Chemistry > p Block Elements > 14.0 Sulphur
p Block Elements
1.0 Group $13$ – The Boron Family
2.0 Boron
3.0 Compounds of boron
4.0 Compounds of Aluminium
5.0 Group $14$ – The Carbon family
6.0 Allotropes of Carbon
7.0 Compounds of Carbon
8.0 Properties of Silicon
9.0 Group $15$-The Nitrogen Family
10.0 Oxides of nitrogen
10.1 Nitrogen Oxide $N_2O$ or Laughing gas (Neutral)
10.2 Nitric Oxide $NO$ (Neutral)
10.3 Nitrogen trioxide $N_2O_3$
10.4 Nitrogen dioxide or Di-nitrogen tetroxide $NO_2$ or $N_2O$
10.5 Nitrogen pentaoxide $N_2O_5$
11.0 Oxyacids of Nitrogen
11.1 Nitric acid $HNO_3$
11.2 Oxidation of Metalloid and Inorganic compounds by Nitric acid
11.3 Action of Metals & Proteins
12.0 Phosphorus
13.0 Oxygen
14.0 Sulphur
14.2 Oxyacids of Sulphur
10.2 Nitric Oxide $NO$ (Neutral)
10.3 Nitrogen trioxide $N_2O_3$
10.4 Nitrogen dioxide or Di-nitrogen tetroxide $NO_2$ or $N_2O$
10.5 Nitrogen pentaoxide $N_2O_5$
11.2 Oxidation of Metalloid and Inorganic compounds by Nitric acid
11.3 Action of Metals & Proteins
Oxyacids with $S-S$ links are called thioacids. Acids having sulphur in lower oxidation state belong to $-ous$ series while those having sulphur in higher oxidation state belong to $-ic$ series. For example:
Sulphurous acid $(H_2SO_3)O.N$, sulphur, $+4$
Sulphuric acid $(H_2SO_4)O.N$, sulphur, $+6$
The following are main oxyacids of sulphur.
1. Sulphurous acid series:
(i) Sulphurous acid, $H_2SO_4$
(ii) Thiosulphurous acid, $H_2S_2O_3$
(iii) Hyposulphurous acid, $H_2S_2O_4$
(iv) Pyrosulphurous acid, $H_2S_2O_5$
(i) Sulphurous acid, $H_2SO_4$
This acid is known in solution. The solution is obtained by dissolving sulphur dioxide in water.
The solution gives a smell of suphur dioxide which is evolved completly on heating. it is thus, belived that the acid is present in equillibrium with the free gas.
Like $SO_2$, it acts as a reducing agent and shows bleaching properties. These properties have been described under sulphur dioxide.The acid also acts as an oxidising agnet particularly when treated with strong reducing agents.
(i) The preperation of sulphurous acid by the action of $H_2O$ on thionyl chloride.
(ii) The molecule of sulphurous acid readily takes up oxugen or sulphur atom. This can ne explained if symmetrical formula is accepted.
2. Sulphuric acid series:
(i) Sulphuric acid, $H_2SO_4$
(ii) Thiosulphuric acid $H_2S_2O_3$
(iii) Pyrosulphuric acid $H_2S_2O_7$
Sulphuric acid (oil of vitriol) $H_2SO_4$
In ancient days, it was called oil of vitriol as it was prepared by distilling ferrous sulphate (Green Vitrol).
Native sulphur is the starting material for the preparation of suplhuric acid. Sulphur is burnt and easily converted into sulphur sioxide. The conversion of sulphur sioxide into suplhur trioxide is a matter of difficulty. It requires either a catalyst or a strong oxidising agent. The trioxide dissolves in water to form sulphuric acid. The whole process may be summarised as:
For the oxidation of sulphur dioxide into sulphur trioxie, the following reactions may be used:
(i) By atmospheric oxygen in presence of a catalyst, $V_2O_3$ at $400^\circ C$
(ii) By $Cl_2$ or $Br_2$ in presence of moisture.
(iii) By $H_2O$
(iv) By ozone $(O_3)$
(v) By Conc. $HNO_3$
Manufacture: sulphuric acid is manufactured these days by the following two processes:
(a) Lead Chamber process
Principle: The mixture containing sulphur dioxide, air and nitrite oxide hen treated with steam, sulphuric acid is formed.
Nitric oxide acts as a catalyst in this reaction.
(b) Contact Process
Principle: The process involves the oxidation of sulphur dioxide by air in the presence of a catalyst.
Sulphur trioxide dissolved in $98\%$ sulphuric acid when oleum is formed.
Sulphur acid of any desired concentration can be obtained from oleum by dilution eith water.
The oxidation of suplhur dioxide is reversible and exothermic.
Comparison between the Lead Chamber and Contact Process
| Lead Chamber Process | Contact Process |
| The catalyst used in a gas-nitric oxide | catalyst used is a solid vanadium pentoxide or platinum asbestos. |
| Acid obtained is impure. | Acid obtained is pure. |
| Acid obtained is 80%. | Oleum is obtained. Sulphuric acid of any concentrtion can be obtained from oleum. |
| Methods of concentration of the acid are troublesome. | Further concentration is not necessary. |
| It is not necessary to purify the gases. | It is absolutely necessary to purify the gases as impurities act as a poison to the catalyst. |
| It is a cheaper method. | It is a costlier method. The cost had been reduced by the use of $V_2O_5$ in place of platinum. |
Physical properties:
(a) It is a colourless syrupy liquid. It contains $98.3\%$ sulphuric acid. Its specific gravity is $1.84$ at $15^\circ C$.
(b) It fumes strongly in moist air and is highly corrosive in nature.
(c) It is highly soluble in water. Various hydrates such as ${H_2}S{O_4},{H_2}O,{H_2}S{O_4}.2{H_2}O,{H_2}S{O_4}.4{H_2}O$ are known. Heat is evolved when dissolved in water, usually bumping occurs.
Water should not be added to concentrated sulphuric acid for dilution but concentrated sulphuric acid should be added slowly to cold water with constant stirring. Due to its great affinity for water, It is used as a dehydrating substance.
(d) It is a conductor of heat and electricity.
Chemical Properties:
(a) Dissociation: At $44^\circ C$,the vapours of sulphuric acid dissociate into steam and sulphuric trioxide,
(b) Acidic nature: Dilute sulphuric acid is a strong dibasic acid. It ionizes as:
(i) It forms two series of salts with bases,
(ii) It decomposes carbonates and bicarbonates into carbon dioxide.
(iii) It reacts with electropositive metals evolving hydrogen.
(iv) It displays more volatile acids from their metal salts.
(c) Oxidizing nature: It acts as a strong agent. The potential equation is:
- Non-metals such as carbon and sulphur are oxidized to their oxides. Phosphorous is oxidized to ortho-phosphoric acid.
- metals like copper, silver, mercury etc. are first oxidised to conc. $H_2SO_4$ and then oxides combines with aid to form corresponding sulphates.
- Iodine is liberated from $KI$ ($HI$ is oxidized to $I_2$). Similarly bromine is liberated from $KBr$ ($HBr$ is oxidised to $Br_2$)
- In presence of $Hg$ as a catalyst, naphthalene is oxidized to pthalic acid.
(d) Dehydrating nature: As sulphuric acid has great affinity for water, it acts as a powerful dehydrating agent. Its corrosive action on skins is also due to dehydration of skin. It absorbs water from organic compounds.
Cloth, wood, starch, paper etc, are all charred in concentrated sulphuric acid.
(e) Miscellaneous reactions:
- Sulphonation: Aromatic compounds reacts with sulphuric acid forming sulphonic acid.
- Action of $PCl_5$, The molecule of sulphuric acid contain two hydroxyl groups. One or both hydroxyl groups can be replaced by chlorine when treated with $PCl_5$.
- On heating potassium ferrocyanide with concentrated sulphuric acid, carbon monoxide is evolved.
- On heating potassium chlorate with concentrated sulphuric acid, carbon dioxide is evolved with explosion.
3. Thionic acid series:
(i) Dithionic acid, $H_2S_2O_6$
(ii) Polythionic acid, $H_2S_8O_6$
4. Peroxy acid series:
(i) Peroxy monosulphuric acid, $H_2SO_5$ (Caro's acid)
(ii) Peroxy disulphuric acid, $H_2S_2O_8$ (Marshall's acid)
