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Thermodynamic Principles of Metallurgy
Concepts tested here
- Why sulphide ores are roasted
All Questions
2007 AIPMT-PRE 1 question
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Which of the following statements, about the advantage of roasting of sulphide ore before reduction is not true?The sulphide ore is roasted to the oxide before reduction.
The $\Delta_f G^\circ$ values of most metal sulphides are more negative than those of $CS_2$ and $H_2S$ (in the wording of the paper, they are 'greater'). $CS_2$ is in fact an endothermic compound.
Therefore neither carbon nor hydrogen can reduce a metal sulphide to the metal: the reactions $2MS + C \rightarrow 2M + CS_2$ and $MS + H_2 \rightarrow M + H_2S$ have positive $\Delta G$.
So the statement that carbon and hydrogen are suitable reducing agents for metal sulphides is not true.
On the other hand, the standard free energies of formation of the oxides are much less than those of $SO_2$, so the oxidation (roasting) of metal sulphides to metal oxides has a negative $\Delta G$ and is thermodynamically favourable.
The oxide obtained can then be reduced easily with carbon.