Home / iGCSE Chemistry 0620 Extended – 6.4 Redox- Exam Style Questions Paper 4

iGCSE Chemistry 0620 Extended - 6.4 Redox- Exam Style Questions Paper 4- New Syllabus

iGCSE Chemistry Theory (Extended) Paper 4 - All Chapters

Question

This question is about aluminium and other metallic elements.
(a) Aluminium is extracted from purified bauxite by electrolysis. Fig. 3.1 shows a diagram of the electrolytic cell used.
(i) State the name of the aluminium compound present in bauxite.
(ii) Name the substance that purified bauxite is dissolved in before electrolysis is carried out.
(iii) Name the substance that is used for the positive electrodes.
(iv) Write the ionic half-equation for the production of aluminium at the negative electrode.
(b) Aluminium is above zinc in the reactivity series.
(i) When a piece of aluminium foil is added to aqueous zinc sulfate, a reaction is not immediately observed even though aluminium is above zinc in the reactivity series. Explain why a reaction is not immediately observed.
(ii) After a few minutes, a displacement reaction occurs between aluminium and aqueous zinc ions. Complete the ionic equation for this reaction:
$\dots Al + \dots Zn^{2+} \rightarrow \dots Al^{3+} + \dots Zn$
(iii) State the oxidation number of zinc in $Zn^{2+}$ and $Zn$.
(iv) Define the term reduction in terms of oxidation number.
(c) Aluminium reacts with iron(III) oxide. The equation is shown:
$2Al + Fe_{2}O_{3} \rightarrow Al_{2}O_{3} + 2Fe \quad \Delta H = -850 \text{ kJ/mol}$
(i) State the meaning of the symbol $\Delta H$.
(ii) State what can be deduced about the reaction from the negative sign in $\Delta H = -850 \text{ kJ/mol}$.
(d) Gallium has many similarities to aluminium. Gallium(III) oxide is an amphoteric oxide. State what is meant by the term amphoteric.
(e) The reaction of gallium(III) oxide, $Ga_{2}O_{3}$, with aqueous sodium hydroxide forms a salt which contains the negative ion $GaO_{2}^{-}$. The only other product is water. Write a symbol equation for this reaction.
(f) Deduce the formula of
  • gallium(III) bromide
  • gallium(III) sulfate

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 9.6 — Extraction of metals
• Topic 6.4 — Redox
• Topic 5.1 — Exothermic and endothermic reactions
• Topic 7.2 — Oxides

▶️ Answer/Explanation

(a)(i) Aluminium oxide.

(a)(ii) Cryolite. (Explanation: Cryolite is added as a solvent to substantially lower the operating temperature of the cell from $2000^{\circ}C$ down to around $900^{\circ}C$, saving massive amounts of energy.)

(a)(iii) Carbon (or graphite).

(a)(iv) $$Al^{3+} + 3e^{-} \rightarrow Al$$

(b)(i) Aluminium natively forms a very thin, tightly adhering, and unreactive coating of aluminium oxide on its surface when exposed to air. This layer must first dissolve or be breached before the underlying reactive aluminium metal can come into contact with the zinc ions.

(b)(ii) $$2Al + 3Zn^{2+} \rightarrow 2Al^{3+} + 3Zn$$ (Explanation: Total charge must balance. Left side: $3 \times (+2) = +6$. Right side: $2 \times (+3) = +6$.)

(b)(iii) The oxidation number of zinc in $Zn^{2+}$ is $+2$, and in elemental $Zn$ is $0$.

(b)(iv) Reduction is defined as a decrease in oxidation number.

(c)(i) $\Delta H$ stands for enthalpy change (of the reaction).

(c)(ii) The negative sign indicates that the forward reaction is exothermic, meaning it releases thermal energy to the surroundings.

(d) Amphoteric oxides are oxides that can exhibit both acidic and basic properties; they react with both acids and bases to produce a salt and water.

(e) $$Ga_{2}O_{3} + 2NaOH \rightarrow 2NaGaO_{2} + H_{2}O$$

(f)
Gallium(III) bromide: $GaBr_{3}$ (since Ga is $3+$ and Br is $1-$)
Gallium(III) sulfate: $Ga_{2}(SO_{4})_{3}$ (since Ga is $3+$ and the sulfate ion $SO_{4}$ is $2-$)

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