JEE Main · 2019 · 09 Jan 2019 · Shift 2 · General Principles and Processes of Isolation of Metals

On an Ellingham diagram a metal can reduce another oxide when its own line lies below that oxide's line at the chosen temperature. At $1400^{\circ }\text{C}$ the $\mathrm{Al}/\mathrm{Al}{}_2\mathrm{O}{}_3$ line has dropped below the $\mathrm{Zn}/\mathrm{ZnO}$ line, so $4\mathrm{Al}+6\mathrm{ZnO}\to 2\mathrm{Al}{}_2\mathrm{O}{}_3+6\mathrm{Zn}$ has a negative $\Delta G$ and aluminium can indeed extract zinc from $\mathrm{ZnO}$. The other statements fail the same test: at $500^{\circ }\text{C}$ the carbon line still lies above $\mathrm{Zn}/\mathrm{ZnO}$, carbon does reduce $\mathrm{Cu}{}_2\mathrm{O}$, and the less reactive copper cannot reduce $\mathrm{ZnO}$.