Tamilnadu BoardEnglish MediumSTD 12ChemistryMetallurgy3 Marks
Question
The selection of reducing agent depends on the thermodynamic factor: Explain with an example.
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Answer
From the Ellingham diagram, it is clear that metals for which the standard free energy of formation $\left(\Delta_{ f } G ^0\right)$ of their oxides is more negative can reduce the metal oxides for which the standard free energy of formation ( $\Delta_{ f } S ^0$ ) of oxides is less negative.
Thermodynamic factor has a major role in selecting the reducing agent for a particular reaction. Only that reagent will be preferred which will lead to a decrease in the free energy $\left(\Delta G^0\right)$ at a certain specific temperature.
E.g. - Carbon reduces ZnO to Zn but not CO .
$ZnO+C \longrightarrow Zn+CO \ldots \ldots \text { (1) }$
$ZnO+CO \longrightarrow Zn+CO_2 \ldots \text { (2) }$
In the first case, there is an increase in the magnitude of $\Delta S^0$ while in the second case, it almost remains the same. In other words, $\Delta G^0$ will have more negative value in the first case, when $C$ is the reducing agent then in the second case when CO acts as the reducing agent. Therefore, C is a better reducing agent.
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