ELECTRO CHEMISTRY.

References: 1.Engg.Chemistry by Jain and Jain 2.Engg.Chemistry by Dr. R.V.Gadag and Dr. A.Nithyananda Shetty 3.Principles of Physical Chemistry by Puri and Sharma 4.Engg. Chemistry ( Vol I & II) by J.C. Kuriacose and J Rajaram 5.( Old question papers)

Electrochemistry is a branch of chemistry which deals with the properties and behavior of electrolytes in solution and inter-conversion of chemical and electrical energies.

An electrochemical cell can be defined as a single arrangement of two electrodes in one or two electrolytes which converts chemical energy into electrical energy or electrical energy into chemical energy. It can be classified into two types:  Galvanic Cells.  Electrolytic Cells.

Galvanic Cells: A galvanic cell is an electrochemical cell that produces electricity as a result of the spontaneous reaction occurring inside it. Eg.: Daniel cell.

Daniel Cell.

ELECTROLYTIC CELL An electrolytic cell is an electro –chemical cell in which a non- spontaneous reaction is driven by an external source of current.

Representation of galvanic cell. Anode Representation: Zn │ Zn 2+ or Zn ; Zn 2+ Zn │ ZnSO 4 (1M) or Zn ; ZnSO 4 (1M) Cathode Representation: Cu 2+ /Cu or Cu 2+ ;Cu Cu 2+ (1M) ; Cu or CuSO 4(1M) /Cu Cell Representation: Zn │ ZnSO 4 (1M) ║ CuSO 4(1M) /Cu

Emf of a cell. The difference of potential, which causes a current to flow from the electrode of higher potential to one of lower potential. E cell = E cathode - E anode The E Cell depends on:  the nature of the electrodes.  temperature.  concentration of the electrolyte solutions.

Standard emf of a cell(E o cell ) is defined as the emf of a cell when the reactants & products of the cell reaction are at unit concentration or unit activity, at 298 K and at 1 atmospheric pressure. The emf of a galvanic cell can not be measured accurately using a voltmeter

Weston Cadmium Cell Sealed wax Cork Soturated solution of CdSO 4.8/3H 2 O crystals Cd-Hg 12-14% Cd Paste of Hg 2 SO 4 Mercury, Hg

Salt Bridge. The liquid junction potential can be reduced (to about 1 to 2 mV) by joining the electrolyte compartments through a salt bridge.

Nernst Equation. It is a quantitative relationship between electrode potential and concentration of the electrolyte species. Consider a general redox reaction: M n+ (aq) + ne - → M(s) At 298K, E= E o /n log 1/[M n+ ]

Energetics of Cell Reactions. Δ G = -nFE ( in k cal) ΔH = nF[T(δ E/ δT) P –E] ΔS = nF (δE/ δT) P

Classification of Electrodes.  Gas electrode ( Hydrogen electrode).  Metal-metal insoluble salt (Calomel electrode).  Ion selective electrode.(Glass electrode).

Construction and working of hydrogen electrode.

Representation: Pt,H 2(g) / H + Electrode reaction: H + + e -  1/2 H 2(g) Applications: To determine electrode potential of other unknown electrodes. To determine the pH of a solution.

Limitations. Construction and working is difficult. Pt is susceptible for poisoning. Cannot be used in the presence of oxidising agents. Difficult to maintain the pressure as 1 atmosphere, and concentration of electrolyte as 1 M.

Metal –metal salt ion electrode. These electrodes consist of a metal and a sparingly soluble salt of the same metal dipping in a solution of a soluble salt having the same anion. Eg: Calomel electrode. Ag/AgCl electrode.

Construction.

Representation: Hg; Hg 2 Cl 2 / KCl IAs anode: 2Hg + 2Cl - → Hg 2 Cl 2 + 2e - As Cathode: Hg 2 Cl 2 + 2e - → 2Hg + 2 Cl - E= E o log [Cl - ] at 298 K Its electrode potential depends on the concentration of KCl. Conc. of Cl - Electrode potential 0.1M V 1.0 M V Saturated V

Applications: Since the electrode potential is a constant it can be used as a secondary reference electrode. To determine electrode potential of other unknown electrodes. To determine the pH of a solution.

Ion Selective Electrode. It is sensitive to a specific ion present in an electrolyte. The potential of this depends upon the activity of this ion in the electrolyte. Magnitude of potential of this electrode is an indicator of the activity of the specific ion in the electrolyte. *This type of electrode is called indicator electrode.

Glass Electrode:`

The overall potential of the glass electrode has three components:  The boundary potential E b  Internal reference electrode potential E ref.  Asymetric potential E asy. E g = E b + E ref. + E asy. Glass electrode is mainly used in the determination of pH of a solution.

CONCENTRATION CELLS. Two electrodes of the same metal are in contact with solutions of different concentrations. Emf arises due to the difference in concentrations. Cell Representation: M/ M n+ [C1] ║ M n+ /M [C2]

Construction.

Anode - electrode with lower electrolyte concentration. Cathode – electrode with higher electrolyte concentration. Higher the ratio [C 2 /C 1 ] higher is the emf. Emf becomes zero when [C 1 ] = [C 2 ].

At anode: Zn →Zn 2+ (C1) + 2e - At cathode: Zn 2+ (C2) + 2e - → Zn E cell = E C -E A = E 0 + (2.303RT/ nF)logC 2 - [E 0 +(2.303RT/nF)logC 1 ] E cell = (0.0592/n) log C 2 /C 1 E cell is positive only if C 2 > C 1