Cr	Mn	Fe	Co	Ni	Cu
	-0.91	-1.18	-0.44	-0.28	-0.25	-0.34

From the given data of E⁰ values answer the following Question:

(i) Which is strongest why is  value exceptionally positive?

(ii) Why is value highly negative as compared to other elements?

(iii) Reducing agent  Cr²⁺ or Fe²⁺? Give reason

 

(b) Why do actinoids show wide range f oxidation states? Write on the similarity between the chemistry of lanthanoids and actinoids.

The E⁰ (M²⁺/M) value of a metal depends on the energy changes involved in the formation of the M²⁺ ion:

1. Sublimation: The energy required for converting one mole of an atom from the solid state to the gaseous state

M(s) ---> M (g)            sH (Sublimation energy)

2. Ionisation: The energy required to take out electrons from one mole of atoms in the gaseous state to form the corresponding cation in the gaseous state

M (g) --->M²⁺ (g)             iH (Ionization energy)

3. Hydration: The energy released when one mole of ions are hydrated

 M (g)---> M2+ (aq)             hydH (Hydration energy)

Now, copper has high energy of atomization and low hydration energy. Hence, theE⁰_(Cu²⁺/Cu) is exceptionally positive.

(ii)  The  value is highly negative as compared to other elements because of the extra stability of Mn²⁺ due half -filled d orbitals.

(iii) The following reactions are involved when Cr²⁺ and Fe⁺ act as reducing agent:

Cr²⁺ ---> Cr³⁺ + e-

Fe²⁺ ----> Fe³⁺ + e^-

The value is -0.41 V and   is + 0.77V. This means that Cr²⁺ can be easily oxidised to Cr³⁺ but Fe²⁺ does not get oxidised to Fe³⁺ easily, therefore, Cr²⁺ is a stronger reducing agent than Fe³⁺.

(b) In actinoids, the 5f, 6d, 7s shells are present. These three shells are of comparable energies; therefore electrons can remove from these shells. This gives rise f variable oxidation states in actinoids.

Following are the similarities between actinoids and lanthanoids:

(i) The size of atom: It decreases across the series in both actinoids (due to actinoid contraction) and lanthanoids(due to lanthanoids contraction).

(ii) Oxidation states: Lanthanoids and actinoids generally show +3 oxidation states. However, some element in the actinoids series is capable of exhibiting oxidation states higher than +3.