Block D and F Elements

8.24 Potassium dichromate is prepared from chromite ore (FeCr₂O₄) by the following steps:

Step1: Preparation of sodium dichromate in the reaction of Chromite ore with sodium hydroxide and oxygen gas.

4FeCr₂O₄ + 16NaOH + 7O₂? 8Na₂CrO₄ + 2Fe₂O₃ + 8H₂O

Step2: Conversion of Sodium Chromate on reaction with concentrated Sulfuric acid gives Sodium dichromate as a product.

2Na₂CrO₄ + conc. H₂SO₄? Na₂Cr₂O₇ + Na₂SO₄ + H₂O

Step3: Sodium dichromate on reaction with potassium chloride converts to potassium dichromate as a product.

Na₂Cr₂O₇ + 2KCl? K₂Cr₂O₇ + 2NaCl

As potassium dichromate is less soluble than sodium chloride so, potassium dichromate is obtained in form of orange crystals.

Dichromate ion exists in equilibrium with chromate ion at around pH. However, by changing the pH they can be interconverted.

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)