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]
▶️ Answer/Explanation
(a) Solid Iodine:
- Separation: Molecules are closely packed/touching.
- Motion: Vibrate about fixed positions.
Iodine Gas:
- Separation: Molecules are far apart.
- Motion: Move rapidly and randomly in all directions.
(b) The volume of iodine gas decreases as pressure increases (Boyle’s Law).
(c)(i) Bromine + Potassium Iodide → Iodine + Potassium Bromide.
(c)(ii) Iodine is less reactive than chlorine, so it cannot displace chloride ions from KCl.
(d)(i) Balanced equation:
$\mathrm{2Na_2S_2O_3 + I_2 \rightarrow Na_2S_4O_6 + 2NaI}$.
(d)(ii) The diagram shows the products at a lower energy level than the reactants, indicating energy is released (exothermic).
(e) Test for Iodide Ions:
Test: Add aqueous silver nitrate.
Observation: Yellow precipitate (AgI) forms.
(f) Product at the positive electrode: Iodine (I₂).
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 (Fe₂O₃)
Hematite is the most common and economically important iron ore, containing about 70% iron by mass.
(b) Ans: 2C + O₂ → 2CO
This shows the incomplete combustion of carbon, where limited oxygen produces carbon monoxide rather than CO₂.
(c) Ans: Carbon gains oxygen (oxidation)
In the reaction, each carbon atom gains oxygen to form CO (oxidation state changes from 0 to +2), while iron is reduced (Fe³⁺ to Fe⁰).
(d)(i) Ans: Thermal decomposition
Limestone (CaCO₃) breaks down into simpler substances (CaO and CO₂) when heated, characteristic of decomposition reactions.
(d)(ii) Ans: Basic oxide; Calcium is a metal
Metal oxides like CaO are basic as they react with acids to form salts and water (e.g., CaO + 2HCl → CaCl₂ + H₂O).
(e) Ans:
- High electrical conductivity (due to delocalized electrons)
- Malleability (can be hammered into sheets without breaking)
- High melting point (strong metallic bonds between atoms)
(f) Ans: Fe₂C₉O₉
The structure shows:
- 2 iron atoms (Fe)
- 9 carbon atoms (C)
- 9 oxygen atoms (O)
Each carbonate group (CO₃²⁻) contributes 1C and 3O, with three such groups in the compound.