Phosphine and phosphorus-containing acids

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

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:

1. РН₃ + HI = PH₄I;

2. PH₃ + 2I₂ + 2H₂O = H(PH₂O₂) + 4HI;

3. 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.

4. 2PH₃ + 2Na = 2NaPH₂ + H₂ (the re­ac­tion is con­duct­ed at -40 ᵒC (or ᵒF) in liq­uid am­mo­ni­um).

Hy­pophos­pho­rous acid

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.

Phos­pho­rous acid

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.

Phos­phor­ic acids

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:

1. nР₄О₁₀ + 2n­H₂O = 4(HPO₃)ₙ at 0 ᵒC (32 ᵒF);

2. 4(HPO₃)ₙ + 2n­H₂O = 2n­H₄P₂O₇ at 20 ᵒC (68 ᵒF);

3. 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:

1. 3Mg + 2H₃PO₄ = Mg₃(PO₄)₂ + 3H₂;

2. 3CaO + 2H₃PO₄ = Ca₃(PO₄)₂ + 3H₂O;

3. 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.