Phosphine and phosphorus-containing acids
How many phosphorus-containing acids do you know?
Phosphorus is an element of the third period of the 15ᵗʰ group in the Periodic Table. It is classed in the subgroup of pnictogens, and found in several minerals, the most important of which is apatite Ca₁₀(PO₄)₆(OH,F,Cl)₂ and phosphorite.
Obtaining phosphorus and its properties
Phosphorus is usually obtained by reduction from minerals and salts:
2Ca₃(PO₄)₂ + 10C + 6SiO₂ = P₄ + 10CO + 6CaSiO₃ (sintering at 1500 ᵒC or 2732 ᵒF, white phosphorus vapors form).
Phosphorus can also be reduced from metaphosphoric acid:
4HPO₃ + 12C = 4P + 2H₂ + 12CO.
Chemical properties include the following reactions that are possible for molecular phosphorus (with metals, non-metals and oxidizers):
4P + 3O₂ = P₄O₆ (this oxide is obtained from red phosphorus);
4P + 5O₂ = P₄O₁₀;
2P + 3Cl₂ = 2PCl₃;
2P + 3Ca = Ca₃P₂ (reaction takes place with red phosphorus);
3P + 5HNO₃ + 2H₂O = 3H₃PO₄ + 5NO;
2Р + 8Н₂О = 2Н₃РО₄ + 5H₂ (red phosphorus is necessary for conducting the reaction);
Р₄ + 3KOH + 3Н₂О = РН₃ + 3KН₂РО₂ (here a disproportionation reaction takes place – some of the phosphorus is oxidized, and some of it is reduced);
12P + 10KClO₃ = 10KCl + 3P₄O₁₀ (red phosphorus is used).
Phosphorus exists in the form of several allotropic modifications, among which the most common are white, red and black types. White phosphorus has the highest reactivity, and black phosphorus is the least reactive (it is obtained from white phosphorus at high pressures). Red phosphorus is obtained by heating white phosphorus.
Phosphorus forms several oxygen-containing acids. There is also a hydrogen compound of phosphorus – the gas phosphine.
Phosphine is a compound of phosphorus and hydrogen (formula PH₃). By certain properties, it resembles ammonium, but because of its molecular structure, phosphine dissolves in water much more poorly than ammonium, and displays a lower base capacity (this can be explained by the difference in the valence angles between atoms – as it is greater in phosphine than in ammonia, phosphine does not form donor-acceptor bonds very well – the lone pair in the phosphorus atom mainly occupies the s-orbital, and so it is less accessible for reactions than in ammonia.
As for its physical properties, it is a colorless gas with an unpleasant smell.
Phosphine can be obtained by the following reaction:
4P + 3KOH + 3H₂O = PH₃ + 3KH₂PO₂.
There is also a reaction of water with phosphides:
Ca₃P₂ + 6H₂O = 3Ca(OH)₂ + 2PH₃.
The main chemical properties of phosphine are the following:
РН₃ + HI = PH₄I;
PH₃ + 2I₂ + 2H₂O = H(PH₂O₂) + 4HI;
2PH₃ + 4NaOH + 7H₂O₂ = Na₄P₂O₆↓ + 12H₂O (both the alkaline and hydrogen peroxide must be concentrated);
Click here for some experiments where you can use hydrogen peroxide.
- 2PH₃ + 2Na = 2NaPH₂ + H₂ (the reaction is conducted at -40 ᵒC (or ᵒF) in liquid ammonium).
Hypophosphorous acid H₃PO₂ is an acid in which phosphorus has the oxidation state of +1. It is a good reducer. Salts of hypophosphorous acid are called hypophosphites.
Hypophosphorous acid can be obtained by the disproportionation of white phosphorus:
2P₄ + 3Ba(OH)₂ + 6H₂O = PH₃ + 3Ba(H₂PO₂)₂ (barium hypophosphite);
Ba(H₂PO₂)₂ + H₂SO₄ = 2H₃PO₂ + BaSO₄.
The acid displays reductive properties in the following oxidation-reduction reactions:
5H₃PO₂ + 4KMnO₄ + 6H₂SO₄ = 5H₃PO₄ + 4MnSO₄ + 2K₂SO₄ + 6H₂O.
When heated above 50 ᵒC (122 ᵒF), hypophosphorous acid can decompose into phosphine and phosphorous acid:
3H₃PO₂ = PH₃ + 2H₃PO₃.
Otherwise, hypophosphorous acid displays typical properties of acid – reactions with alkalis and basic oxides with formation of salts.
In phosphorous acid H₃PO₃ the oxidation state of phosphorus is +3. Although there are three protons in the molecule, the acid is dibasic. This can be explained by its structure. A molecule of phosphorous acid has the form of a tetrahedron, at the top of which two hydroxyl groups are located, one oxygen atom and one hydrogen atom. In dissociation, the proton separates from the hydroxyl groups of OH. As there are two of them in the structure, the acid is dibasic. Crystals of this acid have no color. They dissolve well in water and alcohol.
Phosphorous acid can be obtained by hydrolysis:
PCl₃ + 3H₂O = H₃PO₃ + 3HCl.
Phosphorous acid does not have such great reductive abilities as hypophosphorous acid, but they are also pronounced in H₃PO₃:
5H₃PO₃ + 2KMnO₄ + 3H₂SO₄ = 5H₃PO₄ + 2MnSO₄ + K₂SO₄ + 3H₂O.
In heating to 200 ᵒC (392 ᵒF), the acid decomposes:
4H₃PO₃ = PH₃ + 3H₃PO₄.
The acid forms two rows of salts: phosphites and hydrophosphites (because phosphorous acid is dibasic). Both of these types of salts dissolve well in water. Phosphorous acid can react with salts of transition metals.
There are several types of phosphoric acid, the most common of which are metaphosphoric and orthophosphoric.
Metaphosphoric acid exists in the form of polymers. In a free state, because of its instability the compound is not found.
It is a strong acid. It can be obtained in the dissolution of phosphorus pentoxide:
P₄O₁₀ + 2H₂O = 4HPO₃.
This acid does not have oxidation-reduction reactivity, and displays typical acidic properties.
Orthophosphoric acid can be obtained in the heating of metaphosphoric and pyrophosphoric acids:
nР₄О₁₀ + 2nH₂O = 4(HPO₃)ₙ at 0 ᵒC (32 ᵒF);
4(HPO₃)ₙ + 2nH₂O = 2nH₄P₂O₇ at 20 ᵒC (68 ᵒF);
2nH₄P₂O₇ + 2nH₂O = 4nH₃PO₄ with boiling.
By this reaction, orthophosphoric acid can be obtained from apatite:
Ca₅(PO₄)₃F + 5H₂SO₄ + 10H₂O = 5CaSO₄·2H₂O + 3H₃PO₄ + HF.
Orthophosphoric acid is stable and not poisonous. It displays typical acidic properties:
- 3Mg + 2H₃PO₄ = Mg₃(PO₄)₂ + 3H₂;
3CaO + 2H₃PO₄ = Ca₃(PO₄)₂ + 3H₂O;
3Ca(OН)₂ + 2H₃PO₄ = Ca₃(PO₄)₂ + 6Н₂О.
Pyrophosphoric acid Н₄Р₂О₇ is a polymeric acid. It can be obtained from metaphosphoric acid:
4(HPO₃)ₙ + 2nH₂O = 2nH₄P₂O₇ at 20 ᵒC (68 ᵒF).
Phosphorus is an indispensable component in the manufacture of matches, fertilizers and cleaning agents.