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From the Given Data of E^0 Values, Answer the Following Questions - Chemistry

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`E_((M^(2+)/M)`

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 questions :

(1) Why is `E_(((Cu^(2+))/(Cu)))` value exceptionally positive

(2) Why is `E_(((Mn^(2+))/(Mn)))` value is highly negative as compared to other elements

(3) Which is the stronger reducing agents Cr2+ or Fe2+ ? Give Reason.

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Solution

(i) The `E_((M^(2+)/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)             H (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

M2+(g→ M2+(aq)             hydH (Hydration energy)

Now, copper has high energy of atomisation and low hydration energy. Hence, the `E_(((Cu^(2+))/(Cu)))` is exceptionally positive.

(ii) The `E_(((Mn^(2+))/(Mn)))` value is highly negative as compared to other elements because of the extra stability of Mn2+ ion due to half-filled d-orbitals.

(iii) The following reactions are involved when Cr2+ and Fe2+ act as reducing agents:

Cr2+ → Cr3+ + e

Fe2+ → Fe3+ + e

The `E_(Cr^(3+))` value is −0.41 V and `E_(Fe^(3+))` is +0.77 V. 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+.

Concept: General Properties of the Transition Elements (D-block)
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