Phosphorus Oxides
Two phosphorus oxides, phosphorus(III) oxide, P4O6, and phosphorus(V) oxide, P4O10, are considered here.
Phosphorus(III) oxide: Phosphorus(III) oxide reacts with cold water to produce a solution of the weak acid, H3PO3—known as phosphorous acid, orthophosphorous acid or phosphonic acid:
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[P_4O_6 + 6H_2O rightarrow 4H_3PO_3 nonumber ]
The fully-protonated acid structure is shown below:
The protons remain associated until water is added; even then, because phosphorous acid is a weak acid, few acid molecules are deprotonated. Phosphorous acid has a pKa of 2.00, which is more acidic than common organic acids like ethanoic acid (pKa = 4.76).
Phosphorus(III) oxide is unlikely to be reacted directly with a base. In phosphorous acid, the two hydrogen atoms in the -OH groups are acidic, but the third hydrogen atom is not. Therefore, there are two possible reactions with a base like sodium hydroxide, depending on the amount of base added:
[ NaOH + H_3PO_3 rightarrow NaH_2PO_3 + H_2O nonumber ]
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[ 2NaOH + H_3PO_3 rightarrow Na_2HPO_3 + 2H_2O nonumber ]
In the first reaction, only one of the protons reacts with the hydroxide ions from the base. In the second case (using twice as much sodium hydroxide), both protons react.
If instead phosphorus(III) oxide is reacted directly with sodium hydroxide solution, the same salts are possible:
[4NaOH + P_4O_6 + 2H_2O rightarrow 4NaH_2PO_3 nonumber ]
[9NaOH + P_4O_6 rightarrow 4Na_2HPO_3 + 2H_2O nonumber ]
Phosphorus(V) oxide: Phosphorus(V) oxide reacts violently with water to give a solution containing a mixture of acids, the nature of which depends on the reaction conditions. Only one acid is commonly considered, phosphoric(V) acid, H3PO4 (also known as phosphoric acid or as orthophosphoric acid).
[P_4O_{10} + 6H_2O rightarrow 4H_3PO_4 nonumber ]
This time the fully protonated acid has the following structure:
Phosphoric(V) acid is another weak acid with a pKa of 2.15, marginally weaker than phosphorous acid. Solutions of each of these acids with concentrations around 1 mol dm-3 have a pH of about 1.
Phosphoric (V) oxide is also unlikely to be reacted directly with a base, but the hypothetical reactions are considered. In its acid form, molecule has three acidic -OH groups, which can cause a three-stage reaction with sodium hydroxide:
[ NaOH + H_3PO_4 rightarrow NaH_2PO_4 + H_2O nonumber ]
[ 2NaOH + H_3PO_4 rightarrow Na_2HPO_4 + 2H_2O nonumber ]
[ 3NaOH + H_3PO_4 rightarrow Na_3PO_4 + 3H_2O nonumber ]
Similar to phosphorus (III) oxide, if phosphorus(V) oxide reacts directly with sodium hydroxide solution, the same possible salt as in the third step (and only this salt) is formed:
[12NaOH + P_4O_{10} rightarrow 4Na_3PO_4 + 6H_2O nonumber ]
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This post was last modified on Tháng ba 16, 2024 2:15 chiều