Chemistry Class 12 NCERT Solutions: Chapter 6 General Principles and Processes of Isolation of Elements Part 7

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Q: 3. The reaction,

is thermodynamically feasible as is apparent from the Gibbs energy value. Why does it not take place at room temperature?


The change in Gibbs energy is related to the equilibrium constant, K as


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


Increasing the temperature increases the value of making the value of more and more negative. Therefore, the reaction becomes more and more feasible as the temperature is increased.

Q: 4. Is it true that under certain conditions, can reduce and Si can reduce ? What are those conditions?


The temperature range in which lesser than , can reduce

On the other hand, the temperature range in which is less than , Si can reduce MgO to Mg.

The temperature at which curves of these two substances intersect is . Thus, at temperatures less than can reduce and above , can reduce .