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?

Ans:

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?

Answer:

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 .