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Chemistryp-Block Elements
Which of the following statements about $H_3PO_4$ (orthophosphoric acid) are CORRECT?
A. It is a tribasic acid.
B. It is a strong acid.
C. It has three $P-OH$ bonds.
D. It has no $P=O$ bond.
Options
1
A and C only
2
A, B and C
3
A and C only (B is false, D is false)
4
B and D only
Correct Answer
A and C only
Solution
1

H₃PO₄ structure: P bonded to 3 OH groups and 1 =O group.

A. Tribasic — YES. Three ionisable P-OH hydrogens. ✓ TRUE

B. Strong acid — NO. pK₁ = 2.15, weak acid. ✗ FALSE

2

C. Three P-OH bonds — YES. ✓ TRUE

D. No P=O bond — FALSE. H₃PO₄ HAS one P=O bond. ✗ FALSE

Correct: A and C only

H₃PO₄: 3 P–OH (ionisable) + 1 P=O
Tribasic weak acid | pKa₁=2.15, pKa₂=7.20, pKa₃=12.35
Theory: p-Block Elements
1. Oxoacids of Phosphorus

Phosphorus forms several important oxoacids. The key rule: basicity = number of ionisable P-OH bonds. P-H bonds are NOT ionisable. Orthophosphoric acid H₃PO₄: 3 P-OH + 1 P=O. Tribasic. Molecular formula H₃PO₄. pKa₁=2.15, pKa₂=7.20, pKa₃=12.35. Orthophosphorous acid H₃PO₃: 2 P-OH + 1 P-H + 1 P=O. DIBASIC (not tribasic despite formula suggesting 3 H). The P-H bond is not ionisable. Hypophosphorous acid H₃PO₂: 1 P-OH + 2 P-H + 1 P=O. MONOBASIC. Strong reducing agent (two P-H bonds). Pyrophosphoric acid H₄P₂O₇: 4 P-OH + 2 P=O. Tetrabasic. Metaphosphoric acid HPO₃ (polymer): monobasic. The distinction between ionisable OH and non-ionisable P-H is THE key concept tested in NEET.

2. Structure of H₃PO₄

H₃PO₄ has tetrahedral geometry around P. Hybridisation of P: sp³. Four bonds: three P-O(-H) (single bonds to OH groups) and one P=O (double bond to oxygen). The P=O bond has partial double bond character (d-p π bonding). Molar mass: 98 g/mol. Each OH group provides one ionisable proton. First ionisation is the strongest ($K_{a1} = 7.1 \times 10^{-3}$). Second ionisation is much weaker ($K_{a2} = 6.3 \times 10^{-8}$). Third is extremely weak ($K_{a3} = 4.5 \times 10^{-13}$). The pKa values are all > 0 (unlike strong acids), confirming H₃PO₄ is a weak acid. Industrial production: P₄ + 5O₂ → P₄O₁₀; P₄O₁₀ + 6H₂O → 4H₃PO₄. Or: Ca₃(PO₄)₂ + 3H₂SO₄ → 2H₃PO₄ + 3CaSO₄ (wet process). H₃PO₄ is used in fertilisers, detergents, rust removal, food additive (cola drinks, pH control).

3. p-Block Elements — Group 15 Overview

Group 15 (VA): N, P, As, Sb, Bi. General configuration: [core]ns²np³. Three half-filled p orbitals (stable configuration, explains high ionisation energies). N: most electronegative, no d-orbitals → maximum covalency 4 (NH₄⁺). P, As, Sb: have d-orbitals → can expand octet (PCl₅, PF₆⁻). Common oxidation states: -3 (hydrides), 0, +3, +5. Nitrogen: all states from -3 to +5 in various compounds. Anomalous behaviour of N: smaller size, higher EN, no d-orbitals → pπ-pπ multiple bonding (N₂, NO, NO₂). Phosphorus: pπ-dπ bonding possible. Allotropes: white P₄ (tetrahedral, reactive, glows in dark = phosphorescence), red P (amorphous chain, less reactive), black P (layered, least reactive, electrical conductor). White P is toxic, stored under water. Density increases: white < red < black.

4. Oxides and Oxoacids of Nitrogen

Nitrogen forms more oxides than any other element: N₂O (laughing gas, +1 OS). NO (colourless, paramagnetic, +2, important in biology — vasodilator). N₂O₃ (+3). NO₂ (brown gas, +4, paramagnetic). N₂O₄ (+4). N₂O₅ (+5, anhydride of HNO₃). Oxoacids: HNO₂ (nitrous acid, weak, unstable). HNO₃ (nitric acid, strong, +5). HNO₃ properties: colourless fuming liquid, strong acid, strong oxidising agent. Dilute HNO₃ + metal → metal nitrate + water + NO (reduction to +2). Conc. HNO₃ + metal → NO₂ (reduction to +4). Conc. HNO₃ + C → CO₂. HNO₃ dissolves all metals except Au and Pt (which require aqua regia = 3HCl + HNO₃). Passivation: Fe, Cr, Al form oxide coating in conc. HNO₃ → do not dissolve. Brown ring test: Fe²⁺ + NO → [Fe(NO)]²⁺ (brown ring) — test for NO₃⁻.

5. Allotropes of Phosphorus and Their Properties

White phosphorus (P₄): tetrahedral structure with P-P single bonds (bond angle 60° — highly strained). Very reactive (spontaneously ignites in air above 35°C). Highly toxic. Stored under water. Used in incendiary bombs (white smoke = P₄O₁₀). Luminescent in dark (chemiluminescence from oxidation). Soluble in CS₂. Red phosphorus: amorphous, polymeric chains. Formed by heating white P (250°C). Less reactive. Non-toxic. Used in matchbox friction surface (mixed with glass). Does not ignite spontaneously in air. Insoluble in CS₂. Black phosphorus (orthorhombic, rhombohedral, amorphous): most stable, least reactive. Layered structure like graphite → electrical conductor. Only formed under high pressure (12,000 atm). Monobasic property summary: P₄ + 3NaOH + 3H₂O → 3NaH₂PO₂ + PH₃ (phosphine). Disproportionation reaction: P goes from 0 to -1 (in PH₃) and +1 (in NaH₂PO₂).

6. Group 16 (Oxygen Family) — Sulphur Oxoacids

H₂SO₄ (sulphuric acid): dibasic, strong. S in +6 state. Structure: tetrahedral, 2 P-OH and 2 P=O. Thiosulphuric acid H₂S₂O₃: dibasic. One S replaced by O in SO₄²⁻. Na₂S₂O₃ (hypo) is important salt. Peroxodisulphuric acid H₂S₂O₈: dibasic. S in +7 state (peroxo linkage). Strong oxidising agent. Pyrosulphuric acid (oleum) H₂S₂O₇: dibasic. Formed when SO₃ dissolves in H₂SO₄. Sulphurous acid H₂SO₃: dibasic, weak. S in +4. Important in food preservation. Disproportionation: 4H₂SO₃ → H₂S + 3H₂SO₄ (slow). Bisulphite (HSO₃⁻) and metabisulphite (S₂O₅²⁻) are important food preservatives (E221, E223). Reactions of conc. H₂SO₄: dehydrating agent (sucrose → C + H₂O), sulphonating agent (ArH + H₂SO₄ → ArSO₃H), oxidising agent (Cu + 2H₂SO₄ → CuSO₄ + SO₂ + 2H₂O).

7. Halogens — Group 17

Group 17: F, Cl, Br, I, At. Configuration: ns²np⁵. One electron short of noble gas. F: most electronegative, no d-orbitals, most reactive non-metal. Only -1 oxidation state. Forms HF (weak acid due to strong H-F bond, $K_a = 7.1 \times 10^{-4}$). HF etches glass: SiO₂ + 4HF → SiF₄ + 2H₂O. Cl: +1 (HOCl), +3 (HClO₂), +5 (HClO₃), +7 (HClO₄, perchloric acid, strongest acid). Cl₂ + H₂O → HCl + HOCl (bleaching powder). Cl₂ + 2NaOH → NaCl + NaOCl + H₂O (at room temperature). 3Cl₂ + 6NaOH → 5NaCl + NaClO₃ + 3H₂O (at 60°C). Interhalogen compounds: ClF, ClF₃, BrF₃, IF₅, IF₇. Properties intermediate between constituent halogens. Pseudohalogens: CN⁻, SCN⁻, OCN⁻ — behave like halide ions. Ag(CN): white precipitate, soluble in NH₃ — like AgCl.

8. Noble Gases and Xenon Compounds

Group 18: He, Ne, Ar, Kr, Xe, Rn. Completed outer shell → highly stable, generally unreactive. Earlier called "inert gases" — now "noble gases" because Xe and Kr form compounds. Xenon fluorides: XeF₂ (linear, sp³d), XeF₄ (square planar, sp³d²), XeF₆ (distorted octahedral, sp³d³). Xenon oxides: XeO₃ (pyramidal), XeOF₄ (square pyramidal). These are possible because Xe has d-orbitals and is large enough to accommodate F atoms. F is the only element reactive enough to oxidise Xe. XeF₂ + H₂O → Xe + HF + O (slow). XeF₄ + H₂O → Xe + O₂ + HF (slow). Discovery: Neil Bartlett (1962) synthesised O₂[PtF₆] → proposed Xe should react with PtF₆ similarly (Xe and O₂ have similar IE) → Xe[PtF₆] formed. This destroyed the "inert gas" concept. Uses: He: balloons, MRI magnets (liquid He), diving (HeO₂). Ne: neon lights. Ar: welding shielding gas, light bulbs. Kr: laser. Xe: anaesthesia, high-intensity lamps, ion propulsion.

Frequently Asked Questions
1. What is the rule for basicity of phosphorus oxoacids? ⌄
Basicity = number of ionisable P-OH groups. Only P-OH bonds contribute acidic protons; P-H bonds are NOT ionisable (the P-H bond is covalent and P is not electronegative enough to make H ionisable). Examples: H₃PO₄: 3 P-OH → tribasic. H₃PO₃: 2 P-OH + 1 P-H → dibasic. H₃PO₂: 1 P-OH + 2 P-H → monobasic. This rule works because: (a) in P-OH, oxygen draws electron density from H making it partially positive and ionisable. (b) In P-H, phosphorus is less electronegative than oxygen, so H is not acidic. Additional rule: P-H bonds make the acid a reducing agent (H can be oxidised in certain reactions). That is why H₃PO₃ and H₃PO₂ are strong reducing agents but H₃PO₄ is not.
2. Why is H₃PO₄ considered weak compared to HNO₃? ⌄
Both contain P(V) and N(V) in their respective acids, but their strengths differ greatly. HNO₃ is a strong acid (essentially 100% ionised in water, pKa₁ ≈ -1.4). H₃PO₄ is a weak acid (pKa₁ = 2.15, only ~7% ionised at 0.1 M). Reason: in HNO₃, the N=O bond and resonance delocalisation of the NO₃⁻ conjugate base greatly stabilise it → H⁺ is easily given up. In H₃PO₄, the PO₄³⁻ conjugate base is less stabilised by resonance (P uses d-orbital bonding which is less effective than N's p-p π bonding). Also: HNO₃ has only one OH group with strong electron withdrawal from the N⁺ centre and two NO bonds providing resonance. H₃PO₄ has three OH groups — after first ionisation, the remaining negative charge makes subsequent ionisations progressively harder.
3. What is the difference between orthophosphoric, metaphosphoric and pyrophosphoric acids? ⌄
Orthophosphoric acid H₃PO₄: monomer, single PO₄ unit with 3 P-OH and 1 P=O. Tribasic. Most stable, most commonly referred to as "phosphoric acid." Pyrophosphoric acid H₄P₂O₇: dimer, two PO₄ units linked by P-O-P bridge. Formed by heating 2H₃PO₄ → H₄P₂O₇ + H₂O. Tetrabasic (4 OH groups). Used in food industry, detergents. Metaphosphoric acid HPO₃: cyclic polymer (HPO₃)ₙ. Monobasic. Formed by strong heating of H₃PO₄. Very hygroscopic. Tripolyphosphoric acid H₅P₃O₁₀: trimer. Used in detergents (softens water by chelating Ca²⁺, Mg²⁺). Banned in many countries (excessive algae growth in waterways). The -meta, -ortho, -pyro prefixes in chemistry indicate increasing degree of hydration/polymerisation: meta < pyro < ortho (for acids) in terms of water content.
4. How does the structure of H₃PO₄ relate to its biological importance? ⌄
Phosphate groups (derived from H₃PO₄) are fundamental to biology: ATP (adenosine triphosphate): three phosphate groups linked by phosphoanhydride bonds. Hydrolysis of ATP → ADP + Pᵢ releases ~30 kJ/mol (free energy currency of the cell). DNA and RNA backbone: phosphodiester bonds link sugar units. Each nucleotide has a phosphate group. Phospholipids: make up all biological membranes. Phosphate head is polar (hydrophilic); fatty acid tails are hydrophobic → amphiphilic molecules → bilayer formation. Signal transduction: protein phosphorylation by kinases (add phosphate to Ser, Thr, Tyr residues) activates/deactivates enzymes. Dephosphorylation by phosphatases reverses this. Bone mineralisation: hydroxyapatite Ca₁₀(PO₄)₆(OH)₂ is the mineral component of bone and teeth. The tribasic nature and the ability of phosphate to carry -1, -2, or -3 charge at different pH values makes it an excellent biological buffer (phosphate buffer: H₂PO₄⁻/HPO₄²⁻, pKa = 7.2 — ideal for physiological pH 7.4).
5. Compare the thermal stability of group 15 hydrides? ⌄
Hydrides: NH₃, PH₃, AsH₃, SbH₃, BiH₃. Thermal stability decreases down the group: NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃. Reason: M-H bond strength decreases down the group as atomic size increases (bond length increases → bond enthalpy decreases). NH₃ is very stable (boiling point -33°C, produced in billions of tonnes by Haber process). BiH₃ is extremely unstable, decomposes at room temperature. Basic character: NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃ (lone pair availability and N is more electronegative → better base). NH₃ is a reasonable base (pKb = 4.74); PH₃ is extremely weak base (pKb ≈ 28). Boiling points: NH₃ (-33°C) > PH₃ (-87°C) → anomaly because NH₃ has intermolecular H-bonding. Among PH₃, AsH₃, SbH₃: boiling point increases with molecular mass (van der Waals forces dominate).
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