Question

For metal oxides, when $\Delta G^{\circ }$ is plotted against temperature, there is a point where the graph crosses $\Delta G$ - zero line, above this temperature

• A. the oxide is stable
• B. the oxide is unstable
• C. the oxide melts
• D. the oxide is reduced

Answer 1

Answer: (B)

The graph which represents the variation of standard free energy with temperature for the formation of oxides of elements is called Ellingham diagram. In case of certain metals, the graph rises and free energy of formation becomes greater than zero at high temperatures. So these metal oxides are unstable at higher temperatures, for example silver oxide and mercury oxide.