Chemistry Class 12 NCERT Solutions: Chapter 6 General Principles and Processes of Isolation of Elements Part 5

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Q: 23. The choice of a reducing agent in a particular case depends on thermodynamic factor. How far do you agree with this statement? Support your opinion with two examples.

Ans:

Q 23 Thermodynamic Factor

Q 23 Thermodynamic Factor

Q 23 Thermodynamic Factor

The above figure is a plot of Gibbs energy vs. T for formation of some oxides. It can be observed from the above graph that a metal can reduce the oxide of other metals, if the standard free energy of formation of the oxide of the former is more negative than the latter. For example, since is more negative than , can reduce to , but cannot reduce . Similarly, can reduce , but cannot reduce because is more negative than .

Q: 24. Name the processes from which chlorine is obtained as a by-product. What will happen if an aqueous solution of is subjected to electrolysis?

Answer:

In the electrolysis of molten is obtained at the anode as a byproduct.

At cathode:

At anode:

The overall reaction is as follows:

If an aqueous solution of is electrolyzed, will be obtained at the anode but at the cathode, will be obtained (instead of Na). This is because the standard reduction potential of is more negative than that of .

Hence, will get preference to get reduced at the cathode and as a result, is evolved.

At cathode:

At anode:

Q: 25. What is the role of graphite rod in the electrometallurgy of aluminium?

Answer

In the electrometallurgy of aluminium, a fused mixture of purified alumina , cryolite, and fluorspar is electrolysed. In this electrolysis, graphite is used as the anode and graphite-lined iron is used as the cathode. During the electrolysis, is liberated at the cathode, while and are liberated at the anode, according to the following equation.

If a metal is used instead of graphite as the anode, then will be liberated. This will not only oxidise the metal of the electrode, but also convert some of the liberated at the cathode back into . Hence, graphite is used for preventing the formation of at the anode. Moreover, graphite is cheaper than other metals.