Study of Compounds — Sulphuric Acid
Introduction
Sulphuric acid (H₂SO₄) is called the 'king of chemicals' because of its extensive industrial use. In ICSE Class 10 Chemistry, you study its manufacture by the Contact process, its role as a dehydrating and oxidising agent, and its applications.
Contact Process (Manufacture of H₂SO₄)
Steps
Step 1: Burning sulphur or roasting sulphide ores to produce SO₂:
S + O₂ → SO₂ 4FeS₂ + 11O₂ → 2Fe₂O₃ + 8SO₂
Step 2: Catalytic oxidation of SO₂ to SO₃:
2SO₂(g) + O₂(g) ⇌ 2SO₃(g) + Heat
Conditions:
| Parameter | Condition |
|---|---|
| Temperature | 450°C |
| Pressure | 1−2 atm |
| Catalyst | Vanadium pentoxide (V₂O₅) |
| Promoter | K₂SO₄ |
Step 3: Absorption of SO₃ in concentrated H₂SO₄ (not water):
SO₃ + H₂SO₄ → H₂S₂O₇ (oleum) H₂S₂O₇ + H₂O → 2H₂SO₄
Note: SO₃ is NOT directly dissolved in water because it forms a dense mist of H₂SO₄ that is difficult to condense.
Physical Properties
| Property | Observation |
|---|---|
| Colour | Colourless oily liquid |
| Density | 1.84 g/mL (very heavy) |
| Solubility | Highly soluble in water (miscible) |
| Boiling point | 338°C (high — due to hydrogen bonding) |
| Viscosity | Highly viscous (oily) |
| Hygroscopic | Absorbs moisture from air (concentrated acid) |
Dehydrating Agent
Concentrated H₂SO₄ has a strong affinity for water and removes water (H and O in the ratio 2:1) from many compounds.
Examples
| Reaction | Equation |
|---|---|
| Sugar (sucrose) | C₁₂H₂₂O₁₁ → 12C + 11H₂O (black mass of carbon) |
| Copper sulphate crystals | CuSO₄·5H₂O → CuSO₄ + 5H₂O (blue → white) |
| Formic acid | HCOOH → H₂O + CO |
The charring of sugar is a popular demonstration — white sugar turns black.
Oxidising Agent
Concentrated H₂SO₄ acts as an oxidising agent, especially when hot.
With non-metals:
C + 2H₂SO₄ (conc.) → CO₂ + 2SO₂ + 2H₂O S + 2H₂SO₄ (conc.) → 3SO₂ + 2H₂O
With metals:
Cu + 2H₂SO₄ (conc.) → CuSO₄ + SO₂ + 2H₂O
Dilute H₂SO₄ behaves like a typical acid (provides H⁺) and reacts with reactive metals to give H₂:
Zn + H₂SO₄ (dil.) → ZnSO₄ + H₂↑
Uses of Sulphuric Acid
- Manufacture of fertilisers (ammonium sulphate, superphosphate).
- Petroleum refining.
- Manufacture of dyes, drugs, and detergents.
- Lead-acid batteries (electrolyte).
- As a laboratory reagent and dehydrating agent.
Comparison: Concentrated vs Dilute H₂SO₄
| Property | Concentrated H₂SO₄ | Dilute H₂SO₄ |
|---|---|---|
| Nature | Dehydrating + oxidising agent | Typical acid (provides H⁺) |
| Reaction with Cu | Cu + 2H₂SO₄ → CuSO₄ + SO₂ + 2H₂O | No reaction |
| Reaction with Zn | Zn + 2H₂SO₄ → ZnSO₄ + SO₂ + 2H₂O | Zn + H₂SO₄ → ZnSO₄ + H₂ |
| Dilution | Highly exothermic — ALWAYS add acid to water | — |
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| Adding water to concentrated H₂SO₄ for dilution | ALWAYS add H₂SO₄ to water slowly (heat released can cause splashing) |
| Thinking dilute H₂SO₄ is also a strong oxidising agent | Only CONCENTRATED H₂SO₄ is an oxidising agent |
| Confusing V₂O₅ for the Contact process catalyst | V₂O₅ is the catalyst (not Pt used in Ostwald's process) |
| Forgetting SO₃ is absorbed in H₂SO₄, not water | SO₃ + H₂O forms a mist — use H₂SO₄ instead |
ICSE Exam Focus
This chapter carries 4–6 marks. Key topics: Contact process, dehydrating property (sugar charring), oxidising property (Cu + conc. H₂SO₄), uses.
Marks Blueprint: Contact process — 2 marks, Dehydrating agent — 2 marks, Oxidising agent — 2 marks.
Self-Test Questions
-
Describe the Contact process for the manufacture of H₂SO₄.
-
Why is SO₃ absorbed in concentrated H₂SO₄ and not in water?
-
What is observed when concentrated H₂SO₄ is added to sugar? Explain.
-
Write equations for the reaction of concentrated H₂SO₄ with (a) carbon, (b) copper.
-
How would you distinguish between concentrated and dilute H₂SO₄ using copper?
-
State three important uses of sulphuric acid.
