Sulphuric Acid Complete Guide

Sulphuric Acid (H₂SO₄)
Preparation, Properties, Uses, Structure and PYQs

This notes are targeted for exams like NEET, JEE, JAM, GATE, SET, and CSIR. Focus on key reactions, equations, applications, and structure.

1. Preparation

Sulphuric acid is primarily prepared by the Contact Process on an industrial scale. The process involves the following steps:

• Step 1: Burning of Sulphur or Pyrites
  S + O₂ → SO₂ (Sulphur dioxide)
  Or, 4FeS₂ + 11O₂ → 2Fe₂O₃ + 8SO₂
• Step 2: Oxidation of SO₂ to SO₃
  2SO₂ + O₂ ⇌ 2SO₃ (Catalyst: V₂O₅ at 450-500°C, exothermic reaction)
  Conditions: Low temperature (450°C) and high pressure favor forward reaction (Le Chatelier's principle).
• Step 3: Absorption of SO₃
  SO₃ is absorbed in 98% H₂SO₄ to form oleum (H₂S₂O₇).
  SO₃ + H₂SO₄ → H₂S₂O₇ (Oleum or Pyrosulphuric acid)
• Step 4: Dilution of Oleum
  H₂S₂O₇ + H₂O → 2H₂SO₄

Note for exams: Remember the catalyst (V₂O₅), temperature, and why SO₃ is not directly absorbed in water (forms mist, inefficient).

Why SO₃ is not absorbed directly in water?
Because the reaction is highly exothermic, leading to the formation of a corrosive sulfuric acid mist that is difficult to condense. Hence absorbed in 98% H₂SO₄ first.

2. Properties

Physical Properties

• Colorless, odorless, viscous (oily) liquid.
• Molecular weight: 98 g/mol.
• Density: 1.84 g/cm³ (concentrated).
• Boiling point: 337°C; Melting point: 10°C.
• Highly hygroscopic (absorbs moisture) and miscible with water in all proportions (exothermic dissolution).

Chemical Properties

• Acidic Nature: Strong dibasic acid.
  H₂SO₄ ⇌ H⁺ + HSO₄⁻ (First ionization strong)
  HSO₄⁻ ⇌ H⁺ + SO₄²⁻ (Second ionization weak)
  Reacts with bases: 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
  With metals: Zn + H₂SO₄ → ZnSO₄ + H₂ (dilute)
• Dehydrating Agent: Removes water from compounds.
  C₁₂H₂₂O₁₁ (sucrose) → 12C + 11H₂O (charred sugar)
  Used to dry gases like HCl, SO₂.
• Oxidizing Agent: Hot concentrated H₂SO₄ acts as oxidant.
  Cu + 2H₂SO₄ → CuSO₄ + SO₂ + 2H₂O
  C + 2H₂SO₄ → CO₂ + 2SO₂ + 2H₂O
• Reaction with Salts: Displaces volatile acids.
  NaCl + H₂SO₄ → NaHSO₄ + HCl

Note for exams: Distinguish between dilute and concentrated H₂SO₄ reactions. Emphasize dehydrating and oxidizing properties with examples.

3. Uses

• Fertilizers: Manufacture of superphosphate (Ca(H₂PO₄)₂) and ammonium sulphate ((NH₄)₂SO₄).
• Petroleum Refining: Removal of impurities.
• Detergents and Soaps: Sulphonation reactions.
• Pigments and Dyes: Production of TiO₂, paints.
• Explosives: Nitration for TNT, etc.
• Batteries: Electrolyte in lead-acid batteries.
• Metallurgy: Pickling of metals (removing oxides).
• Laboratory Reagent: Dehydrating agent, acid in titrations.

Note for exams: H₂SO₄ is called the "King of Chemicals" due to its wide industrial applications. Know at least 5-6 major uses.

4. Structure of Sulphuric Acid

• Molecular formula: H₂SO₄
• Central atom: Sulphur (S) in +6 oxidation state
• Hybridization of S: sp³
• Geometry around S: Tetrahedral
• Sulphur is bonded to:
  • Two oxygen atoms by double bonds (S=O)
  • Two hydroxyl groups (–OH) by single bonds
• Bond lengths:

  Bond	Length (pm)
  S=O	142 pm
  S–OH	157 pm


• Actual structure shows resonance: All four S–O bonds have double bond character due to delocalization (pπ–dπ bonding).
• Two O–H bonds are responsible for acidic nature (easily donate H⁺).

Important for exams: Draw the structure with two double bonds and two single bonds to –OH groups, and mention tetrahedral geometry & sp³ hybridization.

High-weightage Points for Competitive Exams
• Structure → Tetrahedral, sp³, +6 oxidation state of S
• Catalyst in Contact process → V₂O₅ (not Pt now)
• Temperature compromise in Contact process (not too low to kill rate)
• Difference in reactions of dilute vs conc. H₂SO₄
• Dehydrating action on sugar, formic acid, oxalic acid
• Oxidising action products with Cu, C, S, P, etc.

Sulphuric Acid Previous Year Questions
NEET │ JEE Main │ JEE Advanced │ CSIR-NET │ GATE

Q1. JEE Main 2023 (Jan)
In the Contact process, the catalyst used for the conversion of SO₂ to SO₃ is:

(A) V₂O₅   (B) Fe₂O₃   (C) CuCl₂   (D) Ni

View Detailed Answer

Answer: (A) V₂O₅
Vanadium pentoxide (V₂O₅) is the commercially used catalyst (1–2% on silica support) at 450–500 °C. It works via redox cycle (V⁵⁺ ⇌ V⁴⁺). Earlier platinum was used but it is costly and easily poisoned by arsenic impurities.

Q2. NEET 2022
Sulphuric acid is manufactured by Contact process. Which of the following statements is incorrect?

(A) SO₃ is absorbed in conc. H₂SO₄ to form oleum
(B) Oleum is diluted with water to get H₂SO₄
(C) SO₃ is directly dissolved in water
(D) Catalyst used is V₂O₅

View Detailed Answer

Answer: (C)
SO₃ is never directly absorbed in water because it forms a dense fog/mist of tiny H₂SO₄ droplets which does not condense easily. Hence SO₃ + H₂SO₄ → H₂S₂O₇ (oleum), then oleum is diluted carefully with water to get 98% H₂SO₄.

Q3. JEE Advanced 2021 (Paper-1)
The oxidation state of sulphur in H₂SO₄ and SO₃ respectively are:

(A) +6, +6   (B) +4, +6   (C) +6, +4   (D) +4, +4

View Detailed Answer

Answer: (A)
Calculation:
H₂SO₄ → 2(+1) + x + 4(−2) = 0 ⇒ x = +6
SO₃ → x + 3(−2) = 0 ⇒ x = +6
Sulphur exhibits its highest oxidation state (+6) in both.

Q4. CSIR-NET Chemical Sciences Dec 2020
The geometry and hybridization of sulphur in H₂SO₄ molecule are:

(A) Tetrahedral, sp³   (B) Trigonal planar, sp²   (C) Square planar, dsp²   (D) Octahedral, d²sp³

View Detailed Answer

Answer: (A) Tetrahedral, sp³
Sulphur is surrounded by four oxygen atoms (two S=O and two S–OH). Electron pairs = 4 → sp³ hybridization → tetrahedral geometry. Bond angle ≈ 109.5° (slightly distorted due to double bonds).

Q5. GATE Chemistry 2022
Hot concentrated H₂SO₄ reacts with carbon according to the reaction:
C + 2H₂SO₄ → CO₂ + 2SO₂ + 2H₂O
The change in oxidation number of sulphur in this reaction is:

View Detailed Answer

Answer: From +6 to +4 (decrease by 2)
In H₂SO₄, S = +6
In SO₂, S = +4
Each H₂SO₄ molecule gains 2 electrons (reduction). Carbon is oxidized from 0 to +4 (oxidation). Hence hot conc. H₂SO₄ acts as an oxidising agent.

Q6. JEE Main 2024 (April)
Which of the following is formed when conc. H₂SO₄ is treated with P₄O₁₀?

(A) H₄P₂O₇   (B) HPO₃   (C) H₃PO₄   (D) SO₃

View Detailed Answer

Answer: (D) SO₃
Conc. H₂SO₄ acts as a strong dehydrating agent:
P₄O₁₀ + 6H₂SO₄(conc, hot) → 4H₃PO₄ + 6SO₃↑
Commonly asked dehydration reaction.

Q7. NEET 2021
Oil of vitriol is:

(A) HNO₃   (B) H₂SO₄   (C) HCl   (D) H₃PO₄

View Detailed Answer

Answer: (B) H₂SO₄
Concentrated sulphuric acid is highly viscous and oily in appearance, hence historically called “oil of vitriol”.

Q8. JEE Advanced 2019
The correct order of acid strength of H₂SO₄, H₂SeO₄ and H₂TeO₄ is:

View Detailed Answer

Answer: H₂SO₄ > H₂SeO₄ > H₂TeO₄
Acid strength of oxyacids increases with electronegativity of central atom. Electronegativity order: S > Se > Te ⇒ H₂SO₄ is the strongest.

Q9. CSIR-NET June 2019
In the Contact process, the temperature is maintained around 450 °C because:

(A) Reaction is endothermic
(B) Higher temperature deactivates catalyst
(C) It is a compromise between favourable equilibrium and reasonable rate
(D) SO₃ decomposes above 500 °C

View Detailed Answer

Answer: (C)
2SO₂ + OV ⇌ 2SO₃  ΔH = −196 kJ/mol (exothermic)
Low temperature favours equilibrium but rate becomes too slow. Hence 450–500 °C is the optimum compromise for good yield and sufficient rate.

Q10. GATE Chemistry 2018
The product obtained when copper reacts with cold dilute H₂SO₄ is:

(A) CuSO₄   (B) CuS   (C) No reaction   (D) SO₂

View Detailed Answer

Answer: (C) No reaction
Cold dilute H₂SO₄ does not oxidise copper (Cu lies below hydrogen in electrochemical series). Copper reacts only with hot concentrated H₂SO₄ to give SO₂.

Q11. JEE Main 2015
Concentrated H₂SO₄ cannot be used for drying H₂ because:

(A) It oxidises H₂   (B) It decomposes H₂   (C) It reacts with H₂   (D) It absorbs H₂

Answer & Explanation

Answer: (C)
H₂ + H₂SO₄(conc, hot) → SO₂ + 2H₂O  (H₂ is oxidised to H₂O)
Hence conc. H₂SO₄ cannot be used for drying hydrogen (it reacts instead of just absorbing moisture).

Q12. NEET 2016 (Phase-I)
Hot concentrated sulphuric acid is a moderately strong oxidising agent. Which of the following reactions does not show oxidising behaviour?

(A) Cu + 2H₂SO₄ → CuSO₄ + SO₂ + 2H₂O
(B) C + 2H₂SO₄ → CO₂ + 2SO₂ + 2H₂O
(C) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl
(D) None of the above

Answer & Explanation

Answer: (C)
Reaction (C) is simple double displacement (no change in oxidation state). Reactions (A) & (B) show reduction of S from +6 → +4 (oxidising nature).

Q13. JEE Advanced 2016 (Paper-2)
The correct statement(s) about H₂SO₄ is/are:
(I) It acts as a dehydrating agent
(II) It acts as an oxidising agent
(III) It behaves as a reducing agent
(IV) It acts as a Lewis acid

(A) I, II   (B) I, II, IV   (C) II, III, IV   (D) All

Answer & Explanation

Answer: (B) I, II, IV
H₂SO₄ never acts as reducing agent (S is already in +6 state). It is an excellent dehydrating, oxidising (hot conc.) and Lewis acid (SO₃ part accepts electrons).

Q14. JEE Main 2017
The product obtained when formic acid is treated with conc. H₂SO₄ is:

(A) CO   (B) CO₂   (C) HCHO   (D) CH₃COOH

Answer & Explanation

Answer: (A) CO
HCOOH → CO + H₂O (conc. H₂SO₄ dehydrates formic acid to carbon monoxide)
Oxalic acid gives CO + CO₂ + H₂O.

Q15. CSIR-NET Dec 2017
Among the following, the species in which sulphur has the highest oxidation state is:

(A) H₂SO₅   (B) SO₃   (C) H₂SO₄   (D) SO₂Cl₂

Answer & Explanation

Answer: (A) H₂SO₅ (Peroxomonosulphuric acid/Carot’s acid)
Oxidation state: H₂SO₅ → +8 (peroxide linkage), H₂SO₅/SO₃/SO₂Cl₂ → +6.

Q16. GATE Chemistry 2019
The reaction of conc. H₂SO₄ with NaBr produces:

(A) Br₂   (B) HBr   (C) SO₂   (D) Na₂SO₄ only

Answer & Explanation

Answer: (A) Br₂
2NaBr + 3H₂SO₄ → 2NaHSO₄ + Br₂ + SO₂ + 2H₂O
Conc. H₂SO₄ oxidises bromide to bromine (unlike chloride, which gives only HCl).

Q17. JEE Main 2018
Sulphur dioxide is obtained when conc. H₂SO₄ reacts with:

(A) Cu   (B) Hg   (C) Both (A) and (B)   (D) None

Answer & Explanation

Answer: (C) Both
Cu + 2H₂SO₄ → CuSO₄ + SO₂ + 2H₂O
Hg + 2H₂SO₄ → HgSO₄ + SO₂ + 2H₂O
Classic oxidising action of hot conc. H₂SO₄.

Q18. NEET 2019
Which of the following is known as oil of vitriol?

(A) H₃PO₄   (B) H₂SO₃   (C) H₂SO₄   (D) H₂S₂O₇

Answer & Explanation

Answer: (C) H₂SO₄
Historical name due to its oily viscous nature.

Q19. JEE Advanced 2018 (Paper-1)
In the structure of H₂SO₄, the total number of σ and π bonds are respectively:

Answer & Explanation

Answer: 6σ and 4π
Structure: S is bonded to 4 oxygen atoms → 4 S–O σ bonds + 2 O–H σ bonds = 6 σ bonds
Two S=O contributes 2 π bonds each → 2 × 2 = 4 π bonds (due to resonance).

Q20. CSIR-NET June 2020
The catalyst used in the Contact process is poisoned by:

(A) As₂O₃   (B) P₂O₅   (C) CO₂   (D) N₂

Answer & Explanation

Answer: (A) As₂O₃
Arsenic compounds irreversibly poison V₂O₅ catalyst; that's why sulphur or pyrite must be purified before burning.

Q21. JKSET 2023
Which of the following is incorrect statement for Sulphuric acid?

(A) Higher viscosity than water.    (B) Solutes dissolve slowly in it.    (C) High vapour pressure.    (D) Difficult to remove from the crystallised materials.

Answer & Explanation

Answer: (C) High vapour pressure.
Sulphuric acid exhibits strong intermolecular hydrogen bonding. These strong forces mean that a lot of energy is required for its molecules to escape into the gas phase, resulting in a low vapour pressure and high boiling point.