine.
(a) Chlorine is an element in Group VII of the Periodic Table.
State the meaning of the term element. [1]
(b) State one use of chlorine. [1]
(c) Chlorine reacts with phosphorus to produce phosphorus(V) chloride.
(i) Balance the equation for this reaction.
…..P + …..Cl2 → 2PCl5 [2]
(ii) This reaction is exothermic.
State the meaning of the term exothermic. [1]
(d) Chlorine reacts with aqueous sodium bromide.
(i) Complete the word equation for this reaction.
(ii) Describe a test for bromide ions. [2]
test
observations
(iii) When bromine is mixed with aqueous sodium chloride there is no reaction.
Suggest in terms of chemical reactivity why there is no reaction. [1]
(e) A compound of chlorine has the formula C3H6Cl2.
Complete the table to calculate the relative molecular mass of C3H6Cl2.
relative molecular mass = [2]
▶️ Answer/Explanation
(a) Ans: Substance containing only one type of atom
An element cannot be broken down into simpler substances by chemical means.
(b) Ans: Water treatment/purification (1)
Chlorine kills bacteria in drinking water and swimming pools.
(c)(i) Ans: 2P + 5Cl2 → 2PCl5
Balanced equation shows 2 phosphorus atoms and 10 chlorine atoms (as 5 Cl2 molecules) on both sides.
(c)(ii) Ans: Releases heat/thermal energy (1)
Exothermic reactions give out energy to the surroundings, often as heat.
(d)(i) Ans: chlorine + sodium bromide → bromine + sodium chloride
This displacement reaction occurs because chlorine is more reactive than bromine.
(d)(ii) Ans: Add nitric acid then silver nitrate solution (1), cream precipitate forms (1)
Bromide ions (Br–) react with Ag+ to form insoluble cream AgBr.
(d)(iii) Ans: Bromine is less reactive than chlorine (1)
More reactive halogens can displace less reactive ones from their compounds.
(e) Ans: 113 (2)
Calculation: (3×12) + (6×1) + (2×35.5) = 36 + 6 + 71 = 113
Relative molecular mass is the sum of all atomic masses in the molecule.