The most widely made industrial chemical. It dehydrates sugar, oxidises copper, and powers car batteries.
Sulphur dioxide is made by burning sulphur (S + O₂ → SO₂) or roasting iron pyrites (4FeS₂ + 11O₂ → 2Fe₂O₃ + 8SO₂). After purification it is oxidised over a vanadium pentoxide catalyst at about 450 °C and 1 to 2 atmospheres: 2SO₂ + O₂ ⇌ 2SO₃. The sulphur trioxide is absorbed in concentrated sulphuric acid to give oleum, SO₃ + H₂SO₄ → H₂S₂O₇, which is diluted: H₂S₂O₇ + H₂O → 2H₂SO₄.
What is the catalyst in the contact process, and at what temperature does the converter work?
Vanadium pentoxide (V₂O₅), at about 450 °C
Why is SO₃ dissolved in concentrated sulphuric acid and not directly in water?
Absorbing it in water is violent and forms a fine acid mist that is hard to condense; absorbing it in the acid gives oleum, which is then diluted safely
What mass of SO₂ forms when 16 g of sulphur burns completely? (S = 32, O = 16)
16 g is 0.5 mole of S, which gives 0.5 mole of SO₂, so 0.5 × 64 = 32 g
Concentrated sulphuric acid is a dehydrating agent: it removes water from sugar, leaving carbon (C₁₂H₂₂O₁₁ → 12C + 11H₂O). It is an oxidising agent when hot (Cu + 2H₂SO₄ → CuSO₄ + SO₂ + 2H₂O and C + 2H₂SO₄ → CO₂ + 2SO₂ + 2H₂O). It is also non-volatile, so it displaces volatile acids from their salts (NaCl + H₂SO₄ → NaHSO₄ + HCl). To dilute it, always add the acid slowly to water.
Why must concentrated sulphuric acid be added to water, and not water to the acid?
Dilution gives out a great deal of heat; adding acid slowly to water spreads the heat, but adding water to the acid can make it spit dangerously
What is seen when concentrated sulphuric acid is added to sugar?
The sugar turns black as water is removed, leaving carbon, and steam is given off
Which property of the acid is shown by its reaction with hot copper?
Its oxidising property: the copper is oxidised and the acid is reduced to sulphur dioxide
A sulphate is detected with barium chloride solution, which gives a white precipitate of barium sulphate that does not dissolve in dilute hydrochloric acid: BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl. Sulphuric acid is used to make fertilisers, in lead-acid car batteries, and to make detergents, paints and other chemicals.
How do you test for the sulphate ion?
Add barium chloride solution: a white precipitate of barium sulphate forms that is insoluble in dilute hydrochloric acid
1. Name the process used to manufacture sulphuric acid.
2. What is the catalyst in the contact process?
3. At what temperature does the converter operate?
4. What is oleum?
5. Why is the acid added to water when diluting it?
6. Which property is shown when sugar turns black in concentrated sulphuric acid?
7. Write the equation for copper reacting with hot concentrated sulphuric acid.
8. How do you test for a sulphate?
9. What volume of SO₃ forms from 10 litres of SO₂ with excess oxygen, at the same temperature and pressure?
10. Give one use of sulphuric acid.