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Questions 5.5

Explain the chemical tests and results to confirm presence of:
1. Sulphur (IV) oxide gas (SO₂)
2. Hydrogen sulphide gas (H₂S)
3. Sulphate ions (SO₄^2-)
4. Sulphite ions (SO₃^2-)
Prepare a one-page summary on the chemistry of sulphur as learnt in this topic. Refer to "Answers to Questions" sections where you find it necessary.
With sulphur at the centre, draw a flow diagram to show how sulphur is obtained from its gaseous compounds, and how it forms other substances.

Sample Answers to Questions 5.5

(a) Pass the gas into barium nitrate solution. A white precipitate which dissolves in dilute hydrochloric acid with effervescence confirms that the gas is sulphur (IV) oxide.
(b) Test the gas with lead (II) acetate paper. Blackening of the paper confirms hydrogen sulphide.
(c) Add two to three drops of barium nitrate solution to the suspected sulphate. A white precipitate which is insoluble in dilute hydrochloric acid confirms sulphate ions.
(d) Add barium nitrate solution to the suspected sulphite, followed with three to four drops of dilute hydrochloric acid. A white precipitate which produces effervescence as it dissolves in dilute hydrochloric acid conforms presence of sulphite ions (similar to (a)).
Sulphur and its Compounds
Extraction of sulphur: By Frasch Process: Sink three concentric pipes to underground sulphur deposits. Force hot compressed air and superheated water down two of the tubes. A mixture of molten sulphur and water flows out through the third (middle) pipe.
Physical properties: Sulphur is a soft yellow solid, of moderate reactivity, low melting point, insoluble in water but soluble in organic solvents, has the allotropes: rhombic, monoclinic, plastic and many others.
Chemical properties of sulphur: Burns in with a pale blue flame air to form aulphur (IV) oxide,
e.g. S(s) + O₂(g) → SO₂(g)
Combines directly with metals and non-metals, e.g. Fe(s) + S(s) → FeS(s);
Oxides of sulphur
Sulphur (IV) oxide, SO₂: Prepared by reacting sodium sulphite (Na₂SO₃) with dilute hydrochloric acid, dried using concentrated sulphuric (VI) acid, and collected by downward delivery.
Na₂SO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + SO₂(g)
It is denser than air, of pungent smell, acidic, dissolves in water to form H₂SO₃, which causes bleaching. i.e. H₂SO₃(aq) + Dye+O → H₂SO₄(aq) + Dye
Sulphur (VI) oxide, SO₃: Prepared industrially by burning SO₂ in oxygen, catalyzed by platinum.
Reducing properties of SO₂: Reduces common oxidizers: Fe³⁺, Br₂, HNO₃, MnO₄^-, Cr₂O₇^2-, H₂O₂, NO₂; so it decolourizes Br₂ and MnO₄^- ions, and turns Fe³⁺ and Cr₂O₇^2- to green Fe²⁺ and Cr³⁺ ions respectively.
Oxidizing properties of SO₂: Oxidizes H₂S to sulphur, and metals to their oxides
2Mg(s) + SO₂(g) → 2MgO(s) + S(s); 2H₂S(g) + SO₂(g) → 2H₂O(l) + 3S(s)
Test for sulphite (SO₃^2-) and sulphate (SO₄^2-) ions: React the test substance with dilute hydrochloric acid. Sulphites dissolve, effervesce, sulphates do not.
Uses of sulphur (IV) oxide: To make bleaching agent (Ca(HSO₃)₂) for wood pulp in paper industry, raw material in the manufacture of sulphuric (VI) acid, fumigant against pests and insects, preservative for jam and fruit juice.
Concentrated sulphuric (VI) acid (H₂SO₄(l))
Manufacture of H₂SO₄(l): Use of contact process: S(s) + O₂(g) → SO₂(g);
2SO₂(g) + O₂(g) → (reversible) 2SO₃(g) (with Pt catalyst)
Physical properties of H₂SO₄(l): A liquid at room temperature, colourless, oily, denser than water, and soluble in water.
Oxidizing property of H₂SO₄(l): S(s) + 2H₂SO₄(l) → 2H₂O(l) + 3SO₂(g);
Mg(s) + 2H₂SO₄(l) → MgSO₄(aq) + 2H₂O(l) + SO₂(g)
Metal + Concentrated sulphuric (VI) acid → Metal sulphate + Sulphur (IV) oxide + Water
Non-metal + Concentrated sulphuric (VI) acid → Water + Sulphur (IV oxide + Non-metal oxide
Dehydration property of H₂SO₄(l): Removes elements of water, e.g. in cane sugar, wood, paper, cotton etc to carbon and water; ethanol to ethene, methanoic acid to carbon (II) oxide.
Dilute sulphuric (VI) acid
Behaves in the same way as all the other dilute acids.
Hydrogen sulphide (H₂S)
Sources of H₂S: Naturally from volcanic eruptions, and decaying organic matter. Prepared from FeS and HCl(aq)
Properties of H₂S: Colourless, slightly soluble in cold water, acidic, rotten-egg smell, blackens lead (II) acetate solution, poisonous; burns in ordinary air with a faint blue flame to produce steam and sulphur (yellow deposit), and in excess oxygen to form sulphur (IV) oxide; has reducing properties but, unlike SO₂, leaves yellow residue.
Precipitation by H₂S e.g. Cu²⁺(aq) + S^2-(aq) → CuS(s); Pb²⁺(aq) + S^2-(aq) → PbS(s)
Metal sulphides are insoluble in water, except Na₂S, K₂S, (NH₄)₂S.

This is a sample diagram. Your answer could be more or less comprehensive; but it should get more extensive as your knowledge advances.