Question

The selection of reducing agent depends on the thermodynamic factor: Explain with an example.

Answer 1

From the Ellingham diagram, it is clear that metals for which the standard free energy of formation (∆_fG⁰) of their oxides is more negative can reduce the metal oxides for which the standard free energy of formation (∆_fS⁰) 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 (∆G⁰) at a certain specific temperature.

E.g. – Carbon reduces ZnO to Zn but not CO.

\[\ce{ZnO + C -> Zn + CO}\] ......(1)

\[\ce{ZnO + CO -> Zn + CO2}\] .....(2)

In the first case, there is an increase in the magnitude of ∆S⁰ while in the second case, it almost remains the same. In other words, ∆G⁰ 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.