JEE Main · 2021 · 17 Mar 2021 · Shift 1 · General Principles and Processes of Isolation of Metals

The point of intersection and sudden increase in the slope, in the diagram given below, respectively, indicates:

(A) $Δ G = 0$ and reduction of the metal oxide
(B) $Δ G = 0$ and melting or boiling point of the metal oxide✓ Correct
(C) $Δ G > 0$ and decomposition of the metal oxide
(D) $Δ G < 0$ and decomposition of the metal oxide

Answer: (B)

Solution

Each line on the diagram gives

Δ G^{\circ}

for the oxidation of one substance by one mole of oxygen. Where two lines cross, the two oxidations have the same

Δ G^{\circ}

, so the exchange reaction between them has

Δ G = 0

; above that temperature the lower line takes over and its element can reduce the other oxide. The slope of a line is

- Δ S

for the oxidation, and it changes abruptly only when one of the substances taking part changes its state of aggregation, because melting or boiling alters the entropy sharply. The intersection therefore indicates

Δ G = 0

and the sudden increase in slope indicates the melting or boiling point of the metal oxide.

JEE Main · 2021 · 17 Mar 2021 · Shift 1 · Single correct option

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JEE Main · 2021 · 17 Mar 2021 · Shift 1 · General Principles and Processes of Isolation of Metals

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