\(4H_3PO_3 \longrightarrow 3 H_3PO_4 + PH_3\)
What type of chemical reaction is exhibited by the above chemical reaction?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Disproportionation
In a disproportionation reaction an element in one oxidation state is oxidised and reduced at the same time, giving one product with a higher and one with a lower oxidation state. Oxoacids of phosphorus in the +3 state tend to disproportionate in this way.
Oxidation states of P, from the charge balance of each compound:
\( \ce{H3PO3} \): 3(+1) + x + 3(−2) = 0 ⇒ x = +3
\( \ce{H3PO4} \): 3(+1) + x + 4(−2) = 0 ⇒ x = +5
\( \ce{PH3} \): x + 3(+1) = 0 ⇒ x = −3
\( \displaystyle \ce{4H3PO3} \xrightarrow{\Delta} \ce{3H3PO4 + PH3} \)
Three P atoms rise from +3 to +5 (6 electrons lost) and one falls from +3 to −3 (6 electrons gained). Phosphorus is both oxidised and reduced, so the reaction is a disproportionation.
Why the other options are incorrect
Option (1) Decomposition — Incorrect. One compound does break into two here, but that name does not show the redox change; the reaction is classified by the simultaneous oxidation and reduction of phosphorus.
Option (2) Polymerization — Incorrect. Joining of many small molecules into a large one; here a molecule breaks into smaller ones.
Option (4) Isomerization — Incorrect. A change of one compound into another of the same molecular formula; \( \ce{H3PO4} \) and \( \ce{PH3} \) have different formulas.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Oxoacids of Phosphorus – “… In oxoacids phosphorus is tetrahedrally surrounded by other atoms. All these acids contain one P=O and at least one P–OH bond. The oxoacids in which phosphorus has lower oxidation state (less than +5) contain, in addition to P=O and P–OH bonds, either P–P (e.g., in H4P2O6) or P–H (e.g., in H3PO2) bonds but not both. These acids in +3 oxidation state of phosphorus tend to disproportionate to higher and lower oxidation states. For example, orthophophorous acid (or phosphorous acid) on heating disproportionates to give orthophosphoric acid (or phosphoric acid) and phosphine.”
\( \displaystyle \ce{4H3PO3 -> 3H3PO4 + PH3} \)