For a reaction
4M(s) + nO2(g)→ 2M2On(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 :
(1) the slope changes from positive to zero
(2) the free energy change shows a change from negative to positive value
(3) the slope changes from negative to positive
(4) the slope changes from positive to negative
