[Johnny Mac]

In the standardisation of potassium permanganate solution with oxalic acid solution why is it necessary to heat the oxalic acid solution before adding the potassium permanganate?

[shelanachium]

Presumably because in a titration the reaction must be instantaneous for the titration to work, and at ambient temperature this reaction is not. So heat the oxalic acid solution first so the reaction speeds up to effectively instantaneous.

[Johnny Mac]

But in standard conditions, the reaction produces a voltage of +1.94V, would this not mean that the reaction would occur naturally without heating the solution?

[shelanachium]

Kinetic control, not thermodynamic. Many thermodynamically favourable reactions, e.g. the combustion of hydrocarbon fuels, require high temperatures for initiation. The permanganate/oxalic acid reaction requires higher than ambient temperatures to proceed at a rate sufficient for accurate titration.

Thermodynamically favourable reactions may be kinetically slow because intermediate states needed for the reaction to proceed are high in energy, often because strong bonds must first be weakened or broken. In hydrocarbon combustion strong O-O bonds in O2 and strong C-C and C-H bonds in the hydrocarbons must first break, before even stronger C-O and H-O bonds form so that there is overall release of energy.

[AWK]

This reaction need catalyst (Mn2+ ions). In its absency a wrming over 40 C is needed for start of reaction

[shelanachium]

If Mn2+ acts as a catalyst the reaction should be autocatalytic as Mn2+ is one of the products. Therefore no heat should be needed if an amount of permanganate much less than needed for complete reaction is added initially, then wait for this to decolourise. Or add Mn2+ before starting the titration.

Presumably [MnO4]- and Mn2+ react to give other Mn species - Mn(III) and/or (IV) presumably as the V and VI states are unstable in acid solution. These then presumably react with oxalic acid faster than [MnO4]-

[jm1zz]

the reaction with oxallic acid is quite slow, and kmno4 decomposes quite quickly to from MnO2
To avoid this, oxalic acid is heated, to make the reaction faster, producing Mn2+ ions