Q.The reaction, Cr2O3 + 2Al → Al2O3 + 2Cr (ΔrG° = – 421 kJ) is thermodynamically feasible as is apparent from the Gibbs energy value. Why does it not take place at room temperature?

Thermodynamic feasibility (sign of $\Delta G^\circ$) governs whether a reaction can occur, but the rate at which it occurs is a kinetic property governed by activation energy. At room temperature the reactants ($Cr_2O_3$, Al) lack sufficient kinetic energy to cross this barrier, so although $\Delta_rG^\circ = -421$ kJ predicts the reaction is favourable, it needs to be ignited (e.g. with a burning Mg ribbon) before it proceeds, as in the thermite rea …

Step 1 – Distinguish thermodynamics from kinetics

$\Delta_rG^\circ$ tells us about the feasibility (spontaneity) of a reaction under standard conditions — whether the products are more stable than the reactants. It says nothing about how fast the reaction will actually proceed; that is governed by kinetics, specifically the activation energy ($E_a$) of the reaction pathway.

Step 2 – Apply to this reaction

A large negative $\Delta_rG^\circ$ confirms this reaction is thermodynamically highly favourable — the products are much more stable than the reactants. However, at room temperature, the reactant particles (solid $Cr_2O_3$ and solid Al) simply do not possess enough kinetic/thermal energy on average to overcome the activation energy barrier required to break bonds and initiate the reaction.

Step 3 – Why it needs a trigger

This is exactly analogous to the familiar thermite reaction (Al + $Fe_2O_3$): a mixture of aluminium powder and $Cr_2O_3$ can sit indefinitely at room temperature without reacting, but once ignited (commonly by a burning magnesium ribbon, which supplies the initial burst of energy), the reaction proceeds vigorously and is strongly exothermic, sustaining itself once started.

Step 4 – Conclusion …

Separate thermodynamic feasibility ($\Delta G^\circ$, tells you IF a reaction is favourable) from kinetics (activation energy, tells you HOW FAST/whether it proceeds without help) …

• Assuming negative $\Delta G^\circ$ automatically means the reaction happens readily/instantly — thermodynamics says nothing about rate. …