🧪 Sulphuric Acid (H₂SO₄) – Full Breakdown
1) Preparation of Sulphuric Acid
a) Industrial Method — Contact Process
| Contact Process for H₂SO₄ |
|---|
| Step 1 (Sulphur burning): S(s) + O₂(g) → SO₂(g) (in excess air) |
| Step 2 (Catalytic oxidation): 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) (V₂O₅ catalyst, 450°C, 2 atm) |
| Step 3 (Absorption): SO₃(g) + H₂SO₄(l) → H₂S₂O₇(l) (oleum / pyrosulphuric acid) |
| Step 4 (Dilution): H₂S₂O₇(l) + H₂O(l) → 2H₂SO₄(l) |
| NOTES: SO₃ is absorbed in conc. H₂SO₄ (not water) to avoid forming a corrosive mist of H₂SO₄ droplets. V₂O₅ is the catalyst (vanadium pentoxide — replaces the older Pt which was expensive and easily poisoned). |
Optimum conditions for Step 2:
- Pressure: ~2 atm (forward has 3 vol → 2 vol, so moderate pressure favours forward)
- Temperature: ~450°C (exothermic reaction — lower temp gives better yield but slow rate; 450°C is the compromise; V₂O₅ catalyst works best at this temp)
- Catalyst: V₂O₅ (vanadium pentoxide) — speeds up the reaction, not poisoned by impurities like Pt was
2) Physical Properties of H₂SO₄
- Appearance: Colourless, oily liquid (viscous)
- Density: 1.84 g/cm³ (much heavier than water)
- Boiling point: 338°C (high — doesn't evaporate easily; non-volatile)
- Miscibility: Highly soluble in water — mix with a hissing sound, lots of heat evolved (exothermic)
- Hygroscopic: Absorbs moisture from the air (conc. H₂SO₄ is used as a drying agent)
Always add acid to water, NEVER water to acid.
Why? Because the dilution is highly exothermic. Water is lighter than H₂SO₄. If you add water to conc. H₂SO₄, the water floats on top, the heat generated at the interface is so intense that the water boils instantly, splashing hot acid everywhere. Add acid SLOWLY to water with constant stirring to dissipate the heat.
3) Chemical Properties of H₂SO₄
a) Acidic Nature (Strong Diprotic Acid)
H₂SO₄ ionises in two steps:
- H₂SO₄ → H⁺ + HSO₄⁻ (complete — strong acid, first dissociation)
- HSO₄⁻ ⇌ H⁺ + SO₄²⁻ (partial — weak acid, second dissociation)
| H₂SO₄ as an Acid |
|---|
| With bases: H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O |
| With metal oxides: H₂SO₄ + CuO → CuSO₄ + H₂O |
| With carbonates: H₂SO₄ + Na₂CO₃ → Na₂SO₄ + H₂O + CO₂↑ |
| With bicarbonates: H₂SO₄ + 2NaHCO₃ → Na₂SO₄ + 2H₂O + 2CO₂↑ |
| With NH₃: H₂SO₄ + 2NH₃ → (NH₄)₂SO₄ (ammonium sulphate fertiliser) |
| NOTES: Can form both normal salts (sulphates) and acid salts (bisulphates) because it's diprotic. |
b) Behaviour as a Dehydrating Agent (H₂SO₄ removes H₂O)
Conc. H₂SO₄ has a strong affinity for water — it removes water molecules (H and O in 2:1 ratio) from compounds.
| H₂SO₄ as a Dehydrating Agent |
|---|
| Sugar (C₁₂H₂₂O₁₁): C₁₂H₂₂O₁₁ → 12C + 11H₂O (black charred mass — the sugar gets dehydrated to carbon) |
| CuSO₄·5H₂O (blue): CuSO₄·5H₂O → CuSO₄ + 5H₂O (blue → white — H₂SO₄ removes water of crystallisation) |
| Formic acid: HCOOH → CO + H₂O (dehydrates to CO gas) |
| Oxalic acid: (COOH)₂ → CO + CO₂ + H₂O |
| Ethanol: C₂H₅OH → C₂H₄ + H₂O (dehydration → ethene, 170°C) |
| NOTES: The sugar dehydration is a classic demonstration — white sugar turns black (carbon), the test tube heats up, and steam comes out. |
c) Behaviour as an Oxidising Agent (Conc. H₂SO₄ — hot)
Conc. H₂SO₄ (hot) acts as an oxidising agent. It is reduced to SO₂.
| H₂SO₄ as an Oxidising Agent |
|---|
| With Cu: Cu + 2H₂SO₄(conc.) → CuSO₄ + SO₂↑ + 2H₂O (hot conc.) |
| With Zn: Zn + 2H₂SO₄(conc.) → ZnSO₄ + SO₂↑ + 2H₂O |
| With C: C + 2H₂SO₄(conc.) → CO₂↑ + 2SO₂↑ + 2H₂O (hot conc.) |
| With S: S + 2H₂SO₄(conc.) → 3SO₂↑ + 2H₂O |
| NOTES: Dilute H₂SO₄ is NOT an oxidising agent — it behaves like a normal acid and produces H₂ with metals (e.g., Zn + dil. H₂SO₄ → ZnSO₄ + H₂↑). Only hot, concentrated H₂SO₄ acts as an oxidising agent, producing SO₂. |
d) Reaction with Metals (Dilute vs Concentrated)
| Dilute H₂SO₄ vs Conc. H₂SO₄ |
|---|
| Dilute + Zn: Zn + H₂SO₄(dil.) → ZnSO₄ + H₂↑ (H₂ test: burns with pop sound) |
| Conc. + Cu: Cu + 2H₂SO₄(conc.) → CuSO₄ + SO₂↑ + 2H₂O (SO₂: pungent smell, turns K₂Cr₂O₇ green) |
| NOTES: Dilute H₂SO₄ does NOT react with Cu (Cu is below H in reactivity series). Conc. H₂SO₄ reacts even with Cu because of its oxidising nature — it does not depend on H⁺ ions. |
e) Sulphonation
| Sulphonation – Introduction of -SO₃H group |
|---|
| Benzene → Benzenesulphonic acid: C₆H₆ + H₂SO₄(conc.) → C₆H₅SO₃H + H₂O |
| NOTES: Conc. H₂SO₄ provides the -SO₃H group. This is how detergents and some dyes are made. |
4) Non-Volatile Nature of H₂SO₄
H₂SO₄ has a high boiling point (338°C) — it is non-volatile. This means it can displace more volatile acids from their salts. This is used in the lab preparation of:
- HCl: NaCl + H₂SO₄ → NaHSO₄ + HCl↑ (more volatile — bp -85°C)
- HNO₃: KNO₃ + H₂SO₄ → KHSO₄ + HNO₃↑ (more volatile — bp 86°C)
5) Test for Sulphate Ion (SO₄²⁻)
| Test for Sulphate Ion (BaCl₂ test) |
|---|
| Add BaCl₂ solution + dil. HCl to the test solution |
| A white precipitate of BaSO₄ forms (insoluble in dil. HCl) |
| NOTES: BaCl₂ + H₂SO₄ → BaSO₄↓ + 2HCl. The white precipitate is BaSO₄ — insoluble in dil. HCl (unlike BaCO₃ or BaSO₃ which dissolve in dil. HCl). The dil. HCl is added to eliminate carbonates and sulphites (their Ba salts dissolve in acid). |
6) Uses of Sulphuric Acid
- Fertilisers: (NH₄)₂SO₄, superphosphate fertiliser
- Lab reagent: In preparation of HCl, HNO₃, H₂ — as a drying agent, dehydrating agent, and oxidising agent
- Lead-acid batteries: The electrolyte in car batteries
- Petroleum refining: To remove impurities
- Detergents: Sulphonation of hydrocarbons
- Dyes and pigments: Sulphonation reactions
- Explosives: In nitration reactions (as a dehydrating agent)
- Drying agent: To dry gases (HCl, Cl₂, SO₂, CO₂ — but NOT NH₃ or H₂S)
7) Top ICSE Reasoning Questions
Q1: Why is H₂SO₄ called "King of Chemicals"?
Answer: Because it is the most widely used industrial chemical. It's used in fertilisers, batteries, petroleum refining, detergents, explosives, and as a lab reagent. The production of H₂SO₄ is often used as an indicator of a country's industrial development.
Q2: Why must water never be added to conc. H₂SO₄?
Answer: The dilution of H₂SO₄ is highly exothermic. Water is lighter than H₂SO₄, so it floats on top. The heat generated at the interface causes the water to boil instantly, splashing hot concentrated acid. Always add acid to water slowly with stirring.
Q3: Why is SO₃ absorbed in conc. H₂SO₄ and not in water in the Contact Process?
Answer: SO₃ + H₂O → H₂SO₄ is highly exothermic and creates a fine corrosive mist of H₂SO₄ droplets that is difficult to collect. Instead, SO₃ is absorbed in conc. H₂SO₄ to form oleum (H₂S₂O₇), which is then diluted with water safely.
Q4: Distinguish between dil. H₂SO₄ and conc. H₂SO₄.
Answer: Dil. H₂SO₄ is a normal acid — reacts with metals above H to give H₂ gas. Conc. H₂SO₄ (hot) is an oxidising agent — reacts even with Cu (below H) to give SO₂ (not H₂). Dilute → produces H₂ with Zn; Conc. → produces SO₂ with Cu.
Q5: Why is conc. H₂SO₄ used as a drying agent for HCl but not for NH₃?
Answer: HCl is an acidic gas — no reaction with H₂SO₄. NH₃ is a basic gas — it reacts with H₂SO₄ to form (NH₄)₂SO₄. So NH₃ cannot be dried using conc. H₂SO₄; it needs CaO (quicklime).
Q6: What happens when conc. H₂SO₄ is added to sugar?
Answer: The sugar turns black (charred) — this is due to dehydration. C₁₂H₂₂O₁₁ → 12C + 11H₂O. The black mass is pure carbon. This proves H₂SO₄ is a powerful dehydrating agent.
Q7: Dilute H₂SO₄ produces H₂ with Zn, but conc. H₂SO₄ produces SO₂ with Cu — why?
Answer: Dilute H₂SO₄ behaves as a normal acid (H⁺ ions) — it reacts with metals above H to displace H₂. Conc. H₂SO₄ behaves as an oxidising agent — the S in H₂SO₄ is reduced from +6 to +4 (SO₂), allowing it to react even with metals below H like Cu.
Q8: In the Contact Process, why is V₂O₅ used instead of Pt?
Answer: Pt was the original catalyst but it was (1) very expensive, and (2) easily poisoned by impurities like arsenic in SO₂. V₂O₅ is cheaper and more resistant to poisoning, making the process more economical.
Q9: Why is dil. HCl added in the test for sulphate ions?
Answer: To eliminate interference from carbonate (CO₃²⁻) and sulphite (SO₃²⁻) ions. BaCO₃ and BaSO₃ also form white precipitates with BaCl₂, but they dissolve in dil. HCl. BaSO₄ is the only one that remains insoluble in dil. HCl — confirming sulphate.
Q10: Why can H₂SO₄ be used to prepare HCl and HNO₃ in the lab?
Answer: Because H₂SO₄ is non-volatile (bp 338°C). It can displace more volatile acids (HCl bp -85°C, HNO₃ bp 86°C) from their salts. The volatile acid escapes as vapour and is condensed.
1. Contact Process: S → SO₂ → SO₃ (V₂O₅, 450°C, 2 atm) → absorbed in H₂SO₄ → oleum → dilute. Memorise the sequence.
2. Water NEVER into acid. This is a safety question they love — "add acid to water slowly".
3. Dilute vs Conc: Dilute → H₂ gas (normal acid). Conc (hot) → SO₂ (oxidising agent). This distinction comes every year.
4. Sugar test: White → black charred mass (dehydration). Most common demonstration question.
5. BaSO₄ test: White ppt insoluble in dil. HCl. Add dil. HCl to rule out carbonates/sulphites.
6. King of Chemicals: Most widely used chemical — fertilisers, batteries, petrochemicals, detergents.
7. H₂SO₄ can dry: HCl, Cl₂, SO₂, CO₂. Cannot dry: NH₃, H₂S (they react).