Metallurgy — ICSE Class 10 Chemistry Important Questions

Answer: (a) bauxite.

Bauxite, $Al_2O_3\cdot2H_2O$Al_2O_3·2H_2O, 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, $Na_3AlF_6$Na_3AlF_6 (with some fluorspar), which lowers the melting point from about $2050^\circ\text{C}$2050^ to about $950^\circ\text{C}$950^ and increases conductivity.

Answer: (a) oxygen liberated oxidises the carbon anode to carbon dioxide.

The oxygen set free at the anode reacts with the hot carbon, $C+O_2\rightarrow CO_2$C+O_2→ CO_2, gradually burning it away, so the anodes must be renewed from time to time.

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.

Any two of the following:

1. Bauxite: $Al_2O_3\cdot2H_2O$Al_2O_3·2H_2O

2. Corundum: $Al_2O_3$Al_2O_3

3. Cryolite: $Na_3AlF_6$Na_3AlF_6

(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- → O_2 + 4e^-

(iii) Wearing away of anode: the oxygen liberated attacks the hot carbon anode,
$C + O_2 \rightarrow CO_2$C + O_2 → CO_2
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^ to about $950^\circ\text{C}$950^ 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 metal's position in the reactivity series decides how tightly it holds its ore and hence its method of extraction.

Potassium (very high): extremely reactive, its compounds are very stable, so it is obtained by electrolysis of its fused chloride/hydroxide.

Aluminium (high): too reactive to be reduced by carbon, so it is obtained by electrolytic reduction of fused alumina.

Copper (low): unreactive; its sulphide ore can simply be roasted and reduced (self-reduction/heating), as its compounds decompose easily. Thus more reactive metals need electrolysis while less reactive ones need only reduction or heating.

(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 $Fe_2O_3$Fe_2O_3 and $SiO_2$SiO_2 remain insoluble and are filtered off.
$Al_2O_3 + 2NaOH \rightarrow 2NaAlO_2 + H_2O$Al_2O_3 + 2NaOH → 2NaAlO_2 + H_2O
The filtrate is diluted and seeded with freshly prepared aluminium hydroxide, which precipitates hydrated alumina:
$NaAlO_2 + 2H_2O \rightarrow NaOH + Al(OH)_3\downarrow$NaAlO_2 + 2H_2O → 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) Al_2O_3 + 3H_2O

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

• Electrolyte: pure alumina dissolved in molten cryolite $Na_3AlF_6$Na_3AlF_6 with a little fluorspar $CaF_2$CaF_2.
• 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- → O_2 + 4e^-
Molten aluminium collects at the bottom and is tapped off. The oxygen burns the carbon anodes ($C+O_2\rightarrow CO_2$C+O_2→ CO_2), 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 + CuSO_4 → ZnSO_4 + 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 very unreactive metals (e.g. $Cu$Cu) require only roasting/heating.

(i) Pure alumina melts only near $2050^\circ\text{C}$2050^; dissolving it in molten cryolite (with fluorspar) lowers the working temperature to about $950^\circ\text{C}$950^ and increases the conductivity of the electrolyte, making the process economical.

(ii) Aluminium is obtained at the cathode, because the $Al^{3+}$Al^3+ cations migrate to the negative electrode where they are reduced: $Al^{3+}+3e^-\rightarrow Al$Al^3++3e^-→ Al.

(iii) At the anode: $2O^{2-} \rightarrow O_2 + 4e^-$2O^2- → O_2 + 4e^- (the oxygen then burns the carbon anode).

(iv) Aluminium is a highly reactive metal that holds oxygen very strongly; carbon (coke) is not a powerful enough reducing agent to remove oxygen from alumina, so chemical reduction fails and electrolytic reduction is required.