Electrochem

• Electrochemistry is the branch of chemistry that deals with the study of chemical changes that occur when passing electric current into certain chemical systems and the generation of electricity by carrying out chemical reactions
• Electrochemical Cell is a device used to transform the chemical energy of a spontaneous reaction into electrical energy or to bring about a non-spontaneous chemical change using electrical energy from an external source
• Oxidation is when a species loses one or more electrons, while Reduction is when a species gains one or more electrons
• Galvanic cells generate current, while electrolytic cells require current
• Specific resistance of a conductor is the resistance of a conductor of unit length and unit area of cross-section
• Cell constant (K) is a function of the electrode areas, the distance between the electrodes, and the electrical field pattern between the electrodes
• Molar conductivity of a solution at a given concentration is the conductance of the volume of solution containing one mole of electrolyte kept between two electrodes with the unit area of cross-section and distance of unit length
• Specific conductivity (conductivity) is the measure of the ability of a material to conduct electricity
• An Electrochemical cell consists of two electrodes known as half cells, and the difference between the potential of the two half cells constitutes the electromotive force (emf) or cell potential (Ecell)
• Electrode Potential is the measure of the tendency of a metallic electrode to lose or gain electrons when in contact with a solution of its own salt
• Helmholtz Double Layer (HDL) is formed when the metal passes into the solution and leaves the electrons on the surface of the electrodes, creating a layer of positive charge around the electrode
• Standard Electrode Potential:

  • It is the potential when a metal is dipped in 1M solution of its ions or when an inert electrode is in contact with a gas at a pressure of 1 atmosphere at 298 K
• Nernst Equation for Cell Potential:

  • Ecell = Ecathode - Eanode
  • Ecell = Ecell + 0.059 log [species at cathode] [species at anode]
• Measurement of Standard Electrode Potential:

  • Measured by using Standard Hydrogen Electrode (SHE)
  • Potential of SHE is taken as zero at all temperatures
  • Electrode potential increases with an increase in the concentration of Mn+ ions and an increase in temperature
• Electrochemical Series:

  • Metal ion-Metal electrodes are arranged in the decreasing order of their reduction potential
  • Electrochemical series provides information on relative ease of oxidation and reduction, replacement tendency, predicting spontaneity of redox reactions, calculation of equilibrium constant, predicting the product of electrolysis, and predicting liberation of H2 gas from acids by metals
• Concentration Cells:

  • Electrochemical cell where the two electrodes are the same material
  • Electrolytes on the two half-cells involve the same ions, but the electrolyte concentration differs between the two half-cells
• Types of Single Electrodes:

  • Metal ion-Metal electrode
  • Gas Electrode
  • Metal-Metal insoluble salt electrode
  • Ion-Selective electrode
• Reference Electrodes:

  • Whose potentials are known and are used for the determination of the potential of other electrodes
  • Primary Reference electrode: e.g., Standard Hydrogen Electrode (SHE)
  • Secondary Reference electrode: e.g., Calomel electrode, Ag/AgCl electrode
• Standard Hydrogen Electrode:

  • System with hydrogen ion and gaseous hydrogen in standard states
  • Potential is arbitrarily taken as zero at all temperatures
  • Consists of a platinum electrode with a hydrogen ion concentration of 1.00M submerged in a solution
• Calomel Electrode:

  • Consists of mercury, solid mercurous chloride paste, and saturated KCl solution
  • Half-cell reaction: Hg2Cl2 + 2e- -> 2Hg + 2Cl-
  • Advantages include simplicity in construction, constant and reproducible cell potential, and stability
• Ion Selective Electrodes:

  • Selectively sensitive to certain ions and develop a potential proportional to the concentration of the ions
  • Contains a reference electrode and a membrane sensitive to particular chemical species
• Glass Electrode:

  • Type of ion-selective electrode made of a doped glass membrane sensitive to a specific ion
  • Commonly used for pH measurement
  • Principle involves a thin-walled glass bulb immersed in a solution with H+ ions
• Characteristics of a good membrane:

  • Low electronic conductivity
  • Low solubility in the analyte solution
  • High selectivity
  • High permeability
  • Stability and excellent chemical durability
• Limitations of SHE:

  • Difficulty in setting up electrodes
  • Inactivity in the presence of impure gases in hydrogen
• The glass electrode potential is given by E_G = E_b + E_Ag/AgCl + E_asy
• E_b is a constant that depends on the pH of the solutions taken inside the glass bulb
• c2 = [H+] of the solution, -log [H+] = pH, therefore E_b = L - 0.0591 pH
• The glass electrode is dipped in the test solution, the Na+ ions of glass membrane are exchanged for H+ ions of the test solution
• The potential of the cell, E_cell, is measured using a voltmeter connected to a potentiometer or pH meter
• E_cell = E_G - E_SCE = L1 - 0.0591 pH - E_SCE
• pH = K - E_cell / 0.0591, where K = L1 - E_SCE