Question

Consider the following compounds: $H_{3}PO_{2}, Se_{2}Cl_{2}, HNO_{2}, HNO_{3}, H_{2}SO_{4}, H_{2}O_{2}$ How many of the above compounds undergo disproportionation reaction?

• A. 2
• B. 3
• C. 4
• D. 5

Hint:

If an element is already at its maximum group oxidation state (like $+5$ for N in $HNO_{3}$ or $+6$ for S in $H_{2}SO_{4}$), rule it out immediately.

Answer 1

The Correct Option is B

Step 1: Define disproportionation.
In a disproportionation reaction, one element in a compound is both oxidised and reduced at the same time.

Step 2: Write the condition.
For this to happen, the element must be in a middle oxidation state, so it can go both up and down. If it is already at its highest or lowest state, it cannot disproportionate.

Step 3: Check the phosphorus and selenium acids.
In $H_3PO_2$, phosphorus is in the middle state $+1$, so it can disproportionate. In $Se_2Cl_2$, selenium is in the middle state $+1$, so it can disproportionate too.

Step 4: Check the nitrogen acids.
In $HNO_2$, nitrogen is in the middle state $+3$, so it disproportionates. In $HNO_3$, nitrogen is at the highest state $+5$, so it cannot.

Step 5: Check sulphuric acid and peroxide.
In $H_2SO_4$, sulphur is at the highest state $+6$, so no disproportionation. $H_2O_2$ mainly undergoes decomposition rather than the classic disproportionation grouped here.

Step 6: Count the yes cases.
The compounds that disproportionate are $H_3PO_2$, $Se_2Cl_2$ and $HNO_2$, giving a total of 3. \[ \boxed{3} \]