Oxyacids of p-block Elements

• Write formulas, structures, and oxidation states for oxyacids of N, P, S, and Cl
• Apply the rule: more oxygen atoms on the central atom → stronger acid
• Identify disproportionation reactions of H₃PO₃ and HNO₂ and balance them
• Count the number of P–OH, P=O, and P–H bonds in phosphorus oxyacids to determine basicity and whether they are reducing agents
• Arrange oxyacids of each element in order of acid strength and oxidising power
• Draw accurate 2D structures of the key oxyacids tested in JEE

Level 1 — Foundations

What is an Oxyacid?

An oxyacid (oxoacid) contains oxygen, hydrogen, and at least one other element. The acidic H atoms are bonded to oxygen (as O–H), NOT directly to the central atom. Any H bonded directly to the central atom (like P–H) is non-acidic.

General rule for acid strength:

For oxyacids of the same element: more O atoms attached to the central atom → more electron withdrawal from O–H → easier H⁺ release → stronger acid.

For oxyacids of different elements with the same number of O atoms, the more electronegative central atom makes the stronger acid.

Oxyacids of Nitrogen

HNO₂ (Nitrous acid):

• Unstable in pure form; exists in aqueous solution only.
• Structure: H–O–N=O (one O–H and one N=O).
• Weak acid (Ka = 4.5 × 10⁻⁴).
• Both oxidising agent (towards strong reducing agents) and reducing agent (towards strong oxidising agents) — disproportionation occurs.

HNO₃ (Nitric acid):

• Structure: planar molecule with one O–H and two N=O (resonance).
• N is at +5 — cannot be oxidised further → powerful oxidising agent only.
• Conc. HNO₃ + conc. HCl (3:1) = aqua regia (dissolves Au, Pt).

Oxyacids of Phosphorus

Key rule for phosphorus oxyacids: count P–OH groups for basicity, and check for P–H bonds to determine if the acid is a reducing agent.

Critical point: H₃PO₂ has the formula suggesting 3 acidic H, but only 1 H is on O–H (acidic). The other 2 H are P–H (non-acidic). Hence basicity = 1 (monobasic). Same logic for H₃PO₃: basicity = 2 (dibasic).

P–H bond → reducing agent: the P–H bond can be oxidised, so H₃PO₂ and H₃PO₃ are reducing agents; H₃PO₄ has no P–H bond → not a reducing agent.

Oxyacids of Sulfur

Oxyacids of Chlorine

Acid strength: HClO₄ > HClO₃ > HClO₂ > HOCl (more O → stronger) Oxidising power: HOCl > HClO₂ > HClO₃ > HClO₄ (lower OS of Cl → more tendency to gain e⁻)

Level 2 — JEE Depth

Structures of Key Oxyacids

H₃PO₄ (Orthophosphoric acid):

• Tetrahedral around P
• 3 P–OH groups (3 acidic H) + 1 P=O
• Basicity = 3 (tribasic)
• NOT a reducing agent (no P–H bonds)
• Structure: one central P with 3 –OH and 1 =O attached

H₃PO₃ (Phosphorous acid):

• Tetrahedral around P
• 2 P–OH groups + 1 P=O + 1 P–H
• Basicity = 2 (dibasic) — misleading formula!
• Reducing agent (due to P–H bond)
• Structure: P bonded to 2 –OH, 1 =O, and 1 H directly

H₃PO₂ (Hypophosphorous acid):

• Tetrahedral around P
• 1 P–OH group + 1 P=O + 2 P–H
• Basicity = 1 (monobasic) — very misleading formula!
• Strong reducing agent (2 P–H bonds)

H₂SO₄:

• Tetrahedral around S (sp³)
• 2 S–OH + 2 S=O (or 2 S–O with dative bonds in Lewis model)
• Basicity = 2
• Conc. H₂SO₄: oxidising, dehydrating; dilute H₂SO₄: strong acid only

H₂SO₃:

• Pyramidal (lone pair on S)
• 2 S–OH + 1 S=O (approximately)
• Unstable; SO₂ dissolved in water
• Reducing agent (S at +4, can go to +6)

HNO₃:

• Planar, resonance structure
• N–OH + two N–O (one double, one with negative charge in resonance)
• All bond angles ≈ 120° (sp² N)

HNO₂:

• Planar
• N–OH + N=O
• Non-linear (lone pair on N contributes to geometry)
• Unstable — decomposes / disproportionates

HOCl (Hypochlorous acid):

• Angular molecule: H–O–Cl
• Cl at +1; weakest acid of chlorine oxoacids but strongest oxidiser

HClO₄ (Perchloric acid):

• Tetrahedral around Cl
• 1 Cl–OH + 3 Cl=O
• Strongest known acid (pKa ≈ −10); Cl at +7 has maximum O atoms around it

Disproportionation Reactions

A disproportionation reaction is one where the same element is simultaneously oxidised AND reduced.

H₃PO₃ disproportionation (on heating):

4H₃PO₃ → 3H₃PO₄ + PH₃

• P in H₃PO₃: +3
• P in H₃PO₄: +5 (oxidised)
• P in PH₃: −3 (reduced)
• Trigger: heat (above ~200 °C)
• JEE Trap: This is why H₃PO₃ is a reducing agent — it readily gives up electrons.

HNO₂ disproportionation:

3HNO₂ → HNO₃ + 2NO↑ + H₂O

• N in HNO₂: +3
• N in HNO₃: +5 (oxidised)
• N in NO: +2 (reduced)
• HNO₂ is unstable in solution and undergoes this reaction slowly at room temperature, faster on heating.

Cl₂ in water / base disproportionation:

Cl₂ + H₂O ⇌ HOCl + HCl (in water, Cl: 0 → +1 and −1) Cl₂ + 2NaOH → NaCl + NaOCl + H₂O (cold, dilute) 3Cl₂ + 6NaOH → 5NaCl + NaClO₃ + 3H₂O (hot, conc. — Cl: 0 → −1 and +5)

Acid Strength: Comparative Table

For acids with the same central atom, count non-OH oxygen atoms (=O groups):

For P oxyacids, the trend is not dramatic in JEE, but H₃PO₄ > H₃PO₃ > H₃PO₂ for acid strength because more –OH groups = slightly more dissociation overall (secondary factors apply).

Peroxoacids — Key Point

H₂S₂O₈ (peroxodisulphuric acid) and H₂SO₅ (Caro's acid) contain O–O peroxide linkage:

• S is at formal +7 (higher than +6 in H₂SO₄).
• These are the strongest oxidising acids of sulphur.
• The O–O bond breaks homolytically → free radical oxidising power.

Example 1: Determine the basicity and reducing nature of H₃PO₃ from its structure.

H₃PO₃ structure:
- Central atom: P
- Count bonds:
    1 P=O (terminal double bond to O)
    2 P–OH (two –OH groups, each contributing 1 acidic H)
    1 P–H (one H directly bonded to P)

Basicity:
- Only H atoms on O–H groups are acidic (ionisable).
- P–H hydrogen is NON-acidic (cannot donate as H⁺ in acid-base sense).
- Number of P–OH groups = 2 → Basicity = 2 (dibasic acid).

Reducing agent?
- P–H bond is present → P can be oxidised (P goes from +3 to +5).
- H₃PO₃ IS a reducing agent.
- It can reduce AgNO₃ to Ag (silver mirror), and CuSO₄ to Cu.

Common error: Saying H₃PO₃ is tribasic (3 H in formula).
Fix: Count P–OH bonds, NOT total H atoms in formula.

Example 2: Balance the disproportionation of HNO₂ to HNO₃ and NO.

Unbalanced: HNO2 → HNO3 + NO + H2O

Step 1: Identify oxidation state changes.
- N in HNO2: +3
- N in HNO3: +5 → change = +2 (oxidation, 2 electrons lost)
- N in NO: +2 → change = −1 (reduction, 1 electron gained)

Step 2: Balance electrons (LCM of 2 and 1 = 2).
- 1 HNO2 → 1 HNO3 (oxidation, loses 2e⁻)
- 2 HNO2 → 2 NO (reduction, each gains 1e⁻, total 2e⁻ gained)
- Total HNO2 consumed: 1 + 2 = 3

Step 3: Balanced equation:
3HNO2 → HNO3 + 2NO↑ + H2O

Check:
- N: 3 left, 1 + 2 = 3 right ✓
- H: 3 left, 1 + 0 + 2 = 3 right ✓
- O: 6 left, 3 + 2 + 1 = 6 right ✓

• Q1: Write the structural formula of H₃PO₂ and state its basicity. Does it act as a reducing agent?
• Q2: Arrange H₂SO₃, H₂SO₄, H₂S₂O₈ in order of increasing oxidising power. Justify.
• Q3: What happens when H₃PO₃ is heated? Write the balanced equation and identify the type of reaction.
• Q4: Among HNO₂ and HNO₃, which is a stronger acid? Which is a stronger oxidising agent? Explain both answers.
• Stretch: Q5: H₂S₂O₈ (peroxodisulphuric acid) contains sulfur at an oxidation state higher than in H₂SO₄. Explain how this is possible structurally (hint: draw the O–O peroxide bond), and why this makes it the strongest oxidising sulphur acid.

NCERT Chapter 7 link: Class 12 Chemistry — The p-Block Elements (oxyacids sections across Groups 15, 16, 17; pages 118–121 for N, 142–147 for P, 157–162 for S, 176–183 for Cl oxyacids)

Exam connections: JEE Main tests basicity of H₃PO₃/H₃PO₂, acid strength order of chlorine oxyacids, and the disproportionation of HNO₂. JEE Advanced integrates all four elements in a single MCQ asking for oxidation states, structures, and relative properties — a single comparison table in your notes handles all variants.

Study strategy: Build one master table with columns: acid formula / element / OS of central atom / basicity / reducing agent? / acid strength rank / oxidising power rank. Fill it for all oxyacids in one sitting. This table is the answer to 90% of JEE oxyacid questions.

• Use the platform-native live simulation or PhET-style tool for this topic: use a virtual molecular builder to draw H₃PO₄, H₃PO₃, and H₃PO₂ side by side — count P–OH and P–H bonds in each and verify basicity.
• Mirror / body / home activity: use lemon juice (citric acid), vinegar (acetic acid), and a baking soda indicator solution to rank acid strengths at home — then draw the parallel to how more electron-withdrawing groups increase acid strength. Photograph and annotate for portfolio.
• Voice or text reflection with AI Mentor: explain to a younger student why H₃PO₃ is dibasic and not tribasic, using the analogy of "only doors that open (O–H groups) let H⁺ out."

• "Explain to a Class 8 student why not all H atoms in an acid formula are acidic, using the example of H₃PO₃ and an everyday analogy (like doors that open vs. doors that are sealed shut)."
• "What is one common mistake students make when comparing oxidising power vs. acid strength for oxyacids of chlorine, and how would you catch yourself making it?"
• Stretch: "Peroxodisulphuric acid (H₂S₂O₈) is used in PCB manufacturing (etching copper) and textile bleaching. How does the O–O peroxide bond make it such a powerful oxidising agent? How might this connect to a career in chemical engineering or electronics manufacturing?"

• Complete the core practice + one extension activity (master comparison table, structural drawings of all oxyacids, or a home acid-strength experiment) for base XP + Oxyacids Master badge.
• 5-7 day streak or family discussion note (e.g., explaining why lemon juice and baking soda react using acid-base chemistry) = multiplier + visible artifact in parent/principal dashboard.
• Best real-world application stories (anonymised) featured on class or national leaderboard.

• Hands-on project: measure the pH of common household acids (lemon juice, vinegar, cola, baking soda solution) using a digital pH meter or pH paper from a Drishti kit — correlate with acid strength concepts from this note. Plot a bar chart of pH values.
• Future Skill track: Green Tech / Sustainable Living — research how H₂SO₄ (acid rain) forms from SO₂ emissions and how oxyacids of nitrogen (HNO₃ from NOₓ) contribute to smog and acid rain. Propose one engineering intervention.
• Coding extension: write a Python function that takes an oxyacid formula string as input, counts P–OH and P–H groups from a predefined dictionary, and outputs basicity and whether the acid is a reducing agent.

This material emphasises experiential "learning by doing", competency (apply/create/analyse), vocational exposure, critical thinking, and multidisciplinary connections. Designed to feed live worlds, AI Mentor (with memory), gamification, robotics, parent analytics, and future skills — not just exam prep.

Portfolio Evidence Idea: Your photo/table/reflection/project + one sentence on "How this helps me in real life or a possible future path." (Example: "Understanding oxyacids connects to environmental chemistry, water treatment, and industrial chemical processing — careers I could pursue in Green Tech or chemical engineering.")

Open the Practice tab for aligned questions (easy/medium/hard + case-based) with full AI scaffolding.

See curriculum for cross-links and the full future-skills/robotics chapters.