Introduction :
The potential of individual half cell cannot be measured . We can only measure the difference between the two half cell potentials that gives the EMF of the cell. According to convention, a half cell called the standard hydrogen electrode represented by Pt(s)│H2(g)│H+(aq), is assigned a zero potential at all temperatures corresponding to the reaction
H+(aq) + e- →1/2 H2(g)
The standard hydrogen electrode consists of a platinum electrode coated with platinum black. The electrode is dipped In an acidic solution and pure hydrogen gas is bubbled through it. The concentration of both the reduced and oxidized forms of hydrogen is maintained at unity. This implies that the pressure of hydrogen gas is one bar and the concentration of hydrogen ion in the solution is one molar. At 298 K the emf of the cell, standard hydrogen electrode || second half cell constructed by taking standard hydrogen electrode as anode and the other half cell as cathode, gives the reduction potential of the other half cell. If the concentrations of the oxidized and reduced form of the species in the right hand half cell are unity, then the cell potential is equal to standard electrode potential, E-R of the given half cell.
E- = E-R – E-L
As E-L for standard hydrogen electrode is zero.
E- = E-R – 0 = E-R
The potential of individual half cell cannot be measured . We can only measure the difference between the two half cell potentials that gives the EMF of the cell. According to convention, a half cell called the standard hydrogen electrode represented by Pt(s)│H2(g)│H+(aq), is assigned a zero potential at all temperatures corresponding to the reaction
H+(aq) + e- →1/2 H2(g)
The standard hydrogen electrode consists of a platinum electrode coated with platinum black. The electrode is dipped In an acidic solution and pure hydrogen gas is bubbled through it. The concentration of both the reduced and oxidized forms of hydrogen is maintained at unity. This implies that the pressure of hydrogen gas is one bar and the concentration of hydrogen ion in the solution is one molar. At 298 K the emf of the cell, standard hydrogen electrode || second half cell constructed by taking standard hydrogen electrode as anode and the other half cell as cathode, gives the reduction potential of the other half cell. If the concentrations of the oxidized and reduced form of the species in the right hand half cell are unity, then the cell potential is equal to standard electrode potential, E-R of the given half cell.
E- = E-R – E-L
As E-L for standard hydrogen electrode is zero.
E- = E-R – 0 = E-R
Galvanic Cell Potential
Measurement of Galvanic cell potential:
The measured emf of the cell:
Pt(s) | H2 (g, 1 bar) | H+(aq, 1 M)||Cu2+(aq,1M)| Cu
Is 0.34 V and it is also the value for the standard electrode potential of the half cell corresponding to the reaction:
Cu2+ (aq, 1M) + 2e- →Cu(s)
Similarly, the measured emf of the cell:
Pt(s)│H2(g, 1 bar)|H+(aq, 1M)||Zn2+(aq,1M)|Zn
Is -0.76 V corresponding to the standard electrode potential of the half cell reaction:
Zn2+(aq, 1M) + 2e- →Zn(s)
In view of this convention, the half cell reaction of the daniell cell can be given as:
Left electrode: Zn(s) → Zn2+(aq,1M) + 2e-
Right electrode: Cu2+(aq, 1M) + 2e- →Cu(s)
The overall reaction of the cell is the sum of the above two reactions and we obtain the equation:
Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
Emf of the cell = E0cell = E0R – E0L
= 0.34 – (-0.76) = 1.10 V
No comments:
Post a Comment