Ncert Solutions Chemistry Class 12th

8.21 (i) Transition metals and many of their compounds show paramagnetic behavior- Paramagnetic behaviour is shown by transition metals as paramagnetism is due to the presence of unpaired electrons which have a magnetic moment associated with its spin and angular momentum, as the orbital angular momentum is satisfied in the first transition series. So the paramagnetic is only due to the unpaired electrons.

(ii) The enthalpies of atomisation of the transition metals are high- Enthalpies of atomization is the enthalpy change when 1 mol of gaseous atoms is formed from its element in its defined physical state under standard conditions (298.15K, 1 atm). In case of the transition metals enthalpies are high due to high effective nuclear charge and a large number of valence electrons which also leads to strong metallic bonding.

Eg: 515 KJ/mol for Vanadium,473 KJ/mol for Titanium

(iii) The transition metals generally form coloured compounds- Transition metals are generally coloured because of absorption of radiations which fall in the visible region as on obtaining energy electrons jump from one d-orbital to another.

(iv) Transition metals and their many compounds act as good catalyst- Transition metals and their compounds act as good catalysts due to its ability to show variable oxidation state and form complexes, another reason is that they provide a suitable surface for reaction to take place.

Eg: Vanadium Oxide in contact process, Finely divided iron in Haber's Process.

8.17 On moving along the lanthanoid series, the atomic number increases. Also, with the increase in atomic number, the number of electrons in the 4f orbital also increases.

The 4f electrons have a poor shielding effect. Therefore, the effective nuclear charge experienced by the outer electrons increases. Consequently, the force of attraction between the nucleus and valence electrons increases. This results in a decrease in the size of lanthanoids with the increase in the atomic number. This is termed as lanthanoid contraction.

CONSEQUENCES:

1. The properties of second and third transition series are similar in

2. Separation of lanthanoids from its compounds is possible due to lanthanide

3. The variation in the basic strength of lanthanide hydroxides os due to this (Basic strength decreases from La (OH)₃ to Lu (OH)₃)

7.29

Moist chlorine is a good reducing agent because it can accept electrons from other species as it is very electronegative.

(iii) The bleaching action of chlorine is due to oxidation by nascent oxygen produced. This nascent oxygen can be produced by

Chlorine dissolves in water in absence of sunlight and forms hydrochloric acid and hypochlorous acid (HOCl)

Since hypochlorous acid is not much stable it decomposes giving nascent oxygen. Cl₂ + H₂O  HCl + HOCl

HOCl HCl + [O]

This [O] is called as nascent oxygen. This nascent oxygen is responsible for bleaching action.

NOTE: Since chlorine has the ability to kill harmful micro-organism it also acts as a good disinfecting agent.