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Tetraoxosulphate(VI) Acid, H₂SO₄, Preparation, Properties, and Uses

Tetraoxosulphate(VI) Acid is a chemical compound with the formula H₂SO₄.

Commercial Preparation of Tetraoxosulphate(VI) Acid - H₂SO₄

The industrial or commercial production of H₂SO₄ is through a process known as the contact process. The process involves:

1. The oxidation of sulphur(IV) oxide by air to sulphur(VI) oxide using a catalyst such as vanadium(V) oxide, V₂O₅.

2SO2(g) + O2

2SO3(g)

SO₂ is produced by burning sulphur or by burning pyrite, FeS₂ in air.

2. The absorption of sulphur(VI) oxide, SO₃ in conc. H₂SO₄ to form a fuming liquid called ‘Oleum’ - heptaoxosulphate(VI) acid

H₂SO_4(aq) + SO_3(g) → H₂S₂O_7(aq) - oleum

3. The oleum is diluted with correct amount of water to produce the conc. H₂SO₄.

H₂S₂O_7(aq) + H₂O_(l) → 2H₂SO_4(aq)

Note: a direct absorption of SO₃ in water is not done - the reaction is violently exothermic, and a mist of fine drops of dilute H₂SO₄ will fill the environment.

Properties of H₂SO₄

1. As an acid - H₂SO₄ is dibasic and ionizes almost completely in solution, this makes it a strong acid. Due to it being dibasic, it forms two kinds of salts with alkalis.

2NaOH_(aq) + H₂SO_4(aq) → Na₂SO_4(aq) + 2H₂O_(l) and

NaOH_(aq) + H₂SO_4(aq) → NaHSO_4(aq) + H₂O_(l)

2. Action of dilute H₂SO₄ with metals

Reactive metals would displace hydrogen from dilute tetraoxosulphate(VI) acid.

Zn_(s) + H₂SO_4(aq) → ZnSO_4(aq) + H_2(g)

Note:

* Less reactive metals such as copper will not displace hydrogen from dilute acid.

* Cold concentrated H₂SO₄ is not attacked by any metal in the complete absence of water.

3. Action on trioxocarbonate(IV)

Carbon(IV) oxide is liberated when H₂SO₄ is added onto a trioxocarbonate(IV).

Na₂CO_3(aq) + H₂SO_4(aq) → Na₂SO_4(aq) + H₂O_(l) + CO_2(g)

On a piece of marble, which has the chemical formula CaCO₃, the reaction obtained is prematurely stopped due to the formation of the sparingly soluble salt, CaSO₄, which forms a deposit on the surface of the marble.

4. Conc. H₂SO₄ as an oxidizing agent

When hot and concentrated, the acid accepts electrons from reducing agents such as Cu or Zn.

It also oxidizes non-metals, such as carbon and sulphur, and is reduced to SO₂ in the process.

S_(s) + 2H₂SO_4(aq) → 2H₂O_(l) + 3SO_2(g)

C_(s) + 2H₂SO_4(aq) → 2H₂O_(l) + 2SO_2(g) + CO_2(g)

The SO₂ given off is detected using a strip of filter paper moistened with potassium heptaoxodichromate(VI) solution which turns green - this is one of the tests for SO₂ discussed earlier.

5. As a dehydrating agent

Tetraoxosulphate(VI) acid has a strong affinity for water. The addition of the acid to water (conc. acid should always be gradually added to water, with stirring of the liquid as the acid is added - to prevent the acid from forming a lower layer) produces a great amount of heat, hence the addition is not a physical change, but a chemical change.

H₂SO₄ is hygroscopic, that is, it absorbs water from the air and becomes dilute. When conc. H₂SO₄ is added to many compounds, it decomposes them by removing water molecules from them, that is, it dehydrates them.

An example is the removal of rust from iron, and the dehydration of sugar. The sugar becomes a black mass of carbon.

C₁₂H₂₂O_11(s) + nH₂SO_4(l) → 12C_(s) + (11H₂O_(l) + nH₂SO_4(aq))

In a similar way, conc. H₂SO₄ acts on the skin and cloth.

Test for Tetraoxosulphate(VI) Ion

The characteristic test for any soluble tetraoxosulphate(VI) is the formation of white precipitate (barium sulphate) when a solution of barium chloride, acidified with dilute HCl, or when a solution of barium nitrate, acidified with dilute nitric acid is reacted with it.

BaCl_2(aq) + H₂SO_4(aq) → 2HCl_(aq) + BaSO_4(s)

Ba(NO₃)_2(aq) + H₂SO_4(aq) → 2HNO_3(aq) + BaSO_4(s) - white precipitate

Uses of H₂SO₄

Below are some of the uses of H₂SO₄

1. For the manufacture of fertilizer, such as ammonium tetraoxosulphate(VI), NH₄SO₄, and superphosphates.

2. For the manufacture of paints and pigments.

3. For the manufacture of artificial and natural fibre.

4. For the manufacture of metallic tetraoxosulphate(VI), HCl, HNO₃, HF, and plastics.

5. For the manufacture of detergents, dyes, explosives and drugs.

6. For the extraction of metals, example, in pickling (cleaning) iron and steel before plating them with tin or zinc.

7. As electrolyte in lead-acid storage battery.

8. In petroleum refining, where to is used to wash impurities out of gasoline and other refinery products.