Read the passage carefully and answer the questions.

Sodium dichromate $Na_2Cr_2O_7$ and Potassium dichromate $K_2Cr_2O_7$, are strong oxidizing agents; in acidic solution, its oxidising action can be represented as follows:

$Cr_2{O_7}^{2-}+ 14H^++ 6e^-→2Cr^{3+}+ 7H_2O$ $(E° = 1.33V)$

The chromates and dichromates are interconvertible in aqueous solution depending upon pH of the solution. The chromate ion is tetrahedral whereas the dichromate ion consists of two tetrahedra sharing one corner with Cr-O-Cr bond angle of 126°

Potassium permanganate is prepared by fusion of $MnO_2$ with an alkali metal hydroxide and an oxidising agent like $KNO_3$. This produces the dark green $K_2MnO_4$ which disproportionates in a neutral or acidic solution to give permanganate. It is a strong oxidizing agent. Hydrogen ion concentration of the solution plays an important part in the reduction of permanganate to manganate, manganese dioxide and manganese(II) salt.

Potassium permanganate forms dark purple (almost black) crystals which are isostructural with those of $KClO_4$. The salt is not very soluble in water (6.4 g/100 g of water at 293 K), but when heated it decomposes at 513 K.

When $KMnO_4$ is added to Oxalic acid solution, the decolourisation is slow in the beginning but becomes faster after some time because

$Mn^{2+}$ acts as autocatalyst

The correct answer is Option (2) → $Mn^{2+}$ acts as autocatalyst

Core Concept

The reaction between $KMnO_4$ and oxalic acid in acidic medium is an example of autocatalysis. The product $Mn^{2+}$ ion formed during the reaction itself catalyzes the reaction, so initially the rate is slow, but as $Mn^{2+}$ concentration increases, the reaction speeds up.

Option-wise Explanation

1. Reaction is exothermic

Even if a reaction releases heat, it does not necessarily explain the sudden increase in rate after some time. Temperature rise is not the main controlling factor here.

2. $MnO_4^-$ catalyses the reaction

Permanganate ion is the reactant, not a catalyst. It is consumed during the reaction, so it cannot be responsible for increasing rate later.

3. $Mn^{2+}$ acts as autocatalyst

As $MnO_4^-$ is reduced, $Mn^{2+}$ ions are produced. These $Mn^{2+}$ ions catalyze the reaction between permanganate and oxalic acid. Hence the reaction rate increases after some time.

4. $CO_2$ is formed as the product

Carbon dioxide is just a product of oxidation of oxalic acid. It does not influence the reaction rate.