When `KMnO_4` solution is added to oxalic acid solution, the decolourisation is slow in the beginning but becomes instantaneous after some time because

Options

A. `CO_(2)` is formed as the product
B. Reaction is exothermic .
C. `MnO_(4)^(-)`
D. `Mn^(2+)` acts as autocatalyst.

Explanation

To understand why the decolorization of potassium permanganate (KMnO₄) solution in oxalic acid is slow initially but becomes instantaneous after some time, we can break down the explanation into several steps:

### Step 1: Identify the Reactants
When potassium permanganate (KMnO₄) is added to oxalic acid (H₂C₂O₄), a redox reaction occurs where KMnO₄ acts as an oxidizing agent.

**Hint:** Remember that KMnO₄ is a strong oxidizing agent and will react with reducing agents like oxalic acid.

### Step 2: Understand the Initial Reaction
Initially, the reaction is slow because KMnO₄ (which contains Mn in the +7 oxidation state) needs to be reduced to Mn²⁺ (which is in the +2 oxidation state). This reduction process takes time as it involves the transfer of electrons from oxalic acid to KMnO₄.

**Hint:** Consider the oxidation states of manganese in KMnO₄ and the product formed.

### Step 3: Formation of Mn²⁺
As the reaction proceeds, MnO₄⁻ is reduced to Mn²⁺. At the beginning, there is a limited amount of Mn²⁺ formed, which means the reaction rate is slow.

**Hint:** Think about how the concentration of products affects the rate of reaction.

### Step 4: Autocatalysis Effect
Once a sufficient amount of Mn²⁺ is produced, it starts to act as a catalyst for the reaction. This is known as autocatalysis, where one of the products of the reaction (Mn²⁺) catalyzes the reaction itself, speeding it up significantly.

**Hint:** Recall the definition of autocatalysis and how products can influence the rate of their own formation.

### Step 5: Instantaneous Reaction
As more Mn²⁺ is formed, the reaction rate increases dramatically, leading to a rapid decolorization of the solution. This is why after an initial slow phase, the decolorization becomes instantaneous.

**Hint:** Consider how the presence of a catalyst can change the kinetics of a reaction.

### Conclusion
The decolorization of KMnO₄ in oxalic acid solution starts slowly due to the initial formation of Mn²⁺, but once enough Mn²⁺ is generated, it acts as an autocatalyst, causing the reaction to speed up and become instantaneous.

**Final Answer:** The correct explanation for the observed behavior is that Mn²⁺ acts as an autocatalyst in the reaction.