Electrochemistry

Electrochemistry I Electrochemistry is the branch of chemistry that deals with the production of electricity from the energy released during spontaneous chemical reaction. Electrochemical changes take place in two types of cells: Electrochemical cell (in this type of cell, chemical energy of a spontaneous redox reaction is converted into electrical energy. These are also known as Voltaic cell or Galvanic cell and Electrolytic cell (In this type of cell, chemical changes take place on passing an electric current.) Daniel cell is a special type of a galvanic cell in which the anode is made up of zinc and the cathode is made of copper. The reaction which takes place at anode is called as anodic (or oxidation) half cell reaction and the one which takes place at cathode is known as cathodic (or reduction) half cell reaction. The tendency of an electrode to lose or gain electrons is known as electrode potential. It depends upon the concentration of ions and temperature. If the electrode is placed in 1 molar concentration of its own ions and the temperature is maintained at 298 K, then the electrode potential is called standard electrode potential. The representation of complete formulation of a cell is also known as cell notation. Vertical arrangement of the various electrodes in the decreasing order of their standard reduction potentials is known as electrochemical series. Electrochemical series can be used to determine relative oxidizing and reducing capacity. It can be used to calculate electrode potential and to predict the direction of the reaction. Nernst equation is a mathematical relationship between the concentration of the electrolyte and the electrode potential. It can be used to determine cell potential and equilibrium constant. In an electrochemical cell, the electrical work done is equal to the electrical energy produced which can be written as a product of quantity of electricity and electromotive force. Thus, the electrical work done is equal to the product of moles of electrons transferred and quantity of electricity flowing. Electrical work done is equal to the decrease in the free energy. Electrochemistry II Electrochemistry is the branch of chemistry which deals with the relationship between electrical energy and chemical changes taking place in the redox reactions. Based on the electrical conductivity, substances are classified as conductors and insulators. Substances that do not allow the electric current to pass through them are known as insulators and those which allow the electric current to pass through them are called conductors. Substances in which flow of the electric current is due to the flow of free electrons in the metallic lattice are known as electronic conductors. Substances in which flow of electric current through an electrolytic solution is due to the movement of ions are known as electrolytic conductors. Electrolytic conduction is affected by interionic interaction, solvation of ions, viscosity of the solvent, temperature and concentration of the solution. Ability of the electrolytes to conduct electric current is termed as the conductance or conductivity. The conducting potential of all the ions produced by dissolving one mole of an electrolyte in V cm3 of the solution is known as molar conductivity. The conducting potential of all the ions produced by dissolving one gram equivalent of an electrolyte in the solution is known as equivalent conductivity. Conductance of an electrolytic solution is the reciprocal of its resistance. Resistance can be measured by the principle of Wheatstone bridge method. On the basis of the above observations, Kohlrausch enunciated Kohlrausch Law, which states that molar conductivity at infinite dilution of an electrolyte can be represented as the sum of the individual contributions of the cation and anion of the electrolyte. Limiting Ionic Conductivity is the contribution made by an ion (cation or anion) towards the equivalent conductance of the electrolyte at infinite dilution. Molar conductivity at infinite dilution for weak electrolytes can be obtained by using Kohlrausch law. The process of decomposition of an electrolyte when the electric current is passed through its aqueous or molten state is known as electyrolysis. Michael Faraday gave quantitative relationship between the amount of substance liberated at electrode and the amount of current passed through the electrolyte. He put forward two laws: First Law of Electrolysis (The amount of substance deposited or liberated at any electrode, is directly proportional to the quantity of electricity passed through the electrolyte) and Second Law of Electrolysis (When the same quantity of current is passed through different electrolytes, which are connected in series, the amount of substances liberated at respective electrodes is directly proportional to their equivalent weights). Batteries and corrosion The corrosion of metals and functioning of a battery have one thing in common. Both of them involve electrochemical changes, i.e., chemical reactions that involve oxidation and reduction. Devices that convert chemical energy into electrical energy are known as batteries. Batteries are classified as primary batteries, secondary batteries and fuel cells. Primary batteries are non-rechargeable. In primary battery reaction occurs only once and the cell becomes dead after sometime and cannot be reused again. Secondary batteries are rechargeable and can be used again because the redox reaction occurring in the cell is reversible. Cells in which the energy is produced from the combustion of fuels, like H2, CO, CH4, etc. are known as fuel cells. The energy produced is then converted into electrical energy. Corrosion is a process of deterioration of a metal as a result of its reaction with air or water surrounding it. The most common example of corrosion is the rusting of iron. Corrosion is an electrochemical phenomenon. In corrosion, a metal is oxidised by the loss of electrons and leads to the formation of oxides. Corrosion can be prevented by barrier protection, sacrificial protection, cathodic protection etc.

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