Question

When potassium permanganate solution is added to oxalic acid solution the decolourisation is slow

Answer 1

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In titration of KMnO4 and oxalic acid, KMnO4 is an oxidizing agent and oxalic acid is a reducing agent. To initiate the reaction a little amount of acid is required. So, dil. H2SO4 is used for this purpose. But the reaction is slower at acidic pH. As soon as reaction proceeds more and more Mn2+ is formed in the solution. Mn2+ acts as an auto catalyst in this reaction. So, as the concentration of Mn2+ increases, rate of reaction becomes faster with time. The redox reaction can be written as:

5H2C2O4 + 2MnO4- + 6H+ ------> 10CO2 + 2Mn2+ + 8H2OWhen potassium permanganate solution is added to oxalic acid solution the decolourisation is slow