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Corrosion Of Iron Corrosion is the process in which a metal is destructed as a result of its reaction with environment. Corrosion of iron is known as rusting. Rusting is the hydrated ferric oxide. Other examples for corrosion are tarnishing of silver and development of green coating on copper and bronze. In corrosion metals undergo anodic oxidation to metal oxides. Electrochemical theory of rusting The rusting of iron is an electrochemical process involving the following steps. The moister containing CO2 acts as electrolyte. H 2 O + CO 2 ------> H 2 CO 3 H 2 CO 3 ------> 2H+ + CO 3 2- The iron is oxidized by the removal of electrons and acts as the anode. Fe ------> Fe 2+ + 2electron The H+ ions from the electrolyte accept electrons from the adjacent areas on metal surface and function as cathode. 4H + + 4electron ------> 2H 2 The atmospheric oxygen moves hydrogen as water. 2H 2 + O 2 ------> 2H 2 O Adding the above two equations, 4H+ +O 2 + 4electron ------> ...

Determination of λ0M (limiting molar conductivity) for weak electrolytes It is not possible to determine the λ 0 M for a weak electrolyte by the extrapolation of λ M versus √c plot. But it can be calculated from Kohlrausch’s law. Consider the weak electrolyte CH3COOH. According to Kohlraush’s law, λ 0 CH3COOH = λ 0 H+ + λ 0 CH3COO- ………………………………… (Equation 1) We can experimentally determine the λ 0 M of strong electrolytes such as HCl,CH3COONa and NaCl. From Kohlraush’s law, λ 0 HCl = λ 0 H+ + λ 0 Cl- ……………………………………. (Equation 2) λ 0 CH3COONa = λ 0 Na+ + λ 0 CH3COO- ………………………………… (Equation 3) λ 0 NaCl = λ 0 Na+ + λ 0 Cl- ……………………………………. (Equation 4) Substracting equation 4 from the sum of equations 2 and 3, we get, λ 0 HCl + λ 0 CH3COONa - λ 0 NaCl = λ 0 H+ + λ 0 Cl- + λ 0 CH3COO- + λ 0 Na+ - λ 0 Na+ - λ 0 Cl- = λ 0 H+ + λ 0 CH3COO- = λ 0 CH3COOH Thus by measuring the molar conductance values of NaCl, HCl and CH3COONa, one can easily determi...

Kohlrausch's law of indendent migration of ions states that the molar conductance of infinite dilution (limiting molar conductivity ) is the sum of the individual contributions of the anions and cations of the electrolyte. It can be given in the mathematical form as: λ 0 M = v + λ 0 + + v - λ 0 - λ 0 + and λ 0 - are the limiting molar conductivity of cation and anion respectively. For NaCl, λ 0 M = λ 0 Na+ + λ 0 Cl- And for Al2(SO4)3, λ 0 Al2(SO4)3 =2 λ 0 Al3+ + λ 0 SO4 2-

The energy producing cells are of two types - primary cells and secondary cells. 1) Primary cells In a primary cell the reaction occurs only once and it becomes dead after a period of time and hence cannot be used again. Dry cell, mercury cell etc are examples of primary cells. Dry cell The anode is zinc vessel and the cathode rod. It is surrounded by powdered Mno2. The space between the electrode is filled with a paste of NH4Cl and ZnCl2, the electrolyte. The potential is 1.5 V Zn ------> Zn2+ + 2 electron (anode) NH4+ + MnO2 + electron ------> MnO(OH) + NH3 (cathode) It is of our common experience that dry cells do not have a long life. It is because the acidic ammonium chloride corrodes the zinc container even if the acidic ammonium chloride corrodes the zinc container even if the cell is not in use. In the leak proof cells, the zinc vessel is protected by an outer steel covering. In these cells recharging is not possible. 2) Secondary cells These are cells which can be...