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.
(c)(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, $\mathrm{Na}_2 \mathrm{~S}_2 \mathrm{O}_3$.
(d)(i) Balance the chemical equation for this reaction.
$\mathrm{Na}_2 \mathrm{~S}_2 \mathrm{O}_3+\mathrm{I}_2 \rightarrow \mathrm{Na}_2 \mathrm{~S}_4 \mathrm{O}_6+\ldots . . \mathrm{NaI}$
(d)(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 a fixed arrangement (touching each other).
- Molecules vibrate about fixed positions but cannot move freely.
Iodine gas:
- Molecules are far apart and randomly distributed.
- Molecules move rapidly and randomly in all directions.
(b) Ans: The volume of iodine gas decreases as pressure increases (Boyle’s Law: P ∝ 1/V at constant temperature).
(c)(i) Ans: Bromine + Potassium iodide → Iodine + Potassium bromide.
This is a displacement reaction where bromine (more reactive) displaces iodine from potassium iodide.
(c)(ii) Ans: Iodine is less reactive than chlorine, so it cannot displace chlorine from KCl. Halogen reactivity decreases down Group 17 (F₂ > Cl₂ > Br₂ > I₂).
(d)(i) Ans: Balanced equation:
$\mathrm{2Na_2S_2O_3 + I_2 \rightarrow Na_2S_4O_6 + 2NaI}$
(d)(ii) Ans: The energy level diagram shows the products at a lower energy level than reactants, indicating energy is released (exothermic).
(e) Ans: Test for iodide ions:
Test: Add aqueous silver nitrate (AgNO₃).
Observation: Yellow precipitate (AgI) forms, insoluble in dilute ammonia.
(f) Ans: At the positive electrode (anode), iodine (I₂) is produced by oxidation of iodide ions: $\mathrm{2I^- \rightarrow I_2 + 2e^-}$.
This question is about iron and iron compounds.
(a) Name the main ore of iron. [1]
(b) In a blast furnace used for the extraction of iron, carbon reacts with oxygen from the air to form carbon monoxide.
Complete the chemical equation for this reaction.
$$ ….. \rm{C}+\ldots \rightarrow2 \mathrm{CO} $$ [2]
(c) In the hotter parts of the furnace, carbon reacts with the iron(III) oxide present in the iron ore.
$$ 3 \mathrm{C}+\mathrm{Fe}_2 \mathrm{O}_3 \rightarrow 3 \mathrm{CO}+2 \mathrm{Fe} $$
How does this equation show that carbon is oxidised? [1]
(d) Limestone is added to the blast furnace. The limestone is converted into calcium oxide and carbon dioxide. The reaction is endothermic.
$$ \mathrm{CaCO}_3 \stackrel{\text { heat }}{\longrightarrow} \mathrm{CaO}+\mathrm{CO}_2 $$
(i) What type of chemical reaction is this? [1]
(ii) What type of oxide is calcium oxide?
Give a reason for your answer. [2]
(e) Iron is a metal.
Give three physical properties that are characteristic of metals.
(f) The structure of a compound of iron is shown.
Deduce the molecular formula of this compound to show the number of iron, carbon and oxygen atoms. [1]
▶️ Answer/Explanation
(a) Ans: Hematite
Hematite (Fe2O3) is the primary ore of iron due to its high iron content.
(b) Ans: 2C + O2 → 2CO
The balanced equation shows 2 carbon atoms reacting with 1 oxygen molecule to form 2 carbon monoxide molecules.
(c) Ans: Carbon gains oxygen
Carbon is oxidized as it combines with oxygen from Fe2O3 to form CO (oxidation state increases from 0 to +2).
(d)(i) Ans: Thermal decomposition
Calcium carbonate breaks down into calcium oxide and carbon dioxide upon heating.
(d)(ii) Ans: Basic oxide
Calcium oxide (CaO) is basic because it reacts with acids to form salts and water, typical of metal oxides.
(e) Ans: Three metallic properties
1. Good electrical conductivity (due to delocalized electrons).
2. Malleability (can be hammered into sheets).
3. High thermal conductivity (transfers heat efficiently).
(f) Ans: Fe2C9O9
The structure contains 2 iron (Fe), 9 carbon (C), and 9 oxygen (O) atoms.