Metallurgy — ICSE Class 10 Chemistry Important Questions

Answer: (a) bauxite.

Bauxite, $Al_2O_3\cdot2H_2O$Al₂O₃·2H₂O, is the principal ore from which aluminium is extracted. Haematite is an iron ore, calamine a zinc ore.

Answer: (a) froth flotation.

Sulphide ore particles are preferentially wetted by oil and rise with the froth, while the gangue is wetted by water and sinks, achieving concentration.

Answer: (a) cryolite.

Alumina is dissolved in molten cryolite, $Na_3AlF_6$Na₃AlF₆ (with some fluorspar), which lowers the melting point from about $2050^\circ\text{C}$2050° C to about $950^\circ\text{C}$950° C and increases conductivity.

Answer: (b) calcination. For example, calamine changes to zinc oxide: $ZnCO_3\xrightarrow{\Delta}ZnO+CO_2\uparrow$ZnCO₃→ (Δ)ZnO+CO₂↑. Roasting is heating a sulphide ore in excess air, e.g. $2ZnS+3O_2\xrightarrow{\Delta}2ZnO+2SO_2\uparrow$2ZnS+3O₂→ (Δ)2ZnO+2SO₂↑.

Answer: (a) Because aluminium is high in the reactivity series it holds oxygen strongly and cannot be reduced by carbon, so electrolytic reduction is used. R correctly explains A.

(a) Ore: a naturally occurring mineral from which a metal can be extracted conveniently and profitably.

(b) Gangue (matrix): the earthy and rocky impurities (such as sand and clay) associated with an ore.

Iron: haematite, $Fe_2O_3$Fe₂O₃ (or magnetite, $Fe_3O_4$Fe₃O₄).

Aluminium: bauxite, $Al_2O_3\cdot2H_2O$Al₂O₃·2H₂O (or cryolite, $Na_3AlF_6$Na₃AlF₆).

Zinc: zinc blende, $ZnS$ZnS (or calamine, $ZnCO_3$ZnCO₃).

(i) Cathode (reduction):
$Al^{3+} + 3e^- \rightarrow Al$Al^3+ + 3e^- → Al
Molten aluminium collects at the bottom.

(ii) Anode (oxidation):
$2O^{2-} \rightarrow O_2 + 4e^-$2O^2- → O₂ + 4e^-

(iii) Wearing away of anode: the oxygen liberated attacks the hot carbon anode,
$C + O_2 \rightarrow CO_2$C + O₂ → CO₂
so the carbon anodes are slowly burnt away and must be replaced.

(i) Cryolite: dissolves the alumina, lowers the fusion temperature from about $2050^\circ\text{C}$2050° C to about $950^\circ\text{C}$950° C and increases the electrical conductivity of the melt.

(ii) Carbon lining: it lines the inside of the steel tank and acts as the cathode.

(iii) Powdered coke: it is sprinkled over the surface of the molten electrolyte to reduce heat loss (keeps the bath hot) and to prevent burning of the carbon anodes by keeping out air.

(a)

(i) $2ZnS+3O_2\xrightarrow{\Delta}2ZnO+2SO_2\uparrow$2ZnS+3O₂→ (Δ)2ZnO+2SO₂↑

(ii) $ZnCO_3\xrightarrow{\Delta}ZnO+CO_2\uparrow$ZnCO₃→ (Δ)ZnO+CO₂↑

(iii) $ZnO+C\xrightarrow{\Delta}Zn+CO\uparrow$ZnO+C→ (Δ)Zn+CO↑

(b) Copper(II) oxide is reduced by hydrogen: $CuO+H_2\xrightarrow{\Delta}Cu+H_2O$CuO+H₂→ (Δ)Cu+H₂O (lead(II) oxide also). Aluminium oxide $Al_2O_3$Al₂O₃ (or $MgO$MgO) cannot be reduced by carbon; such metals, and very active ones like sodium, potassium and calcium, are obtained by electrolysis.

(a) Purification of bauxite (Bayer's process): Powdered bauxite is heated with concentrated sodium hydroxide solution under pressure. The amphoteric alumina dissolves as sodium aluminate, while impurities like $Fe_2O_3$Fe₂O₃ and $SiO_2$SiO₂ remain insoluble and are filtered off.
$Al_2O_3 + 2NaOH \rightarrow 2NaAlO_2 + H_2O$Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O
The filtrate is diluted and seeded with freshly prepared aluminium hydroxide, which precipitates hydrated alumina:
$NaAlO_2 + 2H_2O \rightarrow NaOH + Al(OH)_3\downarrow$NaAlO₂ + 2H₂O → NaOH + Al(OH)₃↓
The precipitate is filtered, washed and heated strongly (calcined) to give pure alumina:
$2Al(OH)_3 \xrightarrow{\Delta} Al_2O_3 + 3H_2O$2Al(OH)₃ → (Δ) Al₂O₃ + 3H₂O

(b) Electrolytic reduction (Hall-Heroult process):

• Electrolyte: pure alumina dissolved in molten cryolite $Na_3AlF_6$Na₃AlF₆ with a little fluorspar $CaF_2$CaF₂.
• Cathode: the carbon (graphite) lining of the steel tank.
• Anode: thick carbon (graphite) rods.

Electrode reactions:
$\text{Cathode:}\ Al^{3+} + 3e^- \rightarrow Al$Cathode: Al^3+ + 3e^- → Al
$\text{Anode:}\ 2O^{2-} \rightarrow O_2 + 4e^-$Anode: 2O^2- → O₂ + 4e^-
Molten aluminium collects at the bottom and is tapped off. The oxygen burns the carbon anodes ($C+O_2\rightarrow CO_2$C+O₂→ CO₂), which are replaced periodically.

(a) Reactivity series: an arrangement of metals in the vertical order of their decreasing chemical reactivity (tendency to lose electrons and form positive ions).

(b) Decreasing order of reactivity: $K > Al > Zn > Cu$K > Al > Zn > Cu.

(c) (i) Displacement reactions: a more reactive metal displaces a less reactive metal from the solution of its salt. For example, zinc displaces copper: $Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$Zn + CuSO₄ → ZnSO₄ + Cu, because $Zn$Zn is above $Cu$Cu in the series.

(ii) Method of extraction: metals high in the series (e.g. $K$K, $Al$Al) are extracted by electrolysis of their fused compounds, whereas metals lower down (e.g. $Zn$Zn) are extracted by reduction of their oxides with carbon, and oxides of less active metals such as $CuO$CuO and $PbO$PbO can be reduced even by hydrogen.

(i) An alloy is a homogeneous mixture of two or more metals (or a metal and a non-metal such as carbon). An amalgam is an alloy in which one of the metals is mercury.

(ii) Brass: copper and zinc. Bronze: copper and tin.

(iii) Duralumin (mainly aluminium with copper, and a little magnesium and manganese) is light but strong, and resists corrosion.

(iv) Solder: lead and tin. Its low melting point lets it melt easily and join wires without damaging them. (Stainless steel, iron with chromium and nickel, is used for sinks because it does not rust.)