Phosphorus, phosphine, halides and oxoacids

A match head and a glowing element

Phosphorus was found by a man who boiled urine to look for gold. He got a white solid that glowed in the dark. The glow comes from a slow reaction with the oxygen of the air. The element is never found free in nature. It is in the mineral calcium phosphate, $\text{Ca}_{3}(\text{PO}_{4})_{2}$, in bones and in fertilisers.

Allotropes of phosphorus

Allotropes are different forms of the same element in the same state. Phosphorus has three main ones.

• White phosphorus is made of $\text{P}_{4}$ molecules, each a tetrahedron. It is very reactive and catches fire in air. It dissolves in carbon disulphide. It is highly poisonous and is stored under water.
• Red phosphorus is made of long chains of $\text{P}_{4}$ units joined together, with no strained tetrahedron. It is much less reactive. It does not dissolve in carbon disulphide, it is not poisonous, and it is kept in air.
• Black phosphorus has puckered layers of linked atoms. It is the least reactive form, does not dissolve and is not poisonous.

White phosphorus is reactive because the 60° angle in the $\text{P}_{4}$ tetrahedron is far from the natural angle of a p-orbital bond (90°). The bonds are strained, and they break easily. Red phosphorus is made by heating white phosphorus at about 573 K in an inert atmosphere. Black phosphorus forms when white phosphorus is heated at about 473 K under high pressure. It is a semiconductor. Red phosphorus is used in the striking strip of a safety match box.

Phosphine, PH3

Phosphine is made in the laboratory from white phosphorus and a hot, concentrated solution of sodium hydroxide, in an atmosphere of carbon dioxide or nitrogen. The air must be kept out because phosphine catches fire.

$\text{P}_{4}\ +\ 3\text{NaOH}\ +\ 3\text{H}_{2}\text{O}\ \rightarrow \ \text{PH}_{3}\ +\ 3\text{NaH}_{2}\text{PO}_{2}$

The product also forms when calcium phosphide meets water or dilute acid: $\text{Ca}_{3}\text{P}_{2}\ +\ 6\text{H}_{2}\text{O}\ \rightarrow \ 3\text{Ca}(\text{OH})_{2}\ +\ 2\text{PH}_{3}$.

Properties. Phosphine is a colourless gas with the smell of rotting fish. It is highly poisonous. It is only slightly soluble in water. Its bond angle is about 93.6°, smaller than that of ammonia, because phosphorus is less electronegative and the bonding pairs are held farther from it. It is a much weaker base than ammonia, and it forms phosphonium salts with the strong acids: $\text{PH}_{3}\ +\ \text{HBr}\ \rightarrow \ \text{PH}_{4}\text{Br}$. It reacts with copper(II) sulphate: $3\text{CuSO}_{4}\ +\ 2\text{PH}_{3}\ \rightarrow \ \text{Cu}_{3}\text{P}_{2}\ +\ 3\text{H}_{2}\text{SO}_{4}$. Pure phosphine does not catch fire by itself, but traces of the compound $\text{P}_{2}\text{H}_{4}$ make it burn spontaneously in air: $\text{PH}_{3}\ +\ 2\text{O}_{2}\ \rightarrow \ \text{H}_{3}\text{PO}_{4}$.

The chlorides

Phosphorus trichloride, PCl3. White phosphorus burns in a limited supply of chlorine: $\text{P}_{4}\ +\ 6\text{Cl}_{2}\ \rightarrow \ 4\text{PCl}_{3}$. It can also be made with thionyl chloride: $\text{P}_{4}\ +\ 8\text{SOCl}_{2}\ \rightarrow \ 4\text{PCl}_{3}\ +\ 4\text{SO}_{2}\ +\ 2\text{S}_{2}\text{Cl}_{2}$. It is a colourless fuming liquid. It is hydrolysed by water, and reacts with alcohols and acids that have an OH group:

$\text{PCl}_{3}\ +\ 3\text{H}_{2}\text{O}\ \rightarrow \ \text{H}_{3}\text{PO}_{3}\ +\ 3\text{HCl}$

$3\text{C}_{2}\text{H}_{5}\text{OH}\ +\ \text{PCl}_{3}\ \rightarrow \ 3\text{C}_{2}\text{H}_{5}\text{Cl}\ +\ \text{H}_{3}\text{PO}_{3}$

$3\text{CH}_{3}\text{COOH}\ +\ \text{PCl}_{3}\ \rightarrow \ 3\text{CH}_{3}\text{COCl}\ +\ \text{H}_{3}\text{PO}_{3}$

The molecule is a pyramid, as $\text{NH}_{3}$ is, with a lone pair on phosphorus.

Phosphorus pentachloride, PCl5. It forms when white phosphorus burns in an excess of chlorine: $\text{P}_{4}\ +\ 10\text{Cl}_{2}\ \rightarrow \ 4\text{PCl}_{5}$. Or the trichloride takes up chlorine: $\text{PCl}_{3}\ +\ \text{Cl}_{2}\ \rightleftharpoons \ \text{PCl}_{5}$. It is a yellowish white solid. In the gas it is a trigonal bipyramid, with three equatorial bonds and two longer axial bonds. In the solid it is ionic, $[\text{PCl}_{4}]^{+}[\text{PCl}_{6}]^{-}$. The Indian syllabus gives the bond lengths 202 pm (equatorial) and 219 pm (axial). It hydrolyses in two steps:

$\text{PCl}_{5}\ +\ \text{H}_{2}\text{O}\ \rightarrow \ \text{POCl}_{3}\ +\ 2\text{HCl}$

$\text{POCl}_{3}\ +\ 3\text{H}_{2}\text{O}\ \rightarrow \ \text{H}_{3}\text{PO}_{4}\ +\ 3\text{HCl}$

It also replaces OH groups by Cl, as in $\text{C}_{2}\text{H}_{5}\text{OH}\ +\ \text{PCl}_{5}\ \rightarrow \ \text{C}_{2}\text{H}_{5}\text{Cl}\ +\ \text{POCl}_{3}\ +\ \text{HCl}$, and takes chlorine from metals: $2\text{Ag}\ +\ \text{PCl}_{5}\ \rightarrow \ 2\text{AgCl}\ +\ \text{PCl}_{3}$.

The oxoacids

An oxoacid has the central atom bonded to oxygen atoms, some of them in OH groups. Phosphorus has several. Let us read three of them from the picture.

• $\text{H}_{3}\text{PO}_{2}$, hypophosphorous acid: phosphorus is $+1$, with 1 P-OH group and 2 P-H bonds. Basicity 1.
• $\text{H}_{3}\text{PO}_{3}$, phosphorous acid: phosphorus is $+3$, with 2 P-OH groups and 1 P-H bond. Basicity 2.
• $\text{H}_{3}\text{PO}_{4}$, phosphoric acid: phosphorus is $+5$, with 3 P-OH groups and no P-H bond. Basicity 3.

In each acid, phosphorus has one P=O bond. The other three bonds are P-OH or P-H. Basicity is the number of hydrogen atoms that can be replaced by a metal. Only the hydrogen of a P-OH group can be replaced. The hydrogen bonded to phosphorus is not acidic. That is why $\text{H}_{3}\text{PO}_{3}$, with three hydrogen atoms, is only dibasic, and $\text{H}_{3}\text{PO}_{2}$ is monobasic.

The P-H bond is a sign of a reducing agent. Hypophosphorous acid reduces silver nitrate to silver: $4\text{AgNO}_{3}\ +\ 2\text{H}_{2}\text{O}\ +\ \text{H}_{3}\text{PO}_{2}\ \rightarrow \ 4\text{Ag}\ +\ 4\text{HNO}_{3}\ +\ \text{H}_{3}\text{PO}_{4}$. On heating, phosphorous acid undergoes disproportionation, which is the reaction where one substance is oxidised and reduced at the same time: $4\text{H}_{3}\text{PO}_{3}\ \rightarrow \ 3\text{H}_{3}\text{PO}_{4}\ +\ \text{PH}_{3}$. Phosphorus goes from +3 to +5 and to $-3$.

Two phosphoric acid molecules can lose a molecule of water and join through a P-O-P bridge. They give pyrophosphoric acid, $\text{H}_{4}\text{P}_{2}\text{O}_{7}$, which has four P-OH groups and is tetrabasic. Cyclic metaphosphoric acid, $(\text{HPO}_{3})_{3}$, has three P-OH groups and three P-O-P bridges. Orthophosphoric acid is used in fertilisers, in soft drinks, in detergents and in rust removers.

Set 2 P-OH groups and 10 mmol of acid. Phosphorous acid needs 20 mmol of sodium hydroxide. Now move to 3 groups. The oxidation state goes to +5, and the base needed goes up to 30 mmol. The oxidation state is the same as $2n - 1$ for $n$ P-OH groups, as long as the other bonds of the three-hydrogen acid are P-H.

Practise with new numbers

Try it in Python

The program counts the groups on the phosphorus atom of each acid. From the numbers of hydrogen and oxygen atoms it finds the oxidation state of phosphorus, and it prints the basicity.

Common mistakes

• Counting all hydrogen atoms as acidic. $\text{H}_{3}\text{PO}_{3}$ is dibasic and $\text{H}_{3}\text{PO}_{2}$ is monobasic.
• Mixing up the allotropes. White phosphorus is the reactive and poisonous one. Red phosphorus is the stable one.
• Forgetting the conditions for phosphine. It is made in an inert atmosphere, because it burns in air.
• Calling $\text{PCl}_{5}$ always ionic. In the gas it is molecular and bipyramidal. In the solid it is $[\text{PCl}_{4}]^{+}[\text{PCl}_{6}]^{-}$.

Remember these

• White P: $\text{P}_{4}$ tetrahedra, 60° angle, reactive, stored under water. Red P: chains, stable. Black P: layers, semiconducting.
• $\text{PH}_{3}$: from white P and NaOH or from $\text{Ca}_{3}\text{P}_{2}$ and water, rotten-fish smell, angle 93.6°, weak base.
• $\text{PCl}_{3}$ (pyramid) and $\text{PCl}_{5}$ (trigonal bipyramid) are both hydrolysed. $\text{PCl}_{5}$ replaces OH by Cl.
• Basicity is the number of P-OH groups: $\text{H}_{3}\text{PO}_{2}$ 1, $\text{H}_{3}\text{PO}_{3}$ 2, $\text{H}_{3}\text{PO}_{4}$ 3.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why is white phosphorus more reactive than red phosphorus?

2. How many moles of NaOH neutralise 0.50 mol of $\text{H}_{3}\text{PO}_{3}$?

3. What are the products when $\text{PCl}_{5}$ reacts with an excess of water?

4. What is disproportionation? Give the example for phosphorous acid.

References

• NCERT, Chemistry Part I for Class XII, the chapter The p-Block Elements, on phosphorus, phosphine, the halides and the oxoacids.
• OpenStax, Chemistry 2e, Chapter 18, on phosphorus and its compounds.