Page No 157: - Chapter 6 - General Principles & Process Of Isolation Of Elements - Intext Solutions - Ncert Solutions class 12 - Chemistry

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Question 6.3:

The reaction,

is thermodynamically feasible as is apparent from the Gibbs energy value.

Why does it not take place at room temperature?

Answer:

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.

Question 6.4:

Is it true that under certain conditions, Mg can reduce SiO₂ and Si can reduce MgO? What are those conditions?

Answer:

The temperature range in which   is lesser than , Mg can reduce SiO_{2 to} Si.

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

The temperature at which ΔfG curves of these two substances intersect is 1966 K. Thus, at temperatures less than 1966 K, Mg can reduce SiO₂ and above 1966 K, Si can reduce MgO.