(a) Describe how you could prepare a pure sample of crystals of hydrated copper(II) sulfate using dilute sulfuric acid and an excess of copper(II) oxide. [3]
(b) Anhydrous copper(II) sulfate is used to test for water.
$\begin{aligned} & \mathrm{CuSO}_4+5 \mathrm{H}_2 \mathrm{O} \rightleftharpoons \mathrm{CuSO}_4 \cdot 5 \mathrm{H}_2 \mathrm{O} \\ & \text { anhydrous hydrated } \\ & \text { copper(II) sulfate copper(II) sulfate } \end{aligned}$
(i) What is meant by the symbol $\rightleftharpoons$ ? [1]
(ii) How can hydrated copper(II) sulfate be changed into anhydrous copper(II) sulfate? [1]
(c) Complete the table to calculate the relative formula mass of anhydrous copper(II) sulfate, $\mathrm{CuSO}_4$. Use your Periodic Table to help you.
relative formula mass = …………………………. [2]
(d) Complete the table to show the number of electrons, protons and neutrons in the sulfur atom and copper ion shown.
(e) Alloys of copper are used to make coins.
(i) What is meant by the term alloy? [1]
(ii) Suggest why an alloy of copper is used to make coins instead of using pure copper. [1] [Total: 13]
▶️ Answer/Explanation
(a) Ans:
1. React excess copper(II) oxide with dilute sulfuric acid until no more dissolves. 2. Filter to remove excess copper(II) oxide. 3. Heat the filtrate to saturation point and allow to crystallize. 4. Dry the crystals between filter papers.
(b)(i) Ans: The symbol $\rightleftharpoons$ represents a reversible reaction where the reaction can proceed in both forward and backward directions.
(b)(ii) Ans: By heating the hydrated copper(II) sulfate, the water molecules are removed, converting it to the anhydrous form.
(c) Ans: 160
Calculation: Cu (63.5) + S (32) + 4×O (4×16 = 64) = 63.5 + 32 + 64 = 159.5 ≈ 160.
(d) Ans:
Sulfur atom: 16 protons, 16 electrons, 18 neutrons (since mass number is 34). Copper ion (Cu²⁺): 29 protons, 27 electrons (lost 2), 34 neutrons (since mass number is 63).
(e)(i) Ans: An alloy is a mixture of a metal with one or more other elements, often metals, to enhance properties.
(e)(ii) Ans: Copper alloys are used for coins because they are harder and more wear-resistant than pure copper, making them more durable for circulation.
This question is about iron and iron compounds.
(a) Name the main ore of iron.
(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.
$$ ….. \rm{C}+\ldots \rightarrow2 \mathrm{CO} $$
(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?
(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?
(ii) What type of oxide is calcium oxide?
Give a reason for your answer.
(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.
▶️ Answer/Explanation
(a) Ans: hematite
Hematite (Fe2O3) is the most common and economically important ore of iron.
(b) Ans: 2C + O2 → 2CO
Balancing the equation requires 2 carbon atoms and 1 oxygen molecule to produce 2 molecules of carbon monoxide.
(c) Ans: Carbon gains oxygen.
In the reaction, carbon combines with oxygen from iron(III) oxide, which is the definition of oxidation.
(d)(i) Ans: Thermal decomposition
Calcium carbonate breaks down into calcium oxide and carbon dioxide upon heating.
(d)(ii) Ans: Basic oxide
Calcium oxide is a basic oxide because it reacts with acids to form salts and water, typical of metal oxides.
(e) Ans: Conducts electricity, malleable, ductile
Metals have free electrons that allow conductivity, and their structure enables deformation without breaking.
(f) Ans: Fe2C9O9
The structure shows 2 iron atoms, 9 carbon atoms, and 9 oxygen atoms in the molecular unit.