Standard electrode potential -SEP/SRP

 

Standard electrode potential 

·      Stability of the compounds depends upon electrode potentials. Electrode potential value depends upon

 a). enthalpy of sublimation(atomisation enthalpy) of the metal

b).  ionisation enthalpy and

c).  hydration enthalpy

M(s) ---------> M⁺(aq) + e    

Δ_total H (Total energy change)

Total energy change (Δ_total H) 

= Δ_a H + Δ_i H + Δ_Hyd H

·      The smaller the value of total energy change for a particular oxidation state in aqueous solution, greater will be the stability of that oxidation state.

·      The electrode potentials are the measure the total energy change.

·      The lower the electrode potential, i.e., more negative the standard reduction potential of the electrode, more stable is the oxidation state of the transition metal in the aqueous medium.

Trends in M²⁺/M Standard Electrode Potentials

·      It is evident that there is no regular trend in the E⁰ (M²⁺/M) values. This is due to irregular variation of ionisation energies and sublimation energies (atomisation energy) of the atoms of the members of the transition series.

·      The lower (Less negative) of E⁰ (M²⁺/M) values along the series is due to increase in the first and second ionisation energies.

·      The lower  the SEP value, the more is the stability of O.S. of a metal in aqueous.

·      Mn, Ni and Zn have more negative SRP/SEP (E⁰ (M²⁺/M) values than expected because.

Reason: Mn²⁺& Zn²⁺ have 3d⁵ & 3d¹⁰ stable electronic configuration

  Since  Ni  has highest hydration energy. (Δ_hydH Ni⁺² = –2121 kJ/mole)

·      Copper having positive E⁰ value because the sum of the first and second ionisation enthalpies for copper is very high due to exceptionally high second ionisation enthalpy. This is not compensated by the hydration enthalpy(Δ_hydH).

Therefore, it does not liberate hydrogen from acids. It reacts only with oxidising acids such as HN0₃ and cone. H₂S0₄.

Trends in M³⁺/M²⁺ Standard Electrode Potentials-

·      E⁰ value for Sc³⁺/Sc²⁺ is very low. Hence, Sc³⁺ is stable. This is due to its noble gas configuration [Ar]3d⁰.

·      E⁰value for Mn³⁺/Mn²⁺ is high. This reflects that Mn²⁺ state is stable due to d⁵ configuration.

·      For Fe³⁺/Fe²⁺ couple, the value of E⁰  is comparatively low because Fe³⁺ is extra  stable due to 3d⁵.

·      The comparatively low value for vanadium(V) is due to stability of V²⁺having half-filled t_2g level.

 

·      Problem- 1. Mn²⁺ is most stable than Fe³⁺ while both having same electronic configuration Ans due to low value (less negative) of E⁰ for Mn

     Problem -2 Why is Cr²⁺ reducing and Mn³⁺is  oxidising when both have d⁴ configuration?

Ans- Cr²⁺ ----->Cr³⁺ + e^-

      3d⁴               3d³ having half-filled t_2g level

     Mn³⁺ + e^- --------> Mn

     d⁴                           d⁵

less stable                most stable due to half-filled orbital

Problem-3 Why is Cr²⁺ reducing and Mn³⁺is  oxidising when both have d⁴ configuration

Cr²⁺ is stronger reducing reagent due to negative value of SEP(E⁰value)

Whereas

Cr²⁺----------> Cr³⁺

 d⁴                        d³

Fe²⁺---------> Fe³⁺

d⁶                        d⁵

Problem-4 why the Mn³⁺/Mn²⁺ couple is much more positive than that for Cr³⁺/Cr²⁺ or Fe³⁺/Fe²⁺

Because Mn²⁺ has most stable electronic configuration as 3d⁵ so it requires lager third ionisation enthalpy than others so it has much more positive SEP.

Problem – 5. Cu¹⁺ is less table than Cu²⁺

Cu⁺ (aq) + e^_   -----------------> Cu(s)                            E^o = 0.52V

Cu²⁺ (aq) + e^_   -----------------> Cu(s)                           E^o = 0.34V

Due to large (+)ve standard electrode potential of Cu⁺ is more reduced easily than Cu²⁺

In other word Cu⁺  compound give disproportionate reaction in aqueous

  Cu⁺ (aq) -----------------> Cu(s)   +  Cu⁺²    

Due to large (-)ve  hydration enthalpy of   Cu⁺²     which  more than compensate for second ionisation enthalpy of Cu

                                         

Chemical reactivity and E^o values-

·      The E^o values for M²⁺/M across a series increase toward positive value that indicates a decreasing tendency to form divalent cation across the series.

·      Mn, Ni and Zn have more negative SRP/SEP (E⁰ (M²⁺/M) values than expected because.

  Reason: Mn²⁺& Zn²⁺ have 3d⁵ & 3d¹⁰ stable electronic configuration

  Since Ni has highest hydration energy. (Δ_hydH Ni⁺² = –2121 kJ/mole)

·      Mn³⁺ and Co³⁺ ions are strongest oxidising agents in aqueous solution because their E^o values for M³⁺/M²⁺ are large (+) ve .

While Ti²⁺ V²⁺ and Cr²⁺ are strong reducing agent will liberate hydrogen from a dilute acid, since they have negative SEP value.

Note- Lower (large negative) value of E^o -------> element behave as a reducing reagent

            higher(large positive) value of E^o --------> element behave as a oxidising reagent.