Chemistry Class 12 NCERT Solutions: Chapter 8 The d and f Block Elements Part 4

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Q: 16. Describe the preparation of potassium permanganate. How does the acidified permanganate solution react with (i) iron (II) ions (ii) and (iii) oxalic acid? Write the ionic equations for the reactions.


Potassium permanganate can be prepared from Pyrolusite . The ore is fused with in the presence of either atmospheric oxygen or an oxidising agent, such as or , to give .

The green mass can be extracted with water and then oxidized either electrolytically or by passing chlorine/ozone into the solution. Electrolytic oxidation

At anode, manganite ions are oxidized to permanganate ions.

Oxidation by chlorine

Oxidation by ozone

(i) Acidified solution oxidizes ions i.e., ferrous ions to ferric ions.

(ii) Acidified potassium permanganate oxidizes to sulphuric acid.

(iii) Acidified potassium permanganate oxidizes oxalic acid to carbon dioxide

Q: 17. For and systems, the values for some metals are as follows:

Use this data to comment upon:

(i) The stability of in acid solution as compared to that of and

(ii) The ease with which iron can be oxidised as compared to a similar process for either chromium or manganese metal


(i) The value for is higher than that for and lower than that for . So, the reduction of is easier than the reduction of but not as easy as the reduction of . Hence, is more stable than , but less stable than . These metal ions can be arranged in the increasing order of their stability as:

(ii) The reduction potentials for the given pairs increase in the following order.

So, the oxidation of Fe to is not as easy as the oxidation of and the oxidation of . Thus, these metals can be arranged in the increasing order of their ability to get oxidised as:

Q: 18. Predict which of the following will be coloured in aqueous solution? . Give reasons for each.

Only the ions that have electrons in orbital will be coloured. The ions in which orbital is empty will be colourless.

Q_18_The Ions in D-Orbital Is Empty Will Be Colourless
Q_18_The ions in d-Orbital is Empty will be Colourless


Atomic Number

Ionic State

Electronic configuration in ionic state















From the above table, it can be easily observed that only has an empty orbital. All other ions, except , will be coloured in aqueous solution because of transitions.