1. The Contact process (equations and conditions)
- Step 1: sulphur burnt in air (or iron pyrites roasted) → SO₂, which is purified and dried.
- Step 2 (contact step): SO₂ + excess air over vanadium pentoxide, V₂O₅, at about 450 °C and 1–2 atm; reversible and exothermic.
- Step 3: SO₃ absorbed in concentrated H₂SO₄ → oleum (fuming sulphuric acid, H₂S₂O₇).
- Step 4: oleum diluted with the calculated amount of water → H₂SO₄.
Why not water? SO₃ + H₂O is violently exothermic and forms a dense mist of acid droplets that does not condense easily.
| Step | Condition |
|---|---|
| SO₂ → SO₃ | V₂O₅, about 450 °C, 1–2 atm, excess air |
| SO₃ → oleum | Absorbed in conc. H₂SO₄ |
| Oleum → acid | Diluted with water |
Formulas
- Burning sulphur
- V₂O₅, about 450 °C, 1–2 atm
- Oleum
- Dilution of oleum
Key terms
- Contact process
The industrial manufacture of sulphuric acid in which sulphur dioxide and oxygen combine on the surface of a vanadium pentoxide catalyst.
- Oleum
H₂S₂O₇, fuming sulphuric acid, formed when sulphur trioxide dissolves in concentrated sulphuric acid.
2. Dilute sulphuric acid: a dibasic acid
Each molecule gives two H⁺ ions (H₂SO₄ → 2H⁺ + SO₄²⁻), so the acid is dibasic and forms two series of salts:
- acid salts (one H replaced): NaHSO₄, KHSO₄
- normal salts (both H replaced): Na₂SO₄, K₂SO₄
| Base : acid (moles) | Salt | Type |
|---|---|---|
| 1 : 1 (NaOH) | NaHSO₄ | Acid salt |
| 2 : 1 (NaOH) | Na₂SO₄ | Normal salt |
Formulas
- Acid salt
- Normal salt
Key terms
- Dibasic acid
An acid with two replaceable hydrogen atoms per molecule, which forms both acid salts and normal salts.
- Acid salt
A salt in which only part of the replaceable hydrogen of the acid has been replaced by a metal, such as NaHSO₄.
3. Reactions of the dilute acid
- Active metals (above H): sulphate + hydrogen. Copper does not react.
- Basic oxides and hydroxides: sulphate + water.
- Carbonates, hydrogen carbonates: sulphate + water + CO₂. With marble chips the reaction soon stops: insoluble CaSO₄ coats the chips.
- Sulphites, hydrogen sulphites: sulphate + water + SO₂.
- Sulphides: sulphate + H₂S (rotten-egg smell).
| Salt type | Gas | Test for the gas |
|---|---|---|
| Carbonate | CO₂ | Lime water turns milky |
| Sulphite | SO₂ | Acidified K₂Cr₂O₇ turns green |
| Sulphide | H₂S | Lead acetate paper turns black |
Formulas
- Metal
- Basic oxide
- Hydrogen carbonate
- Sulphite
- Hydrogen sulphite
- Sulphide
Key terms
- Displacement of a weaker acid
A strong acid reacting with a salt of a weaker or more volatile acid, setting that acid (or its gas) free.
4. Concentrated acid as an oxidising agent
Hot concentrated sulphuric acid gives oxygen (H₂SO₄ → H₂O + SO₂ + [O]) and is reduced to sulphur dioxide.
- Carbon is oxidised to CO₂ (gas mixture of CO₂ and SO₂).
- Sulphur is oxidised to SO₂ (the only gas).
| Heated with | Gases given off |
|---|---|
| Carbon | CO₂ and SO₂ |
| Sulphur | SO₂ only |
Formulas
- Carbon, hot conc. acid
- Sulphur, hot conc. acid
Key terms
- Oxidising agent
A substance that supplies oxygen to another substance and is itself reduced.
5. Concentrated acid as a dehydrating agent
Concentrated sulphuric acid has a very strong attraction for water.
- Sugar: turns yellow → brown → black; a black spongy mass of carbon rises; steam; much heat (charring): C₁₂H₂₂O₁₁ → 12C + 11H₂O.
- Blue vitriol: blue crystals turn white as the water of crystallisation is removed: CuSO₄·5H₂O → CuSO₄ + 5H₂O.
Drying removes moisture that is merely present (e.g. moist Cl₂, SO₂, HCl passed through the acid); dehydration removes the elements of water from a compound. The acid cannot dry NH₃ (reacts: 2NH₃ + H₂SO₄ → (NH₄)₂SO₄) or H₂S (oxidised to sulphur).
| Change | Drying or dehydration? |
|---|---|
| Moist chlorine through conc. H₂SO₄ | Drying |
| Sugar charred | Dehydration |
| Blue vitriol turns white | Dehydration |
Formulas
- Charring of sugar by conc. H₂SO₄
- Why ammonia cannot be dried by it
Key terms
- Dehydration
Removal of hydrogen and oxygen in the ratio 2 : 1 (the elements of water) from a compound, or of its water of crystallisation.
- Drying
Removal of moisture that is simply present in a substance, without changing the substance.
6. Non-volatile nature and tests
With a boiling point of about 338 °C, sulphuric acid is non-volatile. Heated with salts of volatile acids, it drives them out: this is how HCl and HNO₃ are prepared.
Tests for sulphuric acid / a sulphate:
- Barium chloride solution → white precipitate of BaSO₄, insoluble in dilute HCl (or HNO₃).
- Lead nitrate solution → white precipitate of PbSO₄.
- The concentrated acid chars sugar, wood or paper.
| Reagent | Observation with H₂SO₄ |
|---|---|
| BaCl₂ solution | White precipitate, insoluble in dil. HCl |
| Pb(NO₃)₂ solution | White precipitate |
| Sugar (conc. acid) | Chars black |
Formulas
- Below 200 °C
- Below 200 °C
- White precipitate
- White precipitate
Key terms
- Non-volatile acid
An acid with a high boiling point that does not easily turn to vapour, such as sulphuric acid.
Exam tips
Common mistakes
- Using platinum (Ostwald) instead of vanadium pentoxide as the Contact catalyst.
- Writing SO₃ + H₂O → H₂SO₄ as the plant step (SO₃ is absorbed in conc. H₂SO₄ first).
- Calling the charring of sugar “oxidation” or “drying” (it is dehydration).
- Forgetting to add dilute HCl in the barium chloride test.
How marks are usually awarded
- Contact process (1 mark per step): equation plus catalyst, temperature and pressure.
- Observations (1 mark each): colour change and the gas or residue.
- Drying versus dehydration (2 marks): one example of each.