Question

The reaction \[C{r_2}{O_3} + 2Al \to A{l_2}{O_3} + 2Cr{\rm{ (}}\Delta {{\rm{G}}^0}{\rm{ = - 421 kJ)}}\] is thermodynamically feasible due to -ve value of △G. Why does this reaction not take place at room temperature?
A. Certain amount of activation energy is essential for thermodynamically feasible reactions also.
B. Due to high melting point of chromium oxide the reaction does not take place.
C. Overall value of\[\Delta G\]for the net reaction becomes positive.
D. Molecules of\[C{r_2}{O_3}\]and\[Al\] are not oriented properly.

Answer 1

Hint: Thermodynamic factors are not the only factors that decide the feasibility of a particular reaction. For a reaction to occur at certain conditions, it must be favourable thermodynamically as well as kinetically.

Complete Step by Step Solution:
Whether a particular reaction occurs or not under certain conditions is governed by two factors, the thermodynamic factor and the kinetic factor.

The thermodynamic factor is the change in Gibbs free energy (\[\Delta G\]) which decides if the reaction will occur spontaneously (on its own) or non-spontaneously (not on its own). For spontaneous reactions, the change in Gibbs free energy must be negative i.e., \[\Delta G < 0\]at constant temperature and pressure. A negative Gibbs free energy change indicates that the products of the reaction are more stable than the reactants and therefore the reaction is thermodynamically favourable.

The kinetic factor involved is the Activation energy of the reaction. For a reaction to occur, the reactant molecules must collide and acquire a certain minimum amount of energy called threshold energy. Most reactant molecules, however, possess much less kinetic energy than the threshold amount. The extra energy that these molecules must acquire to successfully react and form products is called the Activation energy. This activation energy “barrier” must be crossed before the reactants can yield products.

The given reaction \[C{r_2}{O_3} + 2Al \to A{l_2}{O_3} + 2Cr{\rm{ (}}\Delta {{\rm{G}}^0}{\rm{ = - 421 kJ)}}\] has a negative value \[\Delta G\] due to which it is thermodynamically feasible. But this reaction does not occur at room temperature because, in this condition, the reactant molecules (\[C{r_2}{O_3}\]and\[Al\]) are not able to gain sufficient activation energy to collide, cross the energy barrier and form the products (\[A{l_2}{O_3}\]and\[Cr\]).

Thus, option A is correct

Note: Please keep in mind that the thermodynamic condition of\[\Delta G < 0\]only applies at constant temperature and pressure. If they are not constant, then change in Gibbs free energy cannot be used as a criterion for spontaneity of chemical reactions.