Standard electrode potentials of some half–cells involving 3d-series of transition elements and their ions in aqueous solution are given in table,

 

 

 

The negative values of E° for the first series of transition elements (except for Cu²⁺/ Cu ) indicate that,

 

(i) These metals should liberate hydrogen from dilute acids i.e., the reactions,

 

           M + 2H⁺ → M²⁺ + H₂ (g); 2M + 6H⁺ → 2M³⁺ + 3H₂(g)

 

are favourable in the forward direction. In actual practice however, most of these metals react with dilute acids very slowly. Some of these metals get coated with a thin protective layer of oxide. Such an oxide layer prevents the metal to react further.

 

(ii) These metals should act as good reducing agents. There is no regular trend in the E° values. This is due to irregular variation in the ionisation and sublimation energies across the series.

 

Relative stabilities of transition metal ions in different oxidation states in aqueous medium can be predicted from the electrode potential data. To illustrate this, let us consider the following,

 

           M(s) → M(g)          ; ?H₁ = Enthalpy of sublimation, ?H_sub

 

           M(g) → M⁺(g) + e^– ; ?H₂ = Ionisation energy, IE

 

           M⁺(g) → M⁺(aq)    ; ?H₃ = Enthalpy of hydration, ?H_hyd

 

Adding these equations one gets,

 

           M(s) → M+(aq) + e–

 

           ?H = ?H₁ + ?H₂ + ?H₃ = ?H_sub + IE + ?H_hyd

 

The represents the enthalpy change required to bring the solid metal M to the monovalent ion in aqueous medium, M⁺(aq).

 

The reaction, M(s) → M⁺(aq) + e^–, will be favourable only if ?H is negative. More negative is the value is of ?H, more favourable will be the formation of that cation from the metal. Thus, the oxidation state for which ?H value is more negative will be stable in the solution.

 

Electrode potential for a Mⁿ⁺/M half-cell is a measure of the tendency for the reaction, Mⁿ⁺(aq) + ne^–→ M(s)

 

Thus, this reduction reaction will take place if the electrode potential for Mⁿ⁺/M half- cell is positive. The reverse reaction, M(s) → Mⁿ⁺(aq) + ne^–

 

Involving the formation of Mⁿ⁺(aq) will occur if the electrode potential is negative, i.e., the tendency for the formation of Mⁿ⁺(aq) from the metal M will be more if the corresponding E° value is more negative. In other words, the oxidation state for which E° value is more negative (or less positive) will be more stable in the solution.

 

When an elements exists in more than one oxidation states, the standard electrode potential (E°) values can be used in the predicting the relative stabilities of different oxidation states in aqueous solutions. The following rule is found useful.

 

The oxidation state of a cation for which ?H = (?H_sub + lE + ?H_hyd) or E° is more negative (for less positive) will be more stable.