Oxoacids of p-Block Elements

Real-Life Connection: From Stomach Acid to Battery Power

Oxoacids are everywhere! Nitric acid etches circuit boards and makes fertilizers. Sulfuric acid powers car batteries and refines petroleum. Phosphoric acid flavors your cola and prevents rust. Chloric acid bleaches paper pulp. These acids - containing oxygen bonded to a central atom - are industrial workhorses and essential for modern chemistry. Understanding their structures, strengths, and reactions is crucial for JEE!

What Are Oxoacids?

Definition: Acids containing hydrogen, oxygen, and another element (central atom), where acidic hydrogen is attached to oxygen.

General form: HₙXOₘ (where X = central atom)

Key features:

• Acidic H is bonded to O (as OH group)
• Central atom bonded to O (as X=O or X-O-H)
• H directly bonded to central atom (X-H) is NOT acidic!

Examples:

• H₂SO₄ (sulfuric acid): S is central atom
• HNO₃ (nitric acid): N is central atom
• H₃PO₄ (phosphoric acid): P is central atom
• HClO₄ (perchloric acid): Cl is central atom

Memory Trick: “OXO = Oxygen eXtra, Only OH acidic”

General Trends in Oxoacid Strength

Rule 1: More oxygen atoms → Stronger acid

For same central atom:

• More O atoms withdraw electrons from O-H bond
• Weaker O-H bond → easier to release H⁺
• Conjugate base more stable (resonance delocalization)

Example (Chlorine oxoacids): HOCl < HClO₂ < HClO₃ < HClO₄ (+1) (+3) (+5) (+7) ← Oxidation state of Cl

Memory Trick: “More O = More Oxy-power = More Acidic”

Rule 2: Higher oxidation state → Stronger acid

Related to Rule 1 (more O usually means higher oxidation state)

Example (Halogen oxoacids):

• HOCl (Cl +1): pKₐ ≈ 7.5
• HClO₂ (Cl +3): pKₐ ≈ 2
• HClO₃ (Cl +5): pKₐ ≈ -1
• HClO₄ (Cl +7): pKₐ ≈ -10 (strongest)

Rule 3: Smaller central atom → Stronger acid (same group)

For same number of O atoms:

• Smaller atom = higher electronegativity
• More electron withdrawal from O-H
• Easier H⁺ release

Example (Group 17): HOCl > HOBr > HOI

Example (Group 16): H₂SO₄ > H₂SeO₄

Rule 4: Basicity depends on ionizable H (in OH groups only!)

Important: Only H in OH groups can ionize!

• P-H bonds do NOT release H⁺
• Count OH groups, not total H atoms

Example:

• H₃PO₄: 3 OH groups → Tribasic
• H₃PO₃: 2 OH groups (1 P-H) → Dibasic
• H₃PO₂: 1 OH group (2 P-H) → Monobasic

Memory Trick: “Count OH not H” - Only hydroxyl H is acidic

Interactive Demo: Visualize Oxoacid Molecular Structures

See the 3D structures of various oxoacids and how oxygen atoms arrange around the central atom.

Oxoacids of Halogens

General Pattern

For Cl, Br, I (not F - only shows -1 oxidation state):

Naming pattern:

• Lowest (+1): Hypo____ous acid
• Middle (+3): ____ous acid
• Middle (+5): ____ic acid
• Highest (+7): Per____ic acid

Memory Trick: “Hypo Per Ate Icecream” - Hypo(+1) → ous → ic → Per(+7)

Chlorine Oxoacids (Detailed)

1. Hypochlorous Acid (HOCl)

Oxidation state: Cl = +1

Preparation:

Cl₂ + H₂O ⇌ HCl + HOCl (disproportionation)
2Cl₂ + 2HgO → HgO·HgCl₂ + HOCl (Cassel's method)

Structure:

H-O-Cl (bent, like H₂O)

Properties:

• Weak acid (pKₐ = 7.5)
• Unstable (exists only in solution)
• Good oxidizing agent
• Bleaching agent

Reactions:

1. Disproportionation:

3HOCl → HClO₃ + 2HCl

2. Oxidation:

HOCl + 2H⁺ + 2e⁻ → Cl⁻ + H₂O (E° = +1.49 V)

Uses: Water disinfection, bleaching

2. Chlorous Acid (HClO₂)

Oxidation state: Cl = +3

Properties:

• Weak acid (pKₐ ≈ 2)
• Unstable (more stable than HOCl)
• Exists only in solution
• Oxidizing agent

Structure:

    O
    ‖
H-O-Cl

Disproportionation:

4HClO₂ → 2HClO₃ + HClO + HCl

3. Chloric Acid (HClO₃)

Oxidation state: Cl = +5

Preparation:

3Cl₂ + 6NaOH (hot) → 5NaCl + NaClO₃ + 3H₂O
NaClO₃ + H₂SO₄ → NaHSO₄ + HClO₃

Structure:

    O
    ‖
H-O-Cl=O

Properties:

• Strong acid (pKₐ ≈ -1)
• Unstable (decomposes on heating)
• Strong oxidizing agent
• Colorless solution

Decomposition:

8HClO₃ → 4HClO₄ + 2Cl₂ + 3O₂ + 2H₂O

Uses: Chlorate salts for matches, explosives

4. Perchloric Acid (HClO₄)

Oxidation state: Cl = +7

Preparation:

NaClO₄ + H₂SO₄ → NaHSO₄ + HClO₄

Structure:

    O
    ‖
    Cl=O
   / \
  O   O-H

(Tetrahedral around Cl)

Properties:

• Strongest common acid (pKₐ ≈ -10, stronger than H₂SO₄!)
• Colorless, oily liquid
• Hygroscopic
• Powerful oxidizing agent (hot, concentrated)
• Relatively stable (can be distilled)

Why is HClO₄ so strong?

• 4 O atoms withdraw electrons from O-H
• ClO₄⁻ extremely stable (4 equivalent resonance structures)
• Perfect tetrahedral symmetry

Reactions:

1. Complete ionization:

HClO₄ + H₂O → H₃O⁺ + ClO₄⁻ (100% ionized in water)

2. Oxidizing (hot, conc.):

4HClO₄ + P₄ → 4H₃PO₄ + 2Cl₂

Uses: Rocket fuel, analytical chemistry

Comparison Table: Chlorine Oxoacids

Important Trend:

• Acidic strength: HOCl < HClO₂ < HClO₃ < HClO₄
• Oxidizing power: HOCl > HClO₂ > HClO₃ > HClO₄
• Stability: HOCl < HClO₂ < HClO₃ < HClO₄

Memory Trick: “ASO = Acid Strength: Oxygen increases; Oxidizing: Opposite trend; Stability: Same as acid”

Bromine and Iodine Oxoacids

Bromine: Similar to chlorine (HOBr, HBrO₃, HBrO₄)

• Less stable than chlorine oxoacids
• Similar trends

Iodine: Special case

Iodic Acid (HIO₃)

Preparation:

I₂ + 6HNO₃(conc) → 2HIO₃ + 6NO₂ + 2H₂O
I₂ + 5Cl₂ + 6H₂O → 2HIO₃ + 10HCl
3I₂ + 10HNO₃(dil) → 6HIO₃ + 10NO + 2H₂O

Properties:

• White crystalline solid
• Moderately strong acid
• Oxidizing agent

Structure: Similar to HClO₃

Periodic Acid (HIO₄ or H₅IO₆)

Two forms:

1. Metaperiodic acid: HIO₄ (paraperiodic dehydrated)
2. Paraperiodic acid: H₅IO₆ or (HO)₅IO

Preparation:

I₂ + 5Cl₂ + 6H₂O → 2HIO₃ + 10HCl
HIO₃ + Cl₂ + H₂O → HIO₄ + 2HCl

Structure of H₅IO₆:

• Octahedral around I
• 5 OH groups + 1 I=O
• Can lose up to 5 H⁺ (pentabasic!)

Properties:

• White crystalline solid
• Weak acid
• Strong oxidizing agent
• Used in organic chemistry (cleaves 1,2-diols)

Reaction with 1,2-diols:

R-CH(OH)-CH(OH)-R' + HIO₄ → R-CHO + R'-CHO + HIO₃ + H₂O
(Used to determine structure)

Oxoacids of Nitrogen

Hyponitrous Acid (H₂N₂O₂)

Oxidation state: N = +1

Structure:

HO-N=N-OH (trans form, more stable)

Properties:

• Weak acid
• Unstable
• Rarely encountered

Nitrous Acid (HNO₂)

Oxidation state: N = +3

Preparation:

NaNO₂ + HCl → NaCl + HNO₂ (in cold)
NO + NO₂ + H₂O → 2HNO₂

Structure:

    O
    ‖
H-O-N (planar)

Properties:

• Weak acid (pKₐ = 3.3)
• Pale blue solution
• Unstable (exists only in solution and cold)
• Both oxidizing and reducing agent

Disproportionation:

3HNO₂ → HNO₃ + 2NO + H₂O

Reactions:

1. As oxidizing agent:

2HNO₂ + 2HI → I₂ + 2NO + 2H₂O

2. As reducing agent:

2HNO₂ + 2KMnO₄ + 3H₂SO₄ → 2HNO₃ + 2MnSO₄ + K₂SO₄ + 2H₂O

3. With amines (important in organic):

R-NH₂ + HNO₂ → R-OH + N₂ + H₂O (primary amine)

Uses: Diazotization reactions in organic chemistry

Nitric Acid (HNO₃)

Oxidation state: N = +5

Preparation:

Laboratory:
NaNO₃ + H₂SO₄ → NaHSO₄ + HNO₃

Industrial (Ostwald process):
4NH₃ + 5O₂ → 4NO + 6H₂O (Pt catalyst, 800°C)
2NO + O₂ → 2NO₂
4NO₂ + O₂ + 2H₂O → 4HNO₃

Structure:

    O
    ‖
    N-O-H
    |
    O⁻
(Resonance between two N-O bonds)

Properties:

• Strong acid (pKₐ = -1.4)
• Colorless liquid (pure)
• Brown when decomposed (dissolved NO₂)
• Powerful oxidizing agent
• Decomposes on heating or light

Decomposition:

4HNO₃ → 4NO₂ + O₂ + 2H₂O (brown fumes)

Reactions: (See Group 15 notes for detailed reactions)

1. With metals:

3Cu + 8HNO₃(dil) → 3Cu(NO₃)₂ + 2NO + 4H₂O
Cu + 4HNO₃(conc) → Cu(NO₃)₂ + 2NO₂ + 2H₂O

2. Passivation: Fe, Al, Cr (cold, concentrated)

3. With non-metals:

S + 6HNO₃(conc) → H₂SO₄ + 6NO₂ + 2H₂O
P₄ + 20HNO₃(conc) → 4H₃PO₄ + 20NO₂ + 4H₂O

Comparison: HNO₂ vs HNO₃

Memory Trick: “N2 = Nitrous (2 N-O bonds visible in structure)”

Oxoacids of Sulfur

Sulfur forms many oxoacids with different oxidation states and structures!

Classification

Sulfurous Acid (H₂SO₃)

Oxidation state: S = +4

Preparation:

SO₂ + H₂O ⇌ H₂SO₃ (exists only in solution)

Structure:

    O
    ‖
H-O-S-O-H

Properties:

• Weak dibasic acid (pKₐ₁ = 1.9, pKₐ₂ = 7.2)
• Unstable (cannot be isolated)
• Reducing agent
• Bleaching agent (temporary)

Reactions:

1. Neutralization:

H₂SO₃ + 2NaOH → Na₂SO₃ + 2H₂O (sulfite)
H₂SO₃ + NaOH → NaHSO₃ + H₂O (bisulfite)

2. Reducing agent:

H₂SO₃ + Br₂ + H₂O → H₂SO₄ + 2HBr
H₂SO₃ + 2H₂S → 3S + 3H₂O

3. Oxidizing agent (rare):

H₂SO₃ + 2H₂S → 3S + 3H₂O

Decomposition:

H₂SO₃ → SO₂ + H₂O (easily)

Sulfuric Acid (H₂SO₄)

Oxidation state: S = +6

Preparation: Contact Process (see Group 16 notes)

S + O₂ → SO₂
2SO₂ + O₂ ⇌ 2SO₃ (450°C, 2 atm, V₂O₅)
SO₃ + H₂SO₄ → H₂S₂O₇
H₂S₂O₇ + H₂O → 2H₂SO₄

Structure:

    O      O
    ‖      ‖
H-O-S-O-H  (tetrahedral around S)
    ‖
    O

Properties:

• Strong dibasic acid (pKₐ₁ = -3, pKₐ₂ = 2)
• Colorless, oily liquid
• Highly viscous (H-bonding)
• Hygroscopic and dehydrating
• Powerful oxidizing agent (hot, conc.)

Reactions: (See Group 16 notes for details)

1. Dehydration:

C₁₂H₂₂O₁₁ → 12C + 11H₂O (sugar charring)
HCOOH → CO + H₂O

2. Oxidation:

Cu + 2H₂SO₄(conc) → CuSO₄ + SO₂ + 2H₂O

Basicity: Dibasic

H₂SO₄ → H⁺ + HSO₄⁻ (complete, strong)
HSO₄⁻ → H⁺ + SO₄²⁻ (partial, weak, pKₐ₂ = 2)

Thiosulfuric Acid (H₂S₂O₃)

Oxidation state: S = +2 (average), actually -2 and +6

Structure:

    O
    ‖
H-O-S-S-H  (one O replaced by S)
    ‖
    O

Properties:

• Unstable (cannot be isolated)
• Dibasic acid
• Salts (thiosulfates) are stable

Important Salt: Sodium thiosulfate (Na₂S₂O₃·5H₂O) - “hypo”

Preparation of Na₂S₂O₃:

Na₂SO₃ + S → Na₂S₂O₃ (boiling)
Na₂S + Na₂SO₃ → Na₂S₂O₃ + Na₂O

Reactions of Na₂S₂O₃:

1. With iodine (iodometry - volumetric analysis):

2Na₂S₂O₃ + I₂ → Na₂S₄O₆ + 2NaI
(Thiosulfate → Tetrathionate)

2. With dilute acid:

Na₂S₂O₃ + 2HCl → 2NaCl + H₂O + SO₂ + S (yellow turbidity)

3. Photography (fixing agent):

AgBr + 2Na₂S₂O₃ → Na₃[Ag(S₂O₃)₂] + NaBr (soluble complex)

Uses: Photography (hypo), iodometry, bleaching

Memory Trick: “THIO = THis Is One sulfur Oxygen substituted”

Peroxodisulfuric Acid (H₂S₂O₈)

Oxidation state: S = +6

Structure:

    O      O
    ‖      ‖
H-O-S-O-O-S-O-H  (peroxy bridge)
    ‖      ‖
    O      O

Preparation:

2H₂SO₄ + H₂O₂ → H₂S₂O₈ + 2H₂O
or electrolysis of H₂SO₄

Properties:

• Strong dibasic acid
• Powerful oxidizing agent
• Contains peroxy linkage (-O-O-)

Salts: Peroxodisulfates (persulfates)

• K₂S₂O₈ (potassium persulfate)

Pyrosulfuric Acid (H₂S₂O₇)

Oxidation state: S = +6

Common name: Oleum, fuming sulfuric acid

Structure:

    O      O
    ‖      ‖
H-O-S-O-S-O-H  (anhydride of H₂SO₄)
    ‖      ‖
    O      O

Preparation:

SO₃ + H₂SO₄ → H₂S₂O₇

Properties:

• Strong acid
• Fumes in moist air (releases SO₃)
• Used in Contact process

Reaction:

H₂S₂O₇ + H₂O → 2H₂SO₄

Comparison Table: Sulfur Oxoacids

Oxoacids of Phosphorus

Important: Basicity depends on OH groups, not total H atoms!

Hypophosphorous Acid (H₃PO₂)

Oxidation state: P = +1

Structure:

    O
    ‖
H-P-O-H  (2 P-H bonds, 1 P-OH bond)
    |
    H

Basicity: Monobasic (only 1 OH group!)

Preparation:

P₄ + 3NaOH + 3H₂O → 3NaH₂PO₂ + PH₃
Ba(H₂PO₂)₂ + H₂SO₄ → BaSO₄ + 2H₃PO₂

Properties:

• Weak acid
• Strong reducing agent (due to P-H bonds)
• Disproportionates on heating

Reactions:

1. Neutralization (monobasic):

H₃PO₂ + NaOH → NaH₂PO₂ + H₂O (maximum)

2. Reducing agent:

4AgNO₃ + H₃PO₂ + 2H₂O → 4Ag + H₃PO₄ + 4HNO₃ (silver mirror)
2HgCl₂ + H₃PO₂ + 2H₂O → 2Hg + H₃PO₄ + 4HCl

3. Disproportionation:

4H₃PO₂ → 3H₃PO₃ + PH₃ (on heating)

Uses: Reducing agent in organic synthesis

Phosphorous Acid (H₃PO₃)

Oxidation state: P = +3

Structure:

    O
    ‖
H-P-O-H  (1 P-H bond, 2 P-OH bonds)
    |
  H-O

Basicity: Dibasic (2 OH groups!)

Preparation:

P₄O₆ + 6H₂O → 4H₃PO₃
PCl₃ + 3H₂O → H₃PO₃ + 3HCl

Properties:

• Weak dibasic acid (pKₐ₁ = 2.0, pKₐ₂ = 6.6)
• Reducing agent
• Disproportionates on heating

Reactions:

1. Neutralization (dibasic):

H₃PO₃ + NaOH → NaH₂PO₃ + H₂O
H₃PO₃ + 2NaOH → Na₂HPO₃ + 2H₂O (maximum)

2. Reducing agent:

H₃PO₃ + 2AgNO₃ + 2H₂O → 2Ag + H₃PO₄ + 2HNO₃
2HgCl₂ + H₃PO₃ + H₂O → 2HgCl + H₃PO₄ + 2HCl

3. Disproportionation:

4H₃PO₃ → 3H₃PO₄ + PH₃ (on heating)

Orthophosphoric Acid (H₃PO₄)

Oxidation state: P = +5

Structure:

    O
    ‖
H-O-P-O-H  (3 P-OH bonds, 0 P-H bonds)
    |
  H-O

Basicity: Tribasic (3 OH groups!)

Preparation:

P₄ + 5O₂ → P₄O₁₀
P₄O₁₀ + 6H₂O → 4H₃PO₄

Ca₃(PO₄)₂ + 3H₂SO₄ → 2H₃PO₄ + 3CaSO₄

Properties:

• Weak tribasic acid (pKₐ₁ = 2.1, pKₐ₂ = 7.2, pKₐ₃ = 12.4)
• Colorless, syrupy liquid
• Non-oxidizing (unlike HNO₃)
• Hygroscopic

Reactions:

1. Neutralization (tribasic):

H₃PO₄ + NaOH → NaH₂PO₄ + H₂O (primary/monobasic sodium phosphate)
H₃PO₄ + 2NaOH → Na₂HPO₄ + 2H₂O (secondary/dibasic sodium phosphate)
H₃PO₄ + 3NaOH → Na₃PO₄ + 3H₂O (tertiary/tribasic sodium phosphate)

2. Dehydration (on heating):

2H₃PO₄ --heat--> H₄P₂O₇ + H₂O (pyrophosphoric acid)
(Metaphosphoric acid also forms at higher temp)

Uses:

• Food industry (soft drinks, cola - 0.05%)
• Fertilizers
• Rust remover
• Detergents

Pyrophosphoric Acid (H₄P₂O₇)

Oxidation state: P = +5

Structure:

    O      O
    ‖      ‖
H-O-P-O-P-O-H  (anhydride of H₃PO₄)
    |      |
  H-O    O-H

Basicity: Tetrabasic (4 OH groups)

Preparation:

2H₃PO₄ --heat--> H₄P₂O₇ + H₂O (240°C)

Properties:

• Weak tetrabasic acid
• Forms four series of salts

Metaphosphoric Acid (HPO₃)

Oxidation state: P = +5

Forms: Cyclic polymers (HPO₃)ₙ

Structure: Ring of P-O with external OH and P=O

Preparation:

H₃PO₄ --strong heat--> HPO₃ + H₂O (>300°C)

Properties:

• Monobasic (per HPO₃ unit)
• Forms metaphosphates

Comparison Table: Phosphorus Oxoacids

Memory Trick: “1-2-3 PO₂-PO₃-PO₄ = 1-2-3 basic, 3-2-1 reducing power”

Critical Point:

• H₃PO₂ has 3 H but monobasic (1 P-OH, 2 P-H)
• H₃PO₃ has 3 H but dibasic (2 P-OH, 1 P-H)
• H₃PO₄ has 3 H and tribasic (3 P-OH, 0 P-H)

Summary: Acidity Trends

Same element, different oxidation states

Higher oxidation state = Stronger acid

Examples:

• HOCl < HClO₂ < HClO₃ < HClO₄
• H₃PO₂ < H₃PO₃ < H₃PO₄
• H₂SO₃ < H₂SO₄

Reason: More O atoms → more electron withdrawal → weaker O-H → easier H⁺ release

Same oxidation state, different elements

More electronegative element = Stronger acid

Examples (same structure type):

• HOCl > HOBr > HOI
• H₂SO₄ > H₂SeO₄

Oxidizing vs Reducing Power

General trend: Lower oxidation state → Better reducing agent

Examples:

• H₃PO₂ (+1): Strong reducing agent
• H₃PO₃ (+3): Moderate reducing agent
• H₃PO₄ (+5): No reducing power

Exception:

• Oxidizing power in halogen oxoacids: HOCl > HClO₄ (Lower oxidation state, greater tendency to gain electrons)

Common Mistakes to Avoid

1. Mistake: H₃PO₃ is tribasic

   • Correct: Dibasic (only 2 OH groups, 1 P-H)

2. Mistake: All H atoms in oxoacids are acidic

   • Correct: Only H in OH groups (X-H bonds don’t ionize)

3. Mistake: Higher oxidation state = better oxidizing agent

   • Correct: For halogens: Lower oxidation state = better oxidizer (HOCl > HClO₄)

4. Mistake: HClO₄ is weak acid like HOCl

   • Correct: HClO₄ is strongest common acid (pKₐ = -10)

5. Mistake: H₂SO₃ can be isolated like H₂SO₄

   • Correct: H₂SO₃ exists only in solution (unstable)

6. Mistake: All sulfur oxoacids have S in +6 state

   • Correct: H₂SO₃ (+4), H₂S₂O₃ (+2 avg), others (+6)

7. Mistake: More O atoms always means stronger oxidizer

   • Correct: For acidity yes, for oxidizing power depends on element and structure

Practice Problems

Level 1: JEE Main Basics

1. Arrange in increasing order of acidic strength: a) HOCl, HClO₂, HClO₃, HClO₄ b) H₃PO₂, H₃PO₃, H₃PO₄

2. What is the basicity of: a) H₃PO₄ b) H₃PO₃ c) H₃PO₂ d) H₄P₂O₇

3. Write the oxidation state of central atom in: a) H₂SO₄ b) HNO₃ c) HClO₃ d) H₃PO₃

4. Which is strongest acid: HClO, HClO₂, HClO₃, HClO₄?

5. Why is H₂SO₄ dibasic acid? Write stepwise ionization.

Level 2: JEE Main Advanced

6. Explain why: a) HClO₄ is stronger acid than HClO b) H₃PO₃ is dibasic while H₃PO₄ is tribasic c) H₂SO₃ cannot be isolated but H₂SO₄ can

7. Draw the structure of: a) H₃PO₂ b) H₃PO₃ c) H₃PO₄ d) H₂S₂O₃

8. Complete and balance: a) H₃PO₂ + AgNO₃ + H₂O → b) H₃PO₃ (heat) → c) Na₂S₂O₃ + I₂ → d) HNO₂ (heat) →

9. Why is HOCl better oxidizing agent than HClO₄ despite lower oxidation state?

10. Arrange in order of: a) Reducing power: H₃PO₂, H₃PO₃, H₃PO₄ b) Stability: HOCl, HClO₂, HClO₃, HClO₄

Level 3: JEE Advanced

11. Calculate the maximum amount of NaOH (in moles) that can react with: a) 1 mole H₃PO₂ b) 1 mole H₃PO₃ c) 1 mole H₃PO₄ d) 1 mole H₄P₂O₇

12. Explain the trend in oxidizing power of halogen oxoacids: HOCl > HClO₂ > HClO₃ > HClO₄ despite acidic strength following opposite trend.

13. H₃PO₃ on heating undergoes disproportionation. Write the balanced equation and calculate the change in oxidation states to verify.

14. Peroxodisulfuric acid (H₂S₂O₈) contains a peroxy linkage. Draw its structure and calculate: a) Oxidation state of S b) Basicity c) Why it’s a stronger oxidizing agent than H₂SO₄

15. Write equations showing H₃PO₄ as: a) Monobasic acid b) Dibasic acid c) Tribasic acid Identify the salts formed in each case.

Cross-Links to Other Topics

Related to Acids and Bases

• Acid Strength - Factors affecting acidity
• Polyprotic Acids - H₂SO₄, H₃PO₄

Related to Redox Reactions

• Oxidation States - Calculating in oxoacids
• Disproportionation - HNO₂, H₃PO₃

Related to Chemical Bonding

• Resonance - ClO₄⁻, NO₃⁻ stability
• Formal Charges - Oxoacid structures

Related to p-Block Elements

• Group 15 - HNO₃, H₃PO₄
• Group 16 - H₂SO₄, H₂SO₃
• Group 17 - Halogen oxoacids

Memory Palace for Oxoacids

Imagine a Chemical Tower:

Ground Floor - Chlorine Oxoacid Gallery:

• Room 1 (+1): HOCl (weak, unstable, oxidizing)
• Room 3 (+3): HClO₂ (moderate)
• Room 5 (+5): HClO₃ (strong)
• Room 7 (+7): HClO₄ (STRONGEST, most stable)
• Acidity increases with floor number

First Floor - Nitrogen Wing:

• Left: HNO₂ (+3) - pale blue, unstable, amphoteric redox
• Right: HNO₃ (+5) - colorless/brown, strong acid, oxidizer

Second Floor - Sulfur Department:

• Section 1: H₂SO₃ (+4) - unstable, reducing
• Section 2: H₂SO₄ (+6) - king of chemicals, dehydrating, oxidizing
• Special room: H₂S₂O₃ (thiosulfate precursor)
• Corner: H₂S₂O₇ (oleum - fuming)

Third Floor - Phosphorus Laboratory:

• Bench 1: H₃PO₂ (3H, 1 basic, strong reducer) - 2 P-H visible
• Bench 2: H₃PO₃ (3H, 2 basic, moderate reducer) - 1 P-H visible
• Bench 3: H₃PO₄ (3H, 3 basic, no reducing) - 0 P-H, all OH
• Display: “Count OH not H” sign

Roof - Trend Observatory:

• Telescope 1: Acidity trend (more O = stronger)
• Telescope 2: Stability trend (higher oxidation = more stable)
• Telescope 3: Oxidizing trend (varies by element)

Quick Revision Checklist

• Oxoacid = H-O-X (acidic H on O only!)
• More O → Stronger acid (same element)
• Higher oxidation state → Stronger acid
• Halogen oxoacids: HOX (+1) to HXO₄ (+7)
• Acidity: HOCl < HClO₂ < HClO₃ < HClO₄
• HClO₄ strongest common acid (pKₐ = -10)
• Oxidizing: HOCl > HClO₄ (paradox!)
• HNO₂ (+3): unstable, amphoteric redox
• HNO₃ (+5): strong acid, oxidizer, passivates Fe/Al/Cr
• H₂SO₃ (+4): unstable, reducing
• H₂SO₄ (+6): strong, dehydrating, oxidizing
• H₂S₂O₃: thiosulfate (S replaces O)
• H₃PO₂: 1 basic (2 P-H), strong reducing
• H₃PO₃: 2 basic (1 P-H), moderate reducing
• H₃PO₄: 3 basic (0 P-H), no reducing
• Basicity = count OH groups only!

Important Equations Summary

Halogen Oxoacids:
1. HOCl: Cl₂ + H₂O → HCl + HOCl
2. HClO₃: 3Cl₂ + 6NaOH(hot) → 5NaCl + NaClO₃ + 3H₂O
3. HClO₄: NaClO₄ + H₂SO₄ → NaHSO₄ + HClO₄

Nitrogen Oxoacids:
4. HNO₂: NaNO₂ + HCl → NaCl + HNO₂
5. HNO₂ disp: 3HNO₂ → HNO₃ + 2NO + H₂O
6. HNO₃: 4NH₃ + 5O₂ → 4NO + 6H₂O (Ostwald 1)

Sulfur Oxoacids:
7. H₂SO₃: SO₂ + H₂O → H₂SO₃
8. H₂SO₄: Contact process (see Group 16)
9. Na₂S₂O₃: Na₂SO₃ + S → Na₂S₂O₃
10. Iodometry: 2Na₂S₂O₃ + I₂ → Na₂S₄O₆ + 2NaI

Phosphorus Oxoacids:
11. H₃PO₂: P₄ + 3NaOH + 3H₂O → 3NaH₂PO₂ + PH₃
12. H₃PO₂ disp: 4H₃PO₂ → 3H₃PO₃ + PH₃
13. H₃PO₃: PCl₃ + 3H₂O → H₃PO₃ + 3HCl
14. H₃PO₃ disp: 4H₃PO₃ → 3H₃PO₄ + PH₃
15. H₃PO₄: P₄O₁₀ + 6H₂O → 4H₃PO₄
16. H₄P₂O₇: 2H₃PO₄ → H₄P₂O₇ + H₂O (heat)

Last updated: August 2025 Previous: Group 18 Elements Related: Group 15 | Group 16 | Group 17