Potassium Permanganate, Study notes of Chemistry

Download Potassium Permanganate and more Study notes Chemistry in PDF only on Docsity! Potassium Permanganate  Permanganate ion is a strong oxidizing reagent. Half-reaction is, MnO4 - + 8H+ + 5e- ⇄ Mn2+ + 4H2O The half-reaction shown for permanganate ion occurs only in solutions that are 0.1 M or greater in strong acid. In less acidic media, the product may be Mn(III), Mn(IV), or Mn(VI), depending on conditions.  Aqueous solutions of permanganate are not entirely stable because of water oxidation: 4MnO4 - + 2H2O  4MnO2(s) + 3O2(g) + 4OH- Permanganate solutions, when properly prepared, are reasonably stable because the decomposition reaction is slow. It is catalyzed by -light, -heat, -acids, -bases, -manganese(II), -manganese dioxide.  Permanganate solutions are moderately stable provided they are free of manganese dioxide and stored in a dark container. Removal of manganese dioxide by filtration before standardization markedly improves the stability of standard permanganate solutions. Before filtration, the reagent solution is allowed to stand for about 24 hours or is heated for a brief period to hasten oxidation of the organic species generally present in small amounts in distilled and deionized water. Paper cannot be used for filtering because permanganate ion reacts with it to form additional manganese dioxide.  Permanganate solutions oxidize chloride ion and cannot be used with hydrochloric acid solutions unless special precautions are taken to prevent the slow oxidation of chloride ion that leads to overconsumption of the standard reagent. 2MnO4 - + 10Cl- + 16H+ ⇄ 2Mn2+ + 5Cl2 + 8H2O  A useful property of a potassium permanganate solution is its intense purple color, which is sufficient to serve as an indicator for most titrations.  The permanganate end point is not permanent because excess permanganate ions react slowly with the relatively large concentration of manganese(II) ions present at the end point, according to the reaction 2MnO4 - + 3Mn2+ + 2H2O ⇄ 5MnO2(s) + 4H+ The rate at which this equilibrium is approached is so slow that the end point fades only gradually over a period of perhaps 30 seconds. Standardizing Permanganate Solution  Sodium oxalate is a widely used primary standard. In acidic solutions, the oxalate ion is converted to the undissociated acid. Thus, its reaction with permanganate can be described by 2MnO4 - + 5C2O4 2- + 16H+  2Mn2+ + 10CO2(g) + 8H2O The reaction between permanganate ion and oxalic acid is complex and proceeds slowly even at elevated temperature unless manganese(II) is present as a catalyst. Therefore, when the first few milliliters of standard permanganate are added to a hot solution of oxalic acid, several seconds are required before the color of the permanganate ion disappears. As the concentration of manganese(II) builds up, however, the reaction proceeds more and more rapidly as a result of autocatalysis. Autocatalysis is a type of catalysis in which the product of a reaction catalyses the reaction. This phenomenon causes the rate of the reaction to increase as the reaction proceeds. Experimental Procedure: KMnO4 solution  4-5 mL Na2C2O4 solution + 30 mL ~3 M H2SO4 + MnSO4(s)  After the color of the first drop disappears, second drop is added.  The solution is titrated until permanent pink color is obtained for 30 seconds