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
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \(Mn^{2+}\) acts as autocatalyst
In autocatalysis, one of the products of a reaction acts as a catalyst for the same reaction. The reaction is slow at first, when little of that product is present, and becomes faster as the product builds up.
Permanganate oxidises oxalic acid (in acid solution, on warming) as follows:
\( \displaystyle \ce{2MnO4^- + 5C2O4^2- + 16H^+ -> 2Mn^2+ + 10CO2 + 8H2O} \)
At the start there are no \( \ce{Mn^2+} \) ions, so the purple colour fades slowly. The \( \ce{Mn^2+} \) ions formed catalyse the reaction, and once some are present the decolourisation becomes fast. If a little \( \ce{MnSO4} \) is added at the beginning, the colour disappears quickly from the very first drops.
Why the other options are incorrect
Option (1) reaction is exothermic — Incorrect. The speeding up is seen even when the temperature is kept constant, and it disappears when \( \ce{Mn^2+} \) is added at the start, so heat is not the cause.
Option (2) \(MnO_4^-\) catalyses the reaction — Incorrect. Permanganate is a reactant; its amount falls as the reaction goes on, so it cannot make the later stage faster.
Option (4) \(CO_2\) is formed as the product — Incorrect. Carbon dioxide escapes as a gas and has no catalytic action on this reaction.
Note: The question is from beyond the NCERT textbook.