JEE Main 2023 · 12 April · morning · Single correct · +4 / −1
Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R
Assertion A : In the Ellingham diagram, a sharp change in slope of the line is observed for Mg → MgO at ∼ 1120° C
Reason R: There is a large change of entropy associated with the change of state
In the light of the above statements, choose the correct answer from the options given below
- (a) Both A and R are true and R is the correct explanation of A
- (b) A is false but R is true
- (c) Both A and R are true but R is NOT the correct explanation of A
- (d) A is true but R is false
Answer and solution
Answer (a) Both A and R are true and R is the correct explanation of A
Solution
Chemistry ma’am · 4 steps
The slope of an Ellingham line is -Δ S°, so a big entropy jump bends the line sharply.
- Slope means entropy. Each Ellingham line plots Δ G° against T. Since Δ G° = Δ H° - TΔ S°, the slope of the line is -Δ S°.
Δ G° = Δ H° - T Δ S° ⇒ slope = -Δ S°
- Below the boiling point. In 2Mg(s) + O₂(g) → 2MgO(s), one mole of gas is used up. Δ S° is negative, so the line slopes upward.
- Above the boiling point. Near 1120°C, close to the boiling point of magnesium, Mg is a gas: 2Mg(g) + O₂(g) → 2MgO(s). Now three moles of gas turn into a solid. Δ S° becomes much more negative, so the slope rises sharply.
- So. A is true. R is also true, and it is exactly why the slope changes: the change of state brings a large entropy change. The answer is (a).
The idea to keep. Every kink in an Ellingham line marks a phase change of the metal or its oxide. The slope is -Δ S°, and more gas consumed means a steeper upward line.
Answer from the NTA final answer key. Solution by PrepWiser.