14.1 Oxides of Sulphur

  p Block Elements

Oxides of Sulphur

Sulphur forms a number of oxides. These are:

(i) Sulphur suboxide

(ii) Sulphur monoxide

(iii) Sulphur sesquioxide

(iv) Sulphur dioxide

(v) Sulphur trioxide

(vi) Sulphur heptoxide

(vii) Sulphur tetroxide

Out of these, sulphur dioxide and sulphur trioxide are common and important and described here:

1. Sulphur dioxide,

The following reactions can be used for the preparation of sulphur dioxide.

(a) By burning sulphur in air or oxygen.

(b) By heating Sulphur with concentrated sulphuric acid.

(c) By heating copper or silver with concentrated sulphuric acid

(d) By reacting sodium sulphite or sodium hydrosulphite with dil. HCl.

(e) By roasting of sulphides

The reactions $b$, $c$ and $d$ can be used or the preparation of $SO_2$ in the laboratory while the reactions (a) and (e) are used in industry to obtain $SO_2$.

In the places where gypsum or anhydride are found in abundence, sulphur dioxide can be obtained on a large scale by heating gypsum or anhydride with carbon at about $1000^\circ C$.

Physical Properties:

(a) It is a colorless gas with a pungent, suffocating odour.

(b) It is a highly soluble in water.

Chemical Properties:

(a) It neither burns nor helps in burning, However, burning Magnesium and Potassium continue to burn in its atmosphere.

(b) Thermal decomposition: It decomposes at $1200^\circ C$ producing sulphur trioxide and sulphur.

(c) Acidic nature: It is an acidic oxide. It dissolves in water forming sulphurous acid.

The gas is, therefore, known as sulphurous anhydride. It combines with basic oxides and forms sulphites.

(d) Addition reactions:

(i) It combines directly with halogens to form sulphuryl halides.

(ii) It combines with oxygen in presence of a catalyst (Platinesed asbestos)

(iii) It directly combines with lead dioxide to form lead sulphate.

(e) Reducing nature: The aqueous solution of $SO_2$ acts as a reducing agent as sulphurous acid canges into sulphuric acid giving nascent hydrogen.

Examples:

(i) It reduces halogens to their respective hydracids.

(ii) Acidified $KMnO_4$ is reduced, i.e. it is decolorised.

(iii) Acidified $K_2Cr_2O_7$ is turned green due to the formation of chromic sulphates.

(iv) Ferric sulphate is reduced to ferrous sulphate.

(v) it reduces acidified iodate to iodine

(f) Oxidizing nature: It also acts as an oxidizing agent particularly when treated with stronger reducing agents.

(i) it oxidises $H_2S$ into $S$.

(ii) Metals are oxidised by $SO_2$

(iii) $CO$ is oxidised to $CO_2$ by $SO_2$

(iv) At $1000^\circ C$, it oxidises hydrogen into water.

(v) Stannous and mercurous salts are oxidised in presence of excess of $HCl$.

(g) Bleaching action: $SO_2$ in presence of moisture acts as a bleaching agent. This is due to the reducing nature of $SO_2$.

The bleaching is temporary. The bleached matter, when exposed to air, regains its colour due to oxidation.

Uses:

(i) As a refrigent in the form of liquid $SO_2$.

(ii) It is used as an antichlor for removing chlorine from a fabric after bleaching.

2. Sulphur Trioxide, $SO_3$

Preparation: Sulphur trioxide is obtained:

(i) by dehydrating sulphuric acid with excess of phosphorous pentoxide.

(ii) by heating ferric sulphate strongly.

(iii) by oxidation of $SO_2$ with oxygen in presence of a catalyst (Commercial method)

Both $SO_2$ and $O_2$ should be completely dry. The mixture is passed over platinised asbestos at $400^\circ C$, when $SO_3$ is formed.

Properties:

(i) $SO_3$ exists in three allotropic forms.

(a) $\alpha - S{O_3}$ It is chemically active form. It forms long transparent ice like crystals. The melting point of this form is $17^\circ C$.

(b) $\beta - S{O_3}$ It is dimeric $S_2O_6$, It forms needle like silky white crystals. It melts at $32.5^\circ C$. Above $50^\circ C$, it changes to $\alpha $-form.

(c) $\gamma - S{O_3}$ It is like $\beta $-form and obtained by drying $\beta - S{O_3}$. It melts at $62.2^\circ C$ under 2 atmospheric pressure.

(ii) $S{O_3}$, it is an acidic oxide. It dissolve in water forming sulphuric acid with evolution of heat.

(iii) It dissolves in concentrated sulphuric acid forming oleum (fuming sulphuric acid)

(iv) It dissociates on heating at $1000^\circ C$ into $SO_2$ and $O_2$.

(v) It combines with concentrated hydrochloric acid forms chloro-sulphonic acid, a derivative of sulphuric acid.

(vi) $SO_3$ acts as an oxidising agent.

Uses:

(i) In the manufacture of sulphuric acid and oleum.

(ii) It is used as a drying agent for gases.