Unit 6 General Principles And Processes Of Isolation Of Elements (Intext Questions-2)-Deleted
Exercises
Intext Question
6.3 The reaction,
$$ \mathrm{Cr_2} \mathrm{O_3}+2 \mathrm{Al} \rightarrow \mathrm{Al_2} \mathrm{O_3}+2 \mathrm{Cr} \quad\left(\Delta \mathrm{G}^{\ominus}=-421 \mathrm{~kJ}\right) $$
is thermodynamically feasible as is apparent from the Gibbs energy value. Why does it not take place at room temperature?
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Answer
The change in Gibbs energy is related to the equilibrium constant, $K$ as
$\Delta G=-\mathrm{R} T \ln K$
At room temperature, all reactants and products of the given reaction are in the solid state. As a result, equilibrium does not exist between the reactants and the products. Hence, the reaction does not take place at room temperature. However, at a higher temperature, chromium melts and the reaction takes place.
We also know that according to the equation
$\Delta G=\Delta H-T \Delta S$,
Increasing the temperature increases the value of $T \Delta S$, making the value of $\Delta G$ more and more negative.
Therefore, the reaction becomes more and more feasible as the temperature is increased.
