Question

In addition to the small amount of $KMn{O_4}$ to conc.${H_2}S{O_4}$, a green oily compound is obtained which is highly explosive in nature. Identify the compound from the following
a.) $M{n_2}{O_7}$
b.) $Mn{O_2}$
c.) $MnS{O_4}$
d.) $M{n_2}{O_3}$

Answer 1

Hint: Try to recall that $KMn{O_4}$ is an oxidizing agent and when a small amount of $KMn{O_4}$ is added to conc.${H_2}S{O_4}$, then ${H_2}S{O_4}$ will dehydrate $KMn{O_4}$ and a green oily compound is obtained and the oxidation state of manganese remains unchanged in this reaction. Now, by using this you can easily answer the given question.

Complete step by step solution:
-It is known to you that on addition of small amount of potassium permanganate ($KMn{O_4}$) to concentrated sulphuric acid, dehydration of potassium permanganate will take place and a green oily compound is obtained.
-The oxidation state of manganese (Mn) in green oily compound is same as that of manganese (Mn) in $KMn{O_4}$ and the oxidation state of manganese in $KMn{O_4}$ is +7.
-Now, by calculating the oxidation state of manganese in each of the compounds given in options, you can find the correct answer:
$M{n_2}{O_7}$ : Let oxidation state of manganese be x,
$2x + 7 \times ( - 2) = 0$
or, x = + 7
$Mn{O_2}$ : Let oxidation state of manganese be x,
$x + 2 \times ( - 2) = 0$
or, x = + 4
$MnS{O_4}$: Let oxidation state of manganese be x,
$x + ( - 2) = 0$
or, x = + 2
$M{n_2}{O_3}$: Let oxidation state of manganese be x,
$2x + 3 \times ( - 2) = 0$
or, x = + 3
So, from above it is clear that the green oily compound which is obtained by the action of conc.${H_2}S{O_4}$ on $KMn{O_4}$ is $M{n_2}{O_7}$ and it is highly explosive in nature. The reaction is:
${H_2}S{O_4} + 2KMn{O_4} \to {K_2}S{O_4} + M{n_2}{O_7} + {H_2}O$.
Therefore, from above we can easily conclude that option A is the correct option for the given question.

Note: It should be remembered to you that solution of potassium permanganate is used for decolourisation of oils because of its strong oxidising power.
Also, you should remember that alkaline potassium permanganate is used in organic chemistry under the name Baeyer’s reagent.