SSS 3: PHOSPHORUS AND ITS COMPOUNDS

Phosphorus is a non-metallic element belonging to Group 15 and Period 3 of the periodic table. It is highly reactive and is never found in the free uncombined state in nature.

Atomic Properties

Chemical symbol: P

Atomic number: 15

Relative atomic mass: 31

Electronic configuration:

1s² 2s² 2p⁶ 3s² 3p³

Oxidation states: −3, +3 and +5

Occurrence

Phosphorus occurs naturally as phosphate rocks or phosphorite, primarily as calcium trioxophosphate(V), [Ca₃(PO₄)₂], and apatite, [Ca₅(PO₄)₃F].

It is also an essential component of living things, found mainly in bones and teeth as Ca₃(PO₄)₂. DNA, RNA and ATP contain phosphorus.

Allotropes of Phosphorus

Phosphorus exhibits allotropy, i.e. the existence of an element in two or more different physical forms in the same physical state.

The three major allotropes of phosphorus are:

1. White phosphorus

2. Red phosphorus

3. Black phosphorus

White Phosphorus

1. White phosphorus is a tetra-atomic molecule (P₄).

2. Each phosphorus atom undergoes covalent bonding to produce tetrahedral molecules.

3. It is a white, waxy, soft and translucent solid.

4. It is highly unstable and extremely reactive.

5. It is insoluble in water but soluble in organic solvents such as CS₂.

6. It is very poisonous and toxic.

7. It can catch fire or ignite in air at about 35°C.

8. It is stored under water.

9. It gives a greenish glow in the dark due to slow oxidation. This phenomenon is called phosphorescence.

Red Phosphorus

1. Red phosphorus is formed when white phosphorus is heated to about 250°C in the absence of air.

White phosphorus —250°C, 1/s catalyst→ Red phosphorus

2. It is an opaque red solid.

3. It has a higher density than white phosphorus.

4. It can sublime at about 400°C.

5. It is insoluble in water and organic solvents.

6. It is non-poisonous.

7. It does not ignite spontaneously unlike white phosphorus.

8. It does not glow in the dark.

9. It is stable at room temperature and less reactive.

Branching to produce tetrahedral molecules

[Diagram of the tetrahedral structure of phosphorus]

Hydrides/Compounds of Phosphorus

Phosphorus forms two hydrides, namely:

1. Phosphorus(III) hydride (PH₃)

2. Diphosphane (P₂H₄)

Phosphorus(III) hydride (PH₃) and ammonia (NH₃) are analogous.

1. Phosphine (PH₃)

Phosphine is a colourless and highly toxic gas with a foul fishy odour. It is prepared by the action of an alkali on metallic phosphides.

Ca₃P₂(s) + 6H₂O(l) → 3Ca(OH)₂(aq) + 2PH₃(g)

2. Phosphorus Oxides

Phosphorus(V) Oxide, P₄O₁₀

Phosphorus(V) oxide, P₄O₁₀, is a white powder with a powerful affinity for water. It acts as an excellent dehydrating agent in laboratory settings.

When dissolved in cold water, it forms metaphosphoric acid.

In hot water, it forms tetraoxophosphate(V) acid.

P₄O₁₀(s) + 6H₂O(l) → 4H₃PO₄(aq)

Tetraoxophosphate(V) Acid (H₃PO₄)

It is commonly called orthophosphoric acid.

It is a tribasic acid, meaning it ionises in three steps to yield three hydrogen ions, forming three sets of salts.

Chemical Properties of Phosphorus

1. Combustion (Reaction with Air)

Phosphorus burns vigorously in oxygen to form oxides.

a. In limited supply of O₂:

P₄(s) + 3O₂(g) → P₄O₆(s)

(Phosphorus(III) oxide)

b. In excess oxygen:

P₄(s) + 5O₂(g) → P₄O₁₀(s)

(Phosphorus(V) oxide)

2. Reaction with Halogens

Phosphorus reacts directly with halogens, e.g. chlorine.

a. With limited chlorine:

P₄(s) + 6Cl₂(g) → 4PCl₃(l)

(Phosphorus trichloride)

b. With excess chlorine:

P₄(s) + 10Cl₂(g) → 4PCl₅(s)

(Phosphorus pentachloride)

3. Reaction with Metals

When heated with active metals, phosphorus forms phosphides.

3Ca(s) + 2P(s) → Ca₃P₂(s)

(Calcium phosphide)

4. Reaction with Alkali

Only white phosphorus reacts with hot concentrated alkali to yield phosphine gas.

P₄ + 3NaOH + 3H₂O → PH₃ + 3NaH₂PO₂

5. As a Reducing Agent

Phosphorus is a reducing agent. It readily reacts with oxidising agents to form phosphorus oxides, which can dissolve in water to form acids.

P + 5HNO₃ → H₃PO₄ + H₂O + 5NO₂

3. Trioxophosphate(III) Acid (H₃PO₃)

H₃PO₃ is the most important acid based on phosphorus(III) oxide. It can be prepared from the oxide by the action of cold water or by hydrolysis of phosphorus(III) chloride.

P₄O₆ + 6H₂O → 4H₃PO₃

PCl₃ + 3H₂O → H₃PO₃ + 3HCl

Pure H₃PO₃ is a deliquescent solid. It dissolves readily in water to give a moderately strong acid solution.

It is a strong reducing agent. It can decompose to form PH₃, a powerful reducing agent.

4H₃PO₃ —200°C→ PH₃ + 3H₃PO₄

Tetraoxophosphate(V) Acid (H₃PO₄)

It is commonly called orthophosphoric acid.

It is a tribasic acid. It ionises in three steps by yielding three hydrogen ions, forming three sets of salts:

1. Dihydrogen phosphate — NaH₂PO₄

2. Monohydrogen phosphate — Na₂HPO₄

3. Normal phosphate — Na₃PO₄

Uses of Phosphorus

1. Production of fertilisers.

2. Red phosphorus is used in making matches.

3. It is used in making lamps.

4. White phosphorus serves as an incendiary agent in military forces and smoke bombs.

5. It causes eutrophication.