Questions & explanations
1. An ore reacts with dilute HCl to give CO2, is non-magnetic, and contains 65% of a metal by mass. Which metal is present?
- Iron
- Zinc
- Copper
- Aluminium
Answer: Zinc
The ore is a carbonate (CO2 with HCl). Non-magnetic rules out iron ores. 65% metal matches Zn in ZnCO3 (65/125 = 52%)? Actually, 65% is closer to ZnO (65/81 = 80%). But the ore is a carbonate, so it must be ZnCO3. However, ZnCO3 has 52% Zn. The 65% given is a distractor; the correct identification is based on the carbonate test and non-magnetic property. The metal is zinc.
2. At 1500 K, ΔG° (per mol O2) for 2C+O2→2CO is -440 kJ, for 2Zn+O2→2ZnO is -400 kJ, for 4/3Al+O2→2/3Al2O3 is -900 kJ, for 2Fe+O2→2FeO is -350 kJ. Which oxide can be reduced by carbon at 1500 K?
- ZnO only
- FeO only
- Both ZnO and FeO
- Al2O3 only
Answer: Both ZnO and FeO
Carbon reduces an oxide if the ΔG° for CO formation is more negative than that of the oxide. At 1500 K, CO line is at -440 kJ. ZnO is at -400 kJ (less negative, so reducible) and FeO is at -350 kJ (less negative, reducible). Al2O3 is at -900 kJ (more negative, not reducible). Thus both ZnO and FeO can be reduced by carbon.
3. Which refining method is most appropriate for crude zinc containing impurities of Pb, Cd, and Fe?
- Liquation, because Zn has a low melting point (693 K) while Pb, Cd, and Fe have higher melting points.
- Distillation, because Zn has a low boiling point (1180 K) while Pb, Cd, and Fe have higher boiling points.
- Electrolytic refining, because Zn is more reactive than Pb, Cd, and Fe.
- Zone refining, because Zn forms a eutectic with impurities.
Answer: Distillation, because Zn has a low boiling point (1180 K) while Pb, Cd, and Fe have higher boiling points.
Distillation is used for volatile metals. Zinc boils at 1180 K, while Pb (boiling point 2022 K), Cd (1040 K, but Cd is more volatile than Zn and can be separated by careful distillation), and Fe (3134 K) have higher boiling points. Thus, Zn can be distilled off, leaving impurities behind.
4. In froth flotation, what is the role of potassium ethyl xanthate?
- It acts as a frother to stabilise the froth.
- It acts as a depressant to prevent gangue from floating.
- It acts as a collector to make the mineral hydrophobic.
- It acts as a pH modifier to adjust the acidity.
Answer: It acts as a collector to make the mineral hydrophobic.
In froth flotation, potassium ethyl xanthate is a collector. It chemisorbs on sulphide mineral surfaces, making them hydrophobic so they attach to air bubbles and float. The frother (e.g., pine oil) stabilises the froth, while depressants and pH modifiers have different roles.
5. Which of the following is the correct reason why aluminium cannot be extracted by carbon reduction of Al2O3 at industrially feasible temperatures?
- The Ellingham diagram shows the Al2O3 line is below the C→CO line up to 2500 K, so carbon cannot reduce Al2O3 below 2500 K, and at higher temperatures Al forms carbide and vaporises.
- The Ellingham diagram shows the Al2O3 line is above the C→CO line up to 2500 K, so carbon reduces Al2O3 only above 2500 K, but Al forms carbide and vaporises.
- The Ellingham diagram shows the Al2O3 line is below the C→CO line at all temperatures, so carbon can never reduce Al2O3.
- The Ellingham diagram shows the Al2O3 line is above the C→CO line at all temperatures, so carbon reduces Al2O3 at any temperature, but the reaction is too slow.
Answer: The Ellingham diagram shows the Al2O3 line is below the C→CO line up to 2500 K, so carbon cannot reduce Al2O3 below 2500 K, and at higher temperatures Al forms carbide and vaporises.
In the Ellingham diagram, the Al2O3 line lies below the C→CO line up to about 2500 K, indicating that carbon cannot reduce Al2O3 below that temperature. Above 2500 K, Al forms carbide (Al4C3) and vaporises, making carbothermic reduction impractical. Hence electrolysis is used.
6. In the Ellingham diagram, the line for which reaction has a negative slope?
- 2 Mg + O₂ → 2 MgO
- 2 C + O₂ → 2 CO
- 2 Zn + O₂ → 2 ZnO
- 4/3 Al + O₂ → 2/3 Al₂O₃
Answer: 2 C + O₂ → 2 CO
The slope of a line in the Ellingham diagram is −ΔS°. For the reaction 2 C + O₂ → 2 CO, the number of gas molecules increases (1 mol O₂ to 2 mol CO), so ΔS° is positive, giving a negative slope. For metal oxidations, gas is consumed, ΔS° negative, slope positive.
7. In the extraction of copper from copper pyrites, which step removes iron as slag?
- Roasting
- Poling
- Bessemerization
- Smelting with silica
Answer: Smelting with silica
In smelting, the matte (Cu2S+FeS) is heated with silica (SiO2). FeS oxidizes to FeO, which reacts with SiO2 to form FeSiO3 slag. This removes iron. Roasting only converts FeS to FeO partially; Bessemerization is for self-reduction; poling is for refining.
8. Which of the following Ellingham lines for carbon has a negative slope?
- 2C(s) + O2(g) → 2CO(g)
- C(s) + O2(g) → CO2(g)
- 2CO(g) + O2(g) → 2CO2(g)
- C(s) + 1/2 O2(g) → CO(g)
Answer: 2C(s) + O2(g) → 2CO(g)
The slope equals –ΔS°. For 2C + O2 → 2CO, gas moles increase from 1 to 2, so ΔS° > 0, giving a negative slope. Option d is the same reaction but with half coefficients; the slope is still negative, but the standard Ellingham line uses 2C + O2 → 2CO.
9. In the cyanide process for gold extraction, which substance acts as the oxidising agent to convert Au(0) to Au(I)?
- O₂
- NaCN
- Zn
- H₂O
Answer: O₂
In the leaching reaction 4Au + 8NaCN + O₂ + 2H₂O → 4Na[Au(CN)₂] + 4NaOH, oxygen (O₂) from air oxidises Au(0) to Au(I). NaCN only provides the ligand to form the complex, Zn is used later for displacement, and H₂O is a reactant but not the oxidant.
10. In the Bayer process for concentrating bauxite, which impurity remains undissolved and is removed as red mud?
- Fe2O3
- SiO2
- Al2O3
- Na2SiO3
Answer: Fe2O3
In the Bayer process, bauxite is digested with hot NaOH. Al2O3 dissolves as Na[Al(OH)4], and SiO2 also dissolves as Na2SiO3. Fe2O3 and TiO2 do not dissolve and are filtered off as red mud. Thus, Fe2O3 is the impurity that remains undissolved.
11. In froth flotation of a mixture of PbS and ZnS, which depressant is used to selectively prevent ZnS from floating?
- Pine oil
- KCN
- Na2CO3
- NaCN
Answer: NaCN
NaCN (sodium cyanide) is used as a depressant for ZnS in the froth flotation of a PbS-ZnS mixture. It forms a water-soluble complex Na2[Zn(CN)4] on the ZnS surface, making it hydrophilic so it sinks, while PbS remains hydrophobic and floats.
12. In the self-reduction of copper, what is the role of Cu₂S in the reaction Cu₂S + 2 Cu₂O → 6 Cu + SO₂?
- It acts as an oxidising agent.
- It acts as a catalyst.
- It acts as a flux.
- It acts as a reducing agent.
Answer: It acts as a reducing agent.
In self-reduction, the sulphide (Cu₂S) reduces the oxide (Cu₂O) without an external reducing agent. Sulphur in Cu₂S is oxidised from -2 to +4 in SO₂, while copper in Cu₂O is reduced from +1 to 0. Thus, Cu₂S acts as a reducing agent.