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Introduction |
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Electrochemistry is a branch of chemistry, which deals with the relationship between electrical energy and chemical changes taking place in redox reactions. i.e., how chemical energy or how electrical energy can be used to bring about a redox reaction which is otherwise not spontaneous. It has many applications in electrolysis, energy producing cell etc. |
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Conductors - Metallic and Electrolytic |
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Any substance, which allows the electric current to pass through it, is called an electrical conductor. There are two types of conductors. |
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Types of Electrolytes |
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Strong electrolytes
A strong electrolyte is one which undergoes complete ionization when dissolved in water. The solution contains only the ions and not molecules. |
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Factors Influencing Electrolytic Conduction |
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The inter-ionic attraction
It is the intersection between the ions of the solute at low concentrations. It is not much at low concentrations the inter-ionic cone is very less, but at high concentrations it is appreciable. |
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Basic Concepts |
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Ohm's law
The potential difference across the conductor is directly proportional to the current flowing through it. Solutions obey Ohm's law like metallic conductors. |
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Specific Conductance |
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It is the reciprocal of specific resistance.
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Equivalent Conductance |
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It is defined as the conductance of that volume of solution which has one equivalent dissolved in it.
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Relation between Equivalent Conductivity and Specific Conductivity |
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In 1 cm3 of the solution containing 1 gm eq. of the electrolyte is taken in the vessel. The conductance of the solution will be equal to the specific conductivity and the equivalent conductivity. |
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Variation of Molar Conductivity With Concentration |
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It is given according to the equation
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Kohlrausch's Law |
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At infinite dilution when the dissociation of the ions is complete each ion makes a definite contribution to the total molar conductance irrespective of the nature of the other ion. |
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Applications of Kohlrausch's Law |
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Calculation of molar conductivity at infinite dilution (Ao) for weak electrolytes.
Example: CH3COOH |
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Calculation of Degree of Dissociation - Degree of Dissociation [a] |
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It is defined as the fraction of the total number of molecules undergoing dissociation at a given concentration. |
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Calculation Of Dissociation Constant K |
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Degree of dissociation (a) at a particular concentration c is related to the dissociation constant (K) |
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Quantitative Aspect of Electrolysis |
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Decomposition of a compound into its constituents by passing electricity is called electrolysis. It consists of an electrolytic solution in a container. Two electrodes are dipped in it and they are connected to a battery. When the electricity is passed, the anion moves to the anode and gets oxidized. This releases electrons, these electrons pass through the outer circuit jump through the battery and these electrons are available at the cathode for reduction of cations. |
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Faraday's Laws Of Electrolysis |
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The mass of substance deposited is directly proportional to the quantity of current passed. |
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Galvanic Cells (Voltaic Cell) |
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The mass of substance deposited is directly proportional to the quantity of current passed. |
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Construction and Working of an Electrochemical Cell |
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A zinc rod is dipped in ZnSO4 solution and a Cu rod in CuSO4 solution. The Zn rod is externally connected to the copper rod through a rheostat, a galvanometer and a plug key. |
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Electrode Potential |
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Consider a zinc rod being dipped in ZnSO4 solution. The zinc atoms on the surface of the metal with in the solution have tendency to release Zn+2 into solution retaining the electrons on the surface of the metal. This process is called dissolution or solution pressure of the metal and it is oxidative in nature. |
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Metal Electrodes |
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A metal ion system in which the metal is in equilibrium with its own ions in solution is called a metal electrode. |
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Standard Electrode Potential |
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It is the potential developed when the pure metal is in contact with its ions at one molar concentration at a temperature of 25oC or 298 K. |
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Measurement of Electrode Potential |
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The absolute value of single electrode potential cannot be determined because the oxidation half reaction cannot take place without the reduction half. To overcome this difficulty the standard hydrogen electrode (SHE) is taken as the standard or reference. |
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Determination of Single Electrode Potential |
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It is not possible to measure directly the potential developed at interface of the metal gas and its ions in solution using voltmeter. Hence the electrode whose single electrode potential has to be determined is coupled with another electrode of known potential and the emf of the cell is determined by the potentiometeric method. |
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Cell Potential or EMF of a Cell |
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The difference between the electrode potentials of two half cells is known as electromotive force or cell potential or cell voltage. |
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Characteristics of Electrochemical Series |
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Reactive metals are placed on top (e.g., Li) and they have a great tendency to get oxidized. Non-reactive metals like Ag and Au. |
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Application of Electrochemical Series [ECS] |
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Higher the SRP, greater is the tendency to accept e-, higher is the tendency to get reduced and greater is the oxidizing power. Fluorine system (F2/F)- has the highest SRP and hence it possesses highest oxidising power and this increases down the group. |
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Problems |
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The electrode potentials A, B, C and D are 2.52 V, -0.16V, + 1.3V and 3.01V respectively. On the basis of this name the following:
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Standard Reduction Potentials at 298 K |
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Calculation of Cell Potential |
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In an electrochemical cell, electrons flow from negative electrode to the positive electrode. This shows that there is a potential difference between the two electrodes. |
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Nernst Equation |
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Effect of temperature and electrolytic concentration on electrode potential. |
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Nernst Equation for EMF of a Cell |
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Effect of temperature and concentration on the E.M.F. of a cell. |
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Equilibrium Constant From Nernst Equation |
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The concentration of CuSO4 decreases while that of ZnSO4 increases. As electrode potential depends upon concentration, a stage comes where both the electrode potentials are equal. Current stops flowing and the cell is said to have attained equilibrium. |
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Gibb's Free Energy and Cell Potential (EMF) |
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When a cell reaction takes place electrical energy is produced which results in decrease in the free energy of the system. |
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Predicting Products of Electrolysis from Electrode Potential |
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Molten sodium chloride
The only ions present are Na+ and Cl-
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Cells - Primary, Secondary and Fuel Cells |
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For an electrochemical cell to be used as a commercial cell it must
1. Be compact, light and rugged.
2. Voltage should not drop during use. |
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Primary Cells |
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Dry Cells
It is the compact form of the Leclanche cell.
Anode - Cylindrical Zn container.
Cathode - Central Graphite rod.
The space in between is filled with NH4Cl and ZnCl2. The graphite rod is surrounded by MnO2 and carbon. |
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Mercury Cell |
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Used in small electric devices like hearing aids and watches.
Anode - Zinc container
Cathode - Carbon rod
Electrolyte - moist HgO mixed with KOH. |
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Secondary Cell |
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Lead storage battery
Anode - Lead
Cathode - PbO2 |
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Fuel Cell |
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These are devices which convert the energy produced during the combustion of fuels like H2, CO and CH4 directly into electrical energy. The most successful fuel cell is the H2-O2 fuel cell. It was used in the Apollo space programme and the water produced used as drinking water for the astronauts. |
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Corrosion |
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The process of slowly eating away of the metal due to attack of the atmospheric gases on the surface of metals; forming oxides, carbonates, sulphides is called corrosion (e.g., tarnishing of Ag, development of a green coating on Cu and bronze etc.) |
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Manufacture of Caustic Soda (NaOH) |
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It is based on the electrolysis of aqueous solution of sodium chloride. The products of electrolysis are sodium hydroxide, chlorine gas and dihydrogen gas. |
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Manufacture of Sodium Metal |
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Sodium metal is extracted by the electrolysis of molten sodium chloride. Calculated amount of calcium chloride is added to sodium chloride to lower its melting point. |
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Manufacture of Aluminium |
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Aluminium is extracted by the electrolysis of molten alumina (Al2O3) containing cryolite (AlF3 - 3 NaF). Cryolite is added to alumina to lower its melting point and to improve its conductivity. The temperature of the electrolyte is maintained between 1200 - 1300 K. |
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Manufacture of Chlorine |
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The industrial production of chlorine is carried out by electrolysis of natural brines or concentrated aqueous solutions of NaCl. Sodium hydroxide and hydrogen are the byproducts.
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Summary |
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The main difference between metallic conductance and electrolyte conductance is that one is the movement of electrons and the other the movement of ions. This study is mainly about electrolyte conductance. |
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Conclusion |
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In this chapter we have understood the conversion of electrical energy into chemical energy and vice versa. It deals with interactions of matter and electricity. We have learnt about the various applications of electro chemistry which goes a long way in helping mankind to make his life more comfortable. |
