Question

`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.

Answer 1

(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)             ∆_s 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⁺.