Question

Write the chemical equations for the preparation of KMnO$_4$ from MnO$_2$. Why does purple colour of acidified permanganate solution decolourise when it oxidised Fe$^{2+}$ to Fe$^{3+}$ ?

Answer 1

Hint: The chemical reaction can be written by the oxidation of manganese dioxide. The formation of potassium permanganate will occur in two steps. The colour change of permanganate solution is also based on the concept of oxidising, and the reducing agent. So, we can explain it.

Complete step by step answer:
-Now, first we will write the chemical reaction of formation of potassium permanganate from the manganese dioxide.
-As mentioned it will be formed in two steps. So, in the first step we have the reaction of manganese dioxide with the potassium hydroxide, in the presence of oxygen.We know that manganese dioxide is also known as pyrolusite.Now, the first chemical reaction can be written as:
2MnO$_2$ + 4KOH + O$_2$ $\xrightarrow{\Delta}$ 2K$_2$MnO$_4$ + 2H$_2$O
Here, we can see the products obtained are potassium manganate, and the water.

Now, the second step of reaction will have the disproportionation of potassium manganate in the presence of acidic solution.The chemical reaction can be written as:
3K$_2$MnO$_4$ $\xrightarrow{4HCl}$ 2KMnO$_4$ + MnO$_2$+ 2H$_2$O + 4KCl
Here, we can see the formation of potassium permanganate, and the by-products are water, and potassium chloride.

Now, the next we have the decolourising of potassium permanganate due to the Fe ions.As we know that Fe$^{2+}$ acts as a reducing agent because it reduces the manganese ion. It is colourless.So, when it oxidises from Fe$^{2+}$ to Fe$^{3+}$, then there is decolourisation of acidified potassium permanganate solution.

 In the last, we can conclude that the oxidation reaction takes while the formation of potassium permanganate, and the reason for decolourisation is the action of ferrous ions as a reducing agent.

Note: Don’t get confused between the ferrous, and the manganese ion. So, here ferrous ions reduce the manganese ion it means the oxidation number is being decreased. The oxidising of ferrous ions means the oxidation number is being increased.