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Group VA and VIA Elements eLearn.Punjab

Properties of Phosphorus Acid
It is a white crystalline solid, which melts at 73.6°C.

Reactions
1. It decomposes into phosphine and orthophophoric acid on heating.

4H3PO3 (s) → 3H3PO4 (l) + PH3 (g)

2. It is a powerful reducing agent and reduces CuSO4, AgNO3, etc. to the
metallic state.

H3PO3 (s) + CuSO4 (aq) + H2O(l) → H3PO4 (aq) + H2SO4 (aq) + Cu(s)

H3PO3(s) + 2AgNO3(aq) + 2NH4OH (aq) → H3PO4 (aq) + 2NH4 NO3(aq) + H2O(l) + 2 Ag(s)

3. It reacts with oxygen to form orthophosphoric acid.

2H3PO3 (s) + O2 (g) → 2H3PO4 (s)

4. Nascent hydrogen produced by Zn/HCl reduces H3PO3 to phosphine

H3PO3(s) + 6[H](g) → PH3(g) + 3H2O(l)

2. Orthophosphoric Acid (H3PO4)
Preparation
1. It is prepared by dissolving phosphorus pentoxide in hot water.

P2O5 (s) + 3H2O(l) → 2H3PO4 (aq)

2. It is also obtained by heating red phosphorus with concentrated HNO3.

P(s) + 5HNO3(conc.) → H3PO4 (aq) + 5NO2 (g) + H2O(l)

3. Hydrolysis of phosphorus pentachloride also gives orthophosphoric acid.

PC15 (s) + 4H2O(l) → H3PO4 (aq) + 5HC1(aq)

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