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Question

(B) The stability of an oxide is determined by the Gibbs free energy change $(\Delta G)$ of the formation reaction.
For the reaction $4 M (s) + n O_2

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the free energy change shows a change from negative to positive value

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(B) The stability of an oxide is determined by the Gibbs free energy change $(\Delta G)$ of the formation reaction.
For the reaction $4 M (s) + n O_2 (g) \rightarrow 2 M_2 O_n (s)$,the oxide is stable when $\Delta G < 0$.
As the temperature increases,the value of $\Delta G$ increases.
The temperature at which $\Delta G = 0$ is the equilibrium temperature.
Below this temperature,$\Delta G$ is negative,meaning the oxide is stable.
Above this temperature,$\Delta G$ becomes positive,meaning the oxide is unstable and tends to decompose.
Therefore,the point where the free energy change crosses from negative to positive indicates the temperature limit for stability.

For a reaction,$4 M (s) + n O_2 (g) \rightarrow 2 M_2 O_n (s)$,the free energy change is plotted as a function of temperature. The temperature below which the oxide is stable could be inferred from the plot as the point at which :

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