The reaction
\(Cr_2O_3 + 2Al \longrightarrow Al_2O_3 + 2Cr\, \ \Delta G^o = -421 kJ\)
does not take place at room temperature because:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Certain amount of activation energy is required
The negative ΔG° (−421 kJ) shows only that the reaction is thermodynamically feasible. Thermodynamics says nothing about the rate: the reactants are solids and the reaction has a high activation energy, so at room temperature it proceeds at a negligible rate. Once the mixture is ignited (heated strongly) the energy barrier is crossed and the reaction goes vigorously.
Why the other options are incorrect
Option (1) Cr2O3 is unstable — Incorrect. \( \ce{Cr2O3} \) is a stable oxide with a large negative Gibbs energy of formation; its stability is not the reason.
Option (2) Aluminium is less reactive than chromium — Incorrect. Aluminium is more reactive; \( \ce{Al2O3} \) has a more negative ΔfG° than \( \ce{Cr2O3} \), which is why ΔG° of the reaction is negative.
Option (4) Entropy of the reaction decreases — Incorrect. All reactants and products are solids, so the entropy change is small, and the given ΔG° already includes it and is negative.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Limitations of Ellingham Diagram
