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(a) Suggest a condition under which magnesium could reduce alumina. (b) Although thermodynamically feasible, in practice magnesium metal is not used for the reduction of alumina in the metallurgy of aluminium. Why ? ( c) What is the reduction of a metal oxide easier if the metal formed is in liquid state at the temperature of reduction ? (d) At a site, low grade copper ores oare available and zinc and iron scraps are also available. Which of the two scraps would be more suitable for reducing the leached copper ore and why ? |
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Answer» (a) The two equations are : (i) `(4)/(3) Al + O_(2) rarr (2)/(3) Al_(2) O_(3)` (ii) `2Mg + O_(2) rarr 2MgO` All the point of intersection of the `Al_(2) O_(3)` and `MgO` curves, `Delta_(r) G^(Ө)` becomes zer for the reaction : `(2)/(3) Al_(2) O_(3) + 2Mg rarr 2 MgO + (4)/(3) Al` Below that point magnesium can reduce alumina. (b) Temperatures below the point of intersection of `Al_(2) O_(3)` and `MgO` curves, magnesium can reduce alumina. But the process will be uneconomical as magnesium is much costlier than alumina. ( c) The entropy is higher if the metal is in liquid state than when it is in solid state. The value of entropy change `(Delta_(r) G^(Ө))` of the reduction process is more on positive side when the metal formed is in liquid state and the metal oxide being reduced is in solid state. Thus, the value of `Delta_(r) G^(Ө)` becomes more on negative side and the reduction becomes easier. (d) Zinc being above iron in the electrochemical series (more reactive metal is zinc), the reduction will be faster in case zinc scraps are used, But zinc is costlier metal than iron, so using iron scraps will be advisable and advantageous. |
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