How does P2O3 react with water in cold and hot states? Mention the equations involved.
The correct answer and explanation is :
Reaction of P₂O₃ (Diphosphorus Trioxide) with Water:
Diphosphorus trioxide (P₂O₃), also known as phosphorus(III) oxide or phosphorus trioxide, reacts with water to form phosphorous acid (H₃PO₃). This reaction occurs both in cold and hot water, but the nature of the product remains the same, although the reaction may proceed more vigorously with hot water due to increased kinetics.
Reaction Equation:
In both cold and hot water:
$$
\text{P}_2\text{O}_3 + 3\text{H}_2\text{O} \rightarrow 2\text{H}_3\text{PO}_3
$$
Explanation (Approx. 300 Words):
Phosphorus trioxide (P₂O₃) is a white, waxy solid that is the anhydride of phosphorous acid (H₃PO₃). It is a covalent compound formed when elemental phosphorus reacts with a limited supply of oxygen. Structurally, it consists of a P–O–P backbone and lone pairs on phosphorus atoms, which makes it reactive with nucleophiles like water.
When P₂O₃ is exposed to water, whether cold or hot, it undergoes hydrolysis. Water molecules break the P–O–P linkages and add hydroxyl groups to form phosphorous acid. The reaction does not require heating, but the rate of reaction increases with temperature due to enhanced molecular motion and collision frequency. In hot water, the reaction is faster and more complete, but the chemical equation remains unchanged.
Phosphorous acid (H₃PO₃), the product, is a dibasic acid, meaning only two of the three hydrogen atoms are ionizable (acidic). This is because it contains two –OH groups and one =O group double bonded to phosphorus. The third hydrogen atom is directly bonded to the phosphorus atom (as P–H), which does not dissociate in water under normal conditions.
This hydrolysis demonstrates the general behavior of acidic oxides, which form oxyacids upon reacting with water. In this case, P₂O₃ behaves as an acidic oxide. The reaction is important in industrial and laboratory synthesis of phosphorous acid, which is used in agrochemicals, water treatment, and as a reducing agent.
In conclusion, P₂O₃ reacts with both cold and hot water to form phosphorous acid, with the same stoichiometry, although the hot water accelerates the reaction kinetics.