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 (∆_fG⁰) 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 decrease in the free energy (AG°) at a certain specific temperature.

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

• ZnO + C → Zn + CO …………..(1)
• ZnO + CO → Zn + CO₂ ………………(2)

In the first case, there is increase in the magnitude of ∆S° while in the second case, it almost remains the same. In other words, ∆G° 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.