A student performs two separate redox titrations using acidified KMnO ₄ as the titrant. For Reductant A, the decolorization of the first few drops is very slow at room temperature, requiring the flask to be heated to about 60^ C , after which the reaction speeds up. For Reductant B, the decolorization is instantaneous at room temperature, and the lab manual explicitly warns against heating to prevent aerial oxidation

Step-by-step solution

The titration of oxalic acid with KMnO ₄ is slow at room temperature due to high activation energy, so it requires initial heating to about 60^ C . Thus, Reductant A is oxalic acid. Once the reaction begins, the Mn ²⁺ ions formed act as an auto-catalyst, speeding up the subsequent decolorization. The titration of Mohr's salt (FAS) with KMnO ₄ is fast at room temperature. Heating is avoided because the active species, Fe ²⁺ , can undergo aerial oxidation to Fe ³⁺ at elevated temperatures, which introduces an error i