Chemistry · Redox Equilibrium · NEET
Because reducing power comes from P-H bonds, and H3PO2 has two P-H bonds while H3PO4 has none. The H atom bonded directly to phosphorus is loosely held and can be released to reduce another species. H3PO4 has all its H atoms as P-OH (bonded through oxygen), so none can act this way. That is why H3PO2 reduces easily and H3PO4 does not reduce at all.
H3PO2 (hypophosphorous acid) has 2 P-H bonds. H3PO3 (phosphorous acid) has 1 P-H bond. H3PO4 (phosphoric acid) and H4P2O7 (pyrophosphoric acid) have 0 P-H bonds. The formula H3PO2 does not mean all 3 H are the same - only 1 is acidic (P-OH) and 2 are P-H. Always draw the structure to count correctly.
No. In H3PO2 only the 2 P-H hydrogens give reducing power. The third hydrogen is in a P-OH group and is the acidic (ionisable) one. So H3PO2 is a monobasic acid (1 acidic H) but a strong reducing agent (2 P-H). Do not confuse basicity (P-OH count) with reducing power (P-H count).
Oxidation state of P rises from +1 in H3PO2 to +3 in H3PO3 to +5 in H3PO4, and reducing power falls in the same direction. But for NEET the safe, direct rule is: count the P-H bonds. More P-H bonds means stronger reducing agent. The low oxidation state and the P-H count both point the same way, so counting P-H bonds is the fastest correct method.
Which of the following oxoacids of phosphorus has the strongest reducing property?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
H3PO2 > H3PO3 > H3PO4. H3PO2 has 2 P-H bonds, H3PO3 has 1 P-H bond, and H3PO4 has 0 P-H bonds. More P-H bonds means stronger reducing power.
Because H3PO4 has zero P-H bonds. All three of its hydrogens are in P-OH groups (bonded through oxygen), which are acidic but cannot act as reducing hydrogens. With no P-H bond, phosphorus is already at its highest common oxidation state (+5) and cannot easily give electrons.
H3PO2 is monobasic. It has only 1 acidic hydrogen (the P-OH group). The other 2 hydrogens are P-H bonds, which are not acidic but give it strong reducing power. So it is monobasic in acidity but a strong reducing agent.
Basicity is decided by the number of P-OH (ionisable) hydrogens. Reducing power is decided by the number of P-H hydrogens. These are two separate counts. For example H3PO2 is low in basicity (1 P-OH) but high in reducing power (2 P-H).
