Question

The value of ${\Delta }_r{G}^{\circ }$ for $C{r}_2{O}_3$ is $-540{\text{kJ mol}}^{-1}$ and that of $A{l}_2{O}_3$ is $-827{\text{kJ mol}}^{-1}$. Is the reduction of $C{r}_2{O}_3$ by $Al$ is feasible reaction?

• A. The data is incomplete
• B. The reaction is feasible
• C. The reaction is not feasible
• D. The reaction may or may not be feasible

Answer 1

The correct option is B The reaction is feasible

The value of ${\Delta }_r{G}^{\circ }$ for $C{r}_2{O}_3(-540{\text{kJ mol}}^{-1})$ is higher than that of $A{l}_2{O}_3$. Therefore, $Al$ can reduce $C{r}_2{O}_3$.
Alternatively,
$Al\to A{l}_2{O}_3{\Delta }_r{G}^{\circ }=-827{\text{kJ mol}}^{-1}\cdots \left(i\right)Cr\to C{r}_2{O}_3{\Delta }_r{G}^{\circ }=-540{\text{kJ mol}}^{-1}\cdots \left(ii\right)$
Subract $\left(ii\right)$ from $\left(i\right)$, we have
$Al+C{r}_2{O}_3\to A{l}_2{O}_3+Cr{\Delta }_r{G}^{\circ }=-827-(-540)=-287{\text{kJ mol}}^{-1}$
As the value of ${\Delta }_r{G}^{\circ }$ is negative, the reaction is feasible.