JEE Mains Chemistry · The p-Block Elements

Phosphorus and Its Oxoacids

The allotropes and chlorides of phosphorus and its oxoacids, from H₃PO₂ to H₄P₂O₇, where only the P–OH hydrogens ionise and any P–H bond makes the acid a reducing agent.

All The p-Block Elements notes JEE Mains Chemistry strategy

Why this matters

Nineteen PYQs, fifteen of them multiple choice, and all from 2021 to 2023. Six test the allotropes and the reactions of white phosphorus and its chlorides; six ask for oxidation states and bond counts in the oxoacids; seven count ionisable hydrogens or pick the acid that reduces silver nitrate.

Concept 1 of 3: Allotropes of phosphorus and reactions of white phosphorus and its chlorides

White phosphorus is made of separate P₄ tetrahedra with strained 60° angles, so it is very reactive. In hot alkali its zero oxidation state splits two ways: some phosphorus goes down to −3 in phosphine and some goes up to +1 in hypophosphite. That is disproportionation. With thionyl chloride it is chlorinated to PCl₃. The chlorides then hydrolyse: every P–Cl becomes P–OH, which gives phosphorous acid from PCl₃ and phosphoric acid from PCl₅.

Definition

Allotropes: white ( $\mathrm{P_4}$ , reactive, glows in air, stored under water); red (polymeric, from heating white P at 573 K in an inert atmosphere); black.
Black phosphorus: α-black from red P heated in a sealed tube at 803 K; β-black from white P heated at 473 K under high pressure.
White P with hot concentrated NaOH: $\mathrm{P_4 + 3NaOH + 3H_2O \rightarrow PH_3 + 3NaH_2PO_2}$ .
With thionyl chloride: $\mathrm{P_4 + 8SOCl_2 \rightarrow 4PCl_3 + 4SO_2 + 2S_2Cl_2}$ .
Hydrolysis: $\mathrm{PCl_3 + 3H_2O \rightarrow H_3PO_3 + 3HCl}$ ; $\mathrm{PCl_5 + 4H_2O \rightarrow H_3PO_4 + 5HCl}$ .
With alcohols: $\mathrm{3C_2H_5OH + PCl_3 \rightarrow 3C_2H_5Cl + H_3PO_3}$ .
Red phosphorus with alkali gives hypophosphoric acid, $\mathrm{H_4P_2O_6}$ , as NCERT's oxoacid table lists it.

Key reactions of phosphorus

\mathrm{P_4 + 3NaOH + 3H_2O \rightarrow PH_3 + 3NaH_2PO_2} \qquad \mathrm{P_4 + 8SOCl_2 \rightarrow 4PCl_3 + 4SO_2 + 2S_2Cl_2} \qquad \mathrm{PCl_3 + 3H_2O \rightarrow H_3PO_3 + 3HCl} \qquad \mathrm{PCl_5 + 4H_2O \rightarrow H_3PO_4 + 5HCl}

Worked example

White phosphorus is boiled with concentrated NaOH in an inert atmosphere. Name the gas and the salt formed, and give the oxidation state of phosphorus in each.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2023 · 25 Jan 2023 · Q43Moderate

Example 1 · The p-Block Elements · Phosphorus and Its Oxoacids

Reaction of thionyl chloride with white phosphorus forms a compound [A], which on hydrolysis gives

\lbrack B\rbrack

, a dibasic acid. [A] and

\lbrack B\rbrack

are respectively

Thionyl chloride gives PCl₃, not PCl₅ or POCl₃

\mathrm{P_4 + 8SOCl_2 \rightarrow 4PCl_3 + 4SO_2 + 2S_2Cl_2}

. The by-products are sulphur dioxide and disulphur dichloride, not chlorine.

Heating red phosphorus gives α-black, not β-black

α-black phosphorus comes from red phosphorus in a sealed tube at 803 K. β-black comes from white phosphorus at 473 K under high pressure.

Drill 5 more on allotropes of phosphorus and reactions of white phosphorus and its chlorides

Concept 2 of 3: Oxoacids of phosphorus: formulas, oxidation states and bonds

Every phosphorus oxoacid is built from tetrahedral phosphorus with one P=O. The other three positions carry OH, H, an O bridge to another phosphorus, or a direct P–P bond. Read the name for the oxidation state: '-ous' acids are +1 or +3, '-ic' acids are +4 or +5, and 'pyro' means two units joined by a P–O–P bridge. Hypophosphorous acid (+1) and hypophosphoric acid (+4) sound alike but are different: the first has two P–H bonds, the second a P–P bond.

Definition

Oxidation state from the formula: H is +1, O is −2. For $\mathrm{H_4P_2O_6}$ : $4 + 2x - 12 = 0$ , so $x = +4$ .
P–O–P bridges: $\mathrm{H_4P_2O_7}$ 1, cyclic $\mathrm{(HPO_3)_3}$ 3, $\mathrm{P_4O_{10}}$ 6.
σ and π bonds: each P=O has one π bond. $\mathrm{H_4P_2O_7}$ has 12 σ and 2 π.
Most oxygen atoms in one formula: pyrophosphoric acid, $\mathrm{H_4P_2O_7}$ , with seven.

Acid	Formula	Oxidation state of P	Bonds in the structure
Hypophosphorous (phosphinic)	$\mathrm{H_3PO_2}$	+1	Two P–H, one P–OH, one P=O
Orthophosphorous (phosphonic)	$\mathrm{H_3PO_3}$	+3	One P–H, two P–OH, one P=O
Pyrophosphorous	$\mathrm{H_4P_2O_5}$	+3	Two P–H, two P–OH, two P=O, one P–O–P
Hypophosphoric	$\mathrm{H_4P_2O_6}$	+4	One P–P, four P–OH, two P=O Hypophosphoric (+4, P–P bond) is not hypophosphorous (+1, two P–H).
Orthophosphoric	$\mathrm{H_3PO_4}$	+5	Three P–OH, one P=O
Pyrophosphoric	$\mathrm{H_4P_2O_7}$	+5	Four P–OH, two P=O, one P–O–P
Cyclotrimetaphosphoric	$\mathrm{(HPO_3)_3}$	+5	A ring with three P–O–P, three P–OH, three P=O
Phosphorus(V) oxide	$\mathrm{P_4O_{10}}$	+5	Six P–O–P bridges and four P=O (the anhydride, not an acid)

Pyrophosphorous is +3 with P–H bonds; pyrophosphoric is +5 with none.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2023 · 15 Apr 2023 · Q37Moderate

Example 2 · The p-Block Elements · Phosphorus and Its Oxoacids

The number of

P - O - P

bonds in

H_{4}P_{2}O_{7}

\left( HPO_{3} \right)_{3}

and

P_{4}O_{10}

are respectively.

Hypophosphorous is +1; hypophosphoric is +4

The '-ous' acid

\mathrm{H_3PO_2}

has phosphorus at +1 with two P–H bonds. The '-ic' acid

\mathrm{H_4P_2O_6}

has phosphorus at +4 with a P–P bond.

Pyrophosphoric acid has only one P–O–P bridge

\mathrm{H_4P_2O_7}

is two

\mathrm{H_3PO_4}

units joined by losing one water, so there is exactly one P–O–P. It is the cyclic trimer

\mathrm{(HPO_3)_3}

that has three.

Drill 5 more on oxoacids of phosphorus: formulas, oxidation states and bonds

Concept 3 of 3: Basicity and reducing power of phosphorus oxoacids

Only a hydrogen on oxygen can leave as H⁺, because the O–H bond is polar. A hydrogen bonded straight to phosphorus stays put. So count P–OH groups for the basicity and ignore P–H. The P–H hydrogens do something else: they make the acid a reducing agent. An acid with a P–H bond reduces silver nitrate to a silver mirror, and on heating it disproportionates.

Definition

Basicity = number of P–OH groups. $\mathrm{H_3PO_2}$ monobasic, $\mathrm{H_3PO_3}$ dibasic, $\mathrm{H_3PO_4}$ tribasic, $\mathrm{H_4P_2O_5}$ dibasic, $\mathrm{H_4P_2O_6}$ and $\mathrm{H_4P_2O_7}$ tetrabasic.
Non-ionisable H = number of P–H bonds: 2 in $\mathrm{H_3PO_2}$ , 1 in $\mathrm{H_3PO_3}$ , 2 in $\mathrm{H_4P_2O_5}$ , none in $\mathrm{H_3PO_4}$ .
P–H makes an acid reducing: $\mathrm{4AgNO_3 + 2H_2O + H_3PO_2 \rightarrow 4Ag + 4HNO_3 + H_3PO_4}$ .
On heating: $\mathrm{4H_3PO_3 \rightarrow 3H_3PO_4 + PH_3}$ .
$\mathrm{PCl_3}$ with phosphorous acid: $\mathrm{5H_3PO_3 + PCl_3 \rightarrow 3H_4P_2O_5 + 3HCl}$ , pyrophosphorous acid.
Neutralisation uses one NaOH per P–OH: $\mathrm{H_3PO_2 + NaOH \rightarrow NaH_2PO_2 + H_2O}$ .

Counting rule for phosphorus oxoacids

\text{basicity} = \text{number of } \mathrm{P{-}OH} \text{ groups} \qquad \text{non-ionisable H} = \text{number of } \mathrm{P{-}H} \text{ bonds} \qquad \mathrm{P{-}H} \text{ present} \Rightarrow \text{reducing}

Worked example

What volume of 0.2 M NaOH exactly neutralises 25 mL of 0.1 M

\mathrm{H_3PO_3}

? What volume neutralises 25 mL of 0.1 M

\mathrm{H_3PO_2}

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2023 · 24 Jan 2023 · Q43Moderate

Example 3 · The p-Block Elements · Phosphorus and Its Oxoacids

Which of the Phosphorus oxoacid can create silver mirror from

AgNO_{3}

solution?

H₃PO₃ is dibasic, not tribasic

One of its three hydrogens is bonded to phosphorus and never ionises. Phosphorous acid gives

\mathrm{NaH_2PO_3}

and

\mathrm{Na_2HPO_3}

, never

\mathrm{Na_3PO_3}

H₃PO₂ with NaOH gives NaH₂PO₂

Hypophosphorous acid has one P–OH, so it takes one NaOH and the salt keeps both P–H hydrogens:

\mathrm{NaH_2PO_2}

. Writing

\mathrm{NaH_2PO_3}

changes the phosphorus compound.

Complete hydrolysis of PCl₃ gives H₃PO₃

Every P–Cl becomes P–OH, but one of the three ends up as P–H after rearrangement to

\mathrm{HP(O)(OH)_2}

. The product has two ionisable hydrogens and one non-ionisable.

Drill 6 more on basicity and reducing power of phosphorus oxoacids

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

Allotropes of phosphorus and reactions of white phosphorus and its chlorides
Key reactions of phosphorus
$\mathrm{P_4 + 3NaOH + 3H_2O \rightarrow PH_3 + 3NaH_2PO_2} \qquad \mathrm{P_4 + 8SOCl_2 \rightarrow 4PCl_3 + 4SO_2 + 2S_2Cl_2} \qquad \mathrm{PCl_3 + 3H_2O \rightarrow H_3PO_3 + 3HCl} \qquad \mathrm{PCl_5 + 4H_2O \rightarrow H_3PO_4 + 5HCl}$
Basicity and reducing power of phosphorus oxoacids
Counting rule for phosphorus oxoacids
$\text{basicity} = \text{number of } \mathrm{P{-}OH} \text{ groups} \qquad \text{non-ionisable H} = \text{number of } \mathrm{P{-}H} \text{ bonds} \qquad \mathrm{P{-}H} \text{ present} \Rightarrow \text{reducing}$

Reference tables (1)

Oxoacids of phosphorus: formulas, oxidation states and bonds8 rows

Acid	Formula	Oxidation state of P	Bonds in the structure
Hypophosphorous (phosphinic)	$\mathrm{H_3PO_2}$	+1	Two P–H, one P–OH, one P=O
Orthophosphorous (phosphonic)	$\mathrm{H_3PO_3}$	+3	One P–H, two P–OH, one P=O
Pyrophosphorous	$\mathrm{H_4P_2O_5}$	+3	Two P–H, two P–OH, two P=O, one P–O–P
Hypophosphoric	$\mathrm{H_4P_2O_6}$	+4	One P–P, four P–OH, two P=O Hypophosphoric (+4, P–P bond) is not hypophosphorous (+1, two P–H).
Orthophosphoric	$\mathrm{H_3PO_4}$	+5	Three P–OH, one P=O
Pyrophosphoric	$\mathrm{H_4P_2O_7}$	+5	Four P–OH, two P=O, one P–O–P
Cyclotrimetaphosphoric	$\mathrm{(HPO_3)_3}$	+5	A ring with three P–O–P, three P–OH, three P=O
Phosphorus(V) oxide	$\mathrm{P_4O_{10}}$	+5	Six P–O–P bridges and four P=O (the anhydride, not an acid)

Pyrophosphorous is +3 with P–H bonds; pyrophosphoric is +5 with none.

Watch out for (7)

Thionyl chloride gives PCl₃, not PCl₅ or POCl₃
→ Allotropes of phosphorus and reactions of white phosphorus and its chlorides
Heating red phosphorus gives α-black, not β-black
→ Allotropes of phosphorus and reactions of white phosphorus and its chlorides
Hypophosphorous is +1; hypophosphoric is +4
→ Oxoacids of phosphorus: formulas, oxidation states and bonds
Pyrophosphoric acid has only one P–O–P bridge
→ Oxoacids of phosphorus: formulas, oxidation states and bonds
H₃PO₃ is dibasic, not tribasic
→ Basicity and reducing power of phosphorus oxoacids
H₃PO₂ with NaOH gives NaH₂PO₂
→ Basicity and reducing power of phosphorus oxoacids
Complete hydrolysis of PCl₃ gives H₃PO₃
→ Basicity and reducing power of phosphorus oxoacids

Test yourself on The p-Block Elements

20 past JEE Mains questions from this chapter, timed at 48 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

Take the The p-Block Elements chapter test Drill all 19 questions