Tetraoxosulphate(VI) Acid as an Oxidizing and Dehydrating Agent

Tetraoxosulphate(VI) acid (sulphuric acid) exhibits strong oxidation properties as hot concentrated tetraoxosulphate(VI) acid accepts electrons from reducing agents. Here, the sulphur is usually transformed from oxidation state +6 to +4 as evident in the reduction of tetraoxosulphate(VI) acid to trioxosulphate(IV) acid or sulphur(IV) oxide. The redox transformation from tetraoxosulphate(VI) acid to sulphur(IV) oxide can be expressed ionically as,

SO₄^2-(aq) + 4H⁺(aq) + 2e^- → 2H₂O(l) + SO₂(g)

Sulphur in the sulphate(VI) ion receives two electrons from the reducing agent so that its oxidation state of +6 is reduced to +4 in sulphur(IV) oxide.

The oxidizing properties of tetraoxosulphate(VI) acid can be shown in its reaction with hydrogen sulphide, metals and non-metals.

With hydrogen sulphide

When hydrogen sulphide, H₂S, is bubbled into concentrated tetraoxosulphate(VI) acid, the sulphide ion, S^2-, is oxidised to sulphur transforming from oxidation state -2 to 0.

H₂S(g) + H₂SO₄(aq) → SO₂(g) + 2H₂O(l) + S(s)

An important aspect of this reaction is that, the reacting acid could be hot or cold.

With Metals

Tetraoxosulphate(VI) acid reacts as oxidizing agents with metals (reducing agents) that are oxidized while they donate their electrons. Here, the metal form metallic ions in corresponding tetraoxosulphate(VI) salts and sulphur(IV) oxide. An example is when zinc, Zn, reacts with tetraoxosulphate(VI) acid to form zinc(II) tetraoxosulphate(VI) salt, sulphur(IV) oxide and water.

Zn(s) + 2H₂SO₄(aq) → ZnSO₄(aq) + 2H₂O(l) + SO₂(g)

The reaction can be explained in ionic terms as,

Zn(s) + 4H⁺ (aq) + SO₄^2-(aq) → Zn²⁺(aq) + 2H₂O(l) + SO₂(g)

Here, zinc metal is oxidised from an oxidation state of zero to +2, as the oxidation state of sulphur in the sulphate ion is transformed from +6 to +4.

With non-metals

Concentrated tetraoxosulphate(VI) acid reacts with non-metals to form their corresponding oxides and sulphur(IV) oxide. An example is when powdered carbon is heated with the concentrated acid to form carbon(IV) oxide, sulphur(IV) oxide and water.

C(s) + 2H₂SO₄(aq) → CO₂(g) + 2H₂O(l) + 2SO₂(g)

Here, carbon is transformed from an oxidation state of zero to +4 as the oxidation state of sulphur is reduced from +6 to +4.

Before discussing the use of tetraoxosulphate(VI) acid as a dehydrating agent it is essential that we differentiate between dehydrating agent and drying agent. Drying is a physical phenomenon that remove the water molecule while allowing the substance to keep its integrity. The basic principle of drying usually involves the absorption of the water molecule into the body of the drying agent as evident in the use of anhydrous calcium chloride as drying agent for the laboratory preparation of oxygen. Using the sun to vapourise water from wet salt crystals is another example of drying.

Dehydration on the other hand, affects the chemical composition of chemicals. Although tetraoxosulphate(VI) acid is also used as a drying agents for certain gases such as oxygen gas, its dehydrating effect is responsible for the corrosive action on human skin, and cellulose materials like wood, paper, cloth, e.t.c.

A unique example of dehydration is the use of the acid to remove water of crystallization from hydrated compounds. An example is the dehydration of blue copper(II) tetraoxosulphate(VI) pentahydrate by concentrated tetraoxosulphate(VI) acid to white anhydrous copper(II) tetraoxosulphate(VI) salt.

( + Conc H₂SO₄) CuSO₄.5H₂O(s) → CuSO₄(s) + 5H₂O(l)

Other examples of the dehydrating actions of tetraoxosulphate(VI) acid are:

The dehydration of sugar (sucrose), C₁₂H₂₂O₁₁, to sugar charcoal (Carbon).

( + Conc. H₂SO₄) C₁₂H₂₂O₁₁(s) → 12C(s) + 11H₂O(l);

The dehydration of ethanol, C₂H₅OH, to ethene gas, C₂H₄.

( + Conc. H₂SO₄) C₂H₅OH(l) → C₂H₄(g) + H₂O(l)

and the dehydration of ethanedioc acid, HOOC-COOH, to carbon(IV) oxide, CO₂, and Carbon(II) oxide, CO.

( + Conc. H₂SO₄) HOOC-COOH(s) → CO₂(g) + CO(g) + H₂O(l)