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: Particles are closely packed in a fixed arrangement (1), vibrating about fixed positions (1).
Iodine gas: Particles are far apart (1), moving rapidly and randomly (1).
(b) Ans: Volume decreases as pressure increases (Boyle’s Law)
The graph shows an inverse relationship: doubling pressure halves volume at constant temperature.
(c)(i) Ans: Bromine + Potassium iodide → Iodine + Potassium bromide
This is a displacement reaction where more reactive bromine (Br₂) displaces iodine from potassium iodide (KI).
(c)(ii) Ans: Iodine is less reactive than chlorine
Chlorine (in KCl) is above iodine in the halogen reactivity series, so iodine cannot displace it.
(d)(i) Ans: $\mathrm{Na}_2 \mathrm{S}_2 \mathrm{O}_3 + \mathrm{I}_2 \rightarrow \mathrm{Na}_2 \mathrm{S}_4 \mathrm{O}_6 + 2\mathrm{NaI}$
Balanced by ensuring equal atoms on both sides: 2NaI forms to balance iodine atoms.
(d)(ii) Ans: Products are at lower energy than reactants
The diagram shows energy is released (ΔH is negative), characteristic of exothermic reactions.
(e) Ans:
Test: Add acidified silver nitrate solution.
Observation: Yellow precipitate (AgI) forms.
(f) Ans: Iodine (I₂)
At the anode (positive electrode), iodide ions (I⁻) are oxidized to iodine molecules.
The table shows some properties of four Group I elements.
(a) (i) Complete the table by estimating:
- the boiling point of sodium
- the relative hardness of lithium. [2]
(ii) Predict the physical state of lithium at 200°C.
Give a reason for your answer. [2]
(b) Potassium reacts with water.
2K + 2H2O → 2KOH + H2
Describe two observations when potassium reacts with water.[2]
(c) Lithium is extracted by the electrolysis of molten lithium chloride.
(i) Name a non-metal used to make the electrodes.[1]
(ii) Give one property, other than the conduction of electricity, that makes this substance suitable for use as an electrode.[1]
(iii) State the products of the electrolysis of molten lithium chloride at:[2]
the negative electrode (cathode)
the positive electrode (anode)
(d) Lithium chloride conducts electricity when molten and when in aqueous solution.
Give two other physical properties of lithium chloride that show it is an ionic compound.[2]
▶️ Answer/Explanation
(a)(i) Ans:
Boiling point of sodium: 800–1300°C (trend increases down Group I).
Hardness of lithium: 0.70–5.0 MPa (harder than sodium but softer than potassium).
(a)(ii) Ans: Liquid
Lithium melts at 180°C and boils at 1342°C. At 200°C, it is above its melting point but below its boiling point.
(b) Ans:
1. Effervescence/bubbles (hydrogen gas is produced).
2. Potassium moves/floats (reacts vigorously, forming KOH solution).
(c)(i) Ans: Carbon/graphite
Graphite is commonly used as it conducts electricity and is inert.
(c)(ii) Ans: Inert/unreactive
Electrodes must not react with the electrolyte or products.
(c)(iii) Ans:
Cathode: Lithium (Li+ gains electrons).
Anode: Chlorine (Cl– loses electrons).
(d) Ans:
1. High melting/boiling point (strong ionic bonds require much energy to break).
2. Soluble in water (polar water molecules disrupt ionic lattice).