A. Account for the following:

(i) is acidic whereas is basic

(ii) Zr and Hf exhibit similar properties.

(iii) Transition metals form a large number of complex compounds.

B. Write the preparation of from pyrolusite ore . Write the type of magnetism shown by KMnO₄ and K₂MnO₄.

OR

A. The elements of 3d transition series are given as :

Sc Ti V Cr Mn Fe Co Ni Cu Zn

Answer the following:

(i) Copper has exceptionally positive value. Why?

(ii) Which element is a strong reducing agent in +2 oxidation state and why?

(iii) salts are colourless. Why?

B. Write the preparation of sodium dichromate from chromite ore

A.

(i)

The acidic or basic nature of a compound is decided on the number of electrons it can accept or donate. The compounds which accept electrons are acids and those who donate electrons are base. In case of manganese has an oxidation state of +7 which is highest for the metal, so it can no longer donate electron from lower shell as it requires large amount of energy. Whereas has a oxidation state of +2 and it can still give 5 electrons thus having a basic nature as compared to .

Thus, is acidic whereas is basic.

(ii)

Atomic radius varies in two ways in the periodic table: it decreases across the period and increases down the group. These general trends are observed in all groups. For period 6, and in the lanthanide series from Z=57 to Z=71 the decrease in atomic radius is more than it should be. The general rule is that as atomic number increases the nuclear charge increases and thus the atomic radius decreases.

When electrons are added in the same shell the electronic repulsion decreases the effective nuclear attraction. This repulsion is generally called the shielding effect. The order of effective shielding of electrons from nuclear charge is

s > p > d > f

As the shielding effect for f orbital in lanthanides is low the decrease in nuclear attraction is less so the net effective nuclear attraction decreases the atomic radii more than expected. This is called as ‘lanthanide contraction’ and the post lanthanide elements have very less atomic radii and almost similar to that in the 6^th period.

Thus Zr(Z=40) and Hf (Z=72) have almost identical radii.

(iii)

The primary reason is that, Transition metals exhibit multiple oxidation states so they can have multiple intermediate forms during a chemical reaction.

Generally transition elements are paramagnetic in nature suggesting that they have valence electrons in their outermost shell. These are loosely bound to the metal itself and can be released during a chemical reaction easily by the heat of reaction. Thus metals lend electrons during a chemical reaction which eases the transfer of electrons between reactants.

Metals also have high porosity which is essential during a reaction to occur (especially for gaseous reactions).

Considering all these factors and the ease with which they can be incorporated in a chemical reaction, (e.g. Presence of Pt metal for reaction of H₂ and O₂ makes it explosive) metal acts as catalyst.

They also have great malleable and grain properties so they can be used under robust conditions and in fine powdered form.

B.

MnO₂ treated with potassium hydroxide or other alkali hydroxide and along with KNO₃ which forms a dense dark green colour compound as K₂MnO₄ potassium manganate. As the +6 oxidation state of Mn is less stable it undergoes disproportionation reaction with HCl to give potassium manganate.

Rxn:

KMnO₄ is paramagnetic as Mn has +7 oxidation state which implies that the metal ion has odd number of electrons thus KMnO₄ will act as paramagnetic and Mn in K₂MnO₄ has +2 oxidation state thus it behaves as diamagnetic.

OR

A.

(i)

When copper compounds are dissolved in water it has low heat of hydration, which results into less release of electrons from the metal ions and thus decrease in the negative flow of electric potential. In short, hydration energy is inversely proportional to electric potential and hence low hydration energy provides higher positive electric potential.

The ionization enthalpy is the energy released when one mole of metal compound dissociate into its component ions. For copper the ionization energy is high and thus favors easy dissociation of ions leading to increase in electrolytic conductivity. Due to these reasons copper has very high positive E° value.

(ii)

Chromium Cr²⁺ is a strong reducing agent Mn³⁺ is an oxidizing agent even though both of them have the same d⁴ configuration. The reason here is the formation of degenerate orbitals under excitation of ions. When the degenerate orbitals are filled according to Crystal field theory, the t_2g level in Mn³⁺ achieves a half filled state and has extra stability thus it resists the acceptance of electrons, (as accepting electrons increases system energy) and shows tendency to undergo oxidation.

Thus chromium with +2 oxidation state shows high tendency to undergo oxidation and act as strong reducing agent.

(iii)

When a photon or energy is given to the metal ions the free electrons absorbs energy and are excited to higher energy band. When they release this energy, it has a certain wavelength and this wavelength imparts color to the salts of d-block elements. In case of Zn as there are no unpaired electrons in the compound it does not show the any color. Therefore, the Zn salts are colorless.

B. This process involves a lot of steps but primary treatment of separation from crude ore is done and then the following procedure is obtained.

Step (1)

The crude ore (chromite) is treated with caustic soda present in excess to convert it into sodium chromate.

Step (2)

Sodium chromate can be further oxidized to give sodium dichromate as follows:

Step (3)

Sodium compounds are difficult to separate due to their high ionization energy under standard conditions. Thus they are converted to potassium dichromate.

The solubility of potassium compounds is less as compared to its sodium counterpart.