Question

Standard potentials are measured against the standard hydrogen electrode (SHE). Because it is not always convenient to use a S.Н.Е., often other reference electrodes are used. The saturated calomel electrode (SCE.) is one commonly used reference electrode, with a reduction potential of +0.242 V versus the S.H.E. Using a table of standard reductions, determine what the standard reduction potential of each reduction half-reaction would be versus the S.C.E Cl2 (g) + 2e-→ 2 Cl-(aq) E1.36 Fe3 + (aq) + 3 e-→ Fe(s) E" = -0.02

Answer 1

Answer:-

As we know that the standard reduction potential of any electrode is determined when it is connected to the standard hydrogen electrode. The half cell of any electrode when connected to the standard hydrogen electrode (S.H.E.) to form a galvanic cell and then the EMF of the cell is determined by the potentiometrically . The EMF of the cell would be equal to the potential of the electrode (on the hydrogen scale) since potential of the standard electrode hydrogen electrode (S.H.E.) is taken as zero.

If the reduction occurs at the standard electrode hydrogen electrode (S.H.E.) i.e formation of H2 gas then oxidation takes place at the other electrode and vice versa.

e.g. Determination of standard reduction potential of Zn, when it is connected to the standard hydrogen electrode (S.H.E.) to form a galvanic cell then

At the anode:-

Zn_(s)   $\rightarrow$ Zn²⁺_(aq) + 2e^- ; (Oxidation) E_{(Zn / Zn2+ )}

At the cathode:-

2H⁺_{(aq) +} 2e^-   $\rightarrow$ H_2(g) ; ( reduction) E_{(H+ / H2)} = 0

As we know that

E⁰_cell = E⁰_cathdoe - E⁰_anode

E⁰_cell = E⁰_{(H+ / H2)}   - E⁰_{(Zn / Zn2+ )}

E⁰_cell = 0 - E⁰_{(Zn / Zn2+ )}

E⁰_cell = - E⁰_{(Zn / Zn2+ )} ( i.e the standard reduction potential of Zn electrode)

The potentiometer reads the 0.76 V of the above cell then

E⁰_cell = - E⁰_{(Zn / Zn2+ )} = 0.76 V

E⁰_{(Zn / Zn2+ )} = - 0.76 V ( i.e the standard reduction potential of Zn electrode)

Note :- The - ve sign of standard electrode potential indicates that it behave as anode when is it is connected to the standard electrode hydrogen electrode (S.H.E.) whereas the + ve sign of standard electrode potential indicates that it behave as cathode when is it is connected to the standard electrode hydrogen electrode (S.H.E.).

As we know that when chlorine electrode is connected with other standard calomel electrode (S.C.E.) then the following half cell reaction takes place each electrode in the galvanic cell.

At the anode:-

2Hg_(l)   $\rightarrow$ 2Hg₂²⁺_{(aq) +} 2e^- ; (Oxidation) ; E_{(Hg2²⁺ / Hg)} = 0.242 V  

At the cathode:-

2Cl^-_(aq) + 2e^-    $\rightarrow$ Cl_2(g) ; ( reduction) ;   E_{(Cl / Cl- )} = 1.36 V

As we know that

E⁰_cell = E⁰_cathdoe - E⁰_anode

E⁰_cell = E_{(Cl / Cl- )} - E_{(Hg2²⁺ / Hg}​​​​​​​₎

E⁰_cell = 1.36 V - 0.242 V

E⁰_cell = 1.118 V ( i.e the standard reduction potential of Cl electrode)

Therefore standard reduction potential of Cl electrode when it is connected with standard calomel electrode (S.C.E.) is 1.118 V.

Similarly

As we know that when iron electrode is connected with other standard calomel electrode (S.C.E.) then the following half cell reaction takes place each electrode in the galvanic cell.

At the anode:-

Fe_(l)   $\rightarrow$ Fe³⁺_{(aq) +} 3e^- ; (Oxidation) ; E_{(Fe³⁺ / Fe}​​​​​​​₎ = - 0.04 V

At the cathode:-

2Hg₂²⁺_{(aq) +} 2e^-    $\rightarrow$ 2Hg_(l)   ; ( reduction) ;   E_{(Hg2²⁺ / Hg}​​​​​​​₎ = 0.242 V  

As we know that

E⁰_cell = E⁰_cathdoe - E⁰_anode

E⁰_cell = E_{(Hg2²⁺ / Hg}​​​​​​​₎ - E_{(Fe³⁺ / Fe}​​​​​​​₎

E⁰_cell = (0.242 V ) - (- 0.04 V)

E⁰_cell = 0.242 V + 0.04 V

E⁰_cell = 0.282 V ( i.e the standard reduction potential of Fe electrode)

Therefore standard reduction potential of Fe electrode when it is connected with standard calomel electrode (S.C.E.) is 0.282 V.

Note :- standard reduction potential of any electrode is an intensive property which means that the it does not depends upon the no. of electrons which involved in the cell reaction.

In the galvanic cell an electrode with higher reduction potential is cathode and an electrode with lower reduction potential is anode.