Galvanic Cell Materials Engineering Dr Lubna Ghalib Galvanic

Galvanic Cell Materials Engineering Dr. Lubna Ghalib

Galvanic Cell not one molar with Electrolytes that are Most of electrolyte solutions of the real galvanic cells are not to be. (1 mole ). But they are always dilute solutions , which they are less than (1 mole ). If the ions concentrates in the electrolyte surround to the anode electrode is less than 1 mole , the reaction motive force to dissolve or corrode the anode will be higher. Because there is less concentrate of ions that causes the reverse reaction. There for it will be more negative of electro-chemical series on the anodic half-cell. M M z+ + ze. Truly the metal ion concentration effects (Cion) on the standards potential of the electrochemical series (Eo)at (25 o)Temperature are given by Nernst equation for the half anodic cell reaction. Where there is one kind of ions are produced

Nernst equation : -

Nernst equation : Where: E = New electrochemical potential for the half cell. Eo = standard electrochemical potential for the half cell. n = No. of electrons transferred (the valence). Cion = molar concentrations of the ions. R = gas constant = 8. 314 J/K o/mole. T = Temperature = 25 + 273 = 298 K o. F = Faraday constant = 96, 500 coulomb.

Electrochemical cell potential for two standard half-cells that are electrically coupled A corrosion reaction can be considered as composed of two half-cell reactions. One of the half-cell reactions corresponds to 'oxidation reaction taking place on the 'anode, ' and the other half cell reaction corresponds to 'reduction reaction' taking place on the 'cathode of the cell. The contribution of each half cell reaction to the Nernst expression can be derived as follows: Oxidation reaction at anode, M 1 z+ + ze. Reduction reaction at cathode M 2 z+ + ze. M 2 Final reaction M 1 + M 2 z+ M 1 z+ + M 2

Electrochemical cell potential for two standard half-cells

Electrochemical cell potential for two standard half-cells

Influence of Concentration and Temperature on Cell Potential

Example 9: A galvanic cell in (25 Co) temperature it is formed from Zinc electrode in (0. 10 mole Zn. SO 4) solution, the other electrode is of Nickel in (0. 05 mole Ni. SO 4) solution. The two electrodes are separated with porous wall and connected with external wire. What is the value of the cell potential, when the circle is connected. Solution: First we suppose that the solutions are in (1 mole) concentrate. So the potentials of Zn & Ni are in standard value of EMF serious. Zn electrode will be the anode because it’s potential (- 0. 763 v) is more negative in the electrochemical cell of Zn – Ni than of Ni (- 0. 250 v).

Example 10:

Example 10: (b) Spontaneous cell reaction is given by: cell reaction = cathodic reaction = anodic reaction Final reaction Cu 2+ + Zn = Zn 2+ + Cu From the table of standard reduction potentials, the reduction potential of Cu is +0. 337 volts and that of Zn is — 0. 763 volts. The emf of the cell is positive. Hence the reduction is spontaneous and it should proceed from left to right. If the position of the electrodes are now interchanged such that the copper electrode is placed on the left and the zinc electrode on the right, the sign of the cell emf will change. Here, zinc electrode being on the right is treated as cathode and the copper on the left as anode. Therefore: Final reaction Zn 2+ + Cu = Cu 2+ + Zn Eo cell =-1. 1 V The emf obtained is now negative which indicates that the reaction is not spontaneous and the current flows from right to left. The left hand electrode where the current originates is, therefore, the anode, and the right-hand electrode, the cathode. The polarity of the cell is, therefore, clearly established.

Example 12:

Example 13:

HW: