Electrode potentials (Eo):

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

Standard electrode potentials for 3d-elements

A negative value of E° for the first series of transition elements except for Cu²⁺/ Cu will be implies the following things:

(a) From dilute acids these metals should liberate hydrogen that is the reactions that was given below,

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

are favorable in the other direction. In fact practice though, many of these metals react with the dilute acids extremely slowly. A few of these metals will be coated with the thin protective layer of oxide element. This category an oxide layer avoids the metal to react additionally.

(b) There is no regular trend in the E° values. The metals should act as good reducing agents. This is because of irregular variation in the sublimation and ionisation energies across the series.

Associative stabilities of transition metal ions in various oxidation states in aqueous medium could be predicted from an electrode potential data. To demonstrate this, now let us assume the following,

Enthalpy of sublimation, DHsub

Adding these equations one than we gets,

Here the ΔH should represents the enthalpy modified that needs 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 just only when ΔH is negative. Many negative value is the value of ΔH, more favourable will be the created of that cation from the metal. Moreover, the oxidation state for ΔH 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 that was given below

 Mⁿ⁺(aq)  + ne^-  → M (s)

Therefore, by the above equation this reduction reaction will take place if the electrode potential for Mⁿ⁺/M half- cell is positive. The reverse reaction is ghiven below,

M(s) →Mⁿ⁺(aq) + ne^-

Including the formation of Mⁿ⁺(aq) will be occur if the electrode potential is negative, which is the tendency  for the creation of Mⁿ⁺(aq) from the metal M will be more if the corresponding E° value is more negative. Or we could say in which the oxidation state for that E° value is more negative (or less positive) will be more stable by the solution.

Here the standard electrode potential (E°) values can be used in the predicting the relative stabilities of different oxidation states in aqueous solutions when an elements exists in more than one oxidation states, the below stated rule is found useful. The oxidation state of a cation for the given equation   or E° is more negative (for less positive) will be more stable.

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