Sulphur and its compounds (O-level chemistry)
Sulphur
Sulphur occurs naturally in three forms:
1. Free form in Japan, Texas and Louisiana.
Extraction of sulphur
In order to extract this underground sulphur (the Frasch process), three concentric pipes are sunk deep into the ground as show in figure 1. Superheated water at 1700C is forced down the outer pipe into the sulphur which is melted; compressed air is blown through the inner pipe forcing sulphur as a liquid to the surface, where it is allowed to solidify. Sulphur of about 99.5% purity is obtained by this process.
2. In the form of sulphides of zinc, lead, copper, iron, hydrogen and mercury.
Sulphur dioxide is obtained as a by-product in the course of extraction of these metals. From hydrogen sulphide, sulphur can be obtained by first, burning hydrogen sulphide to form sulphur dioxide and this is then reacted with more hydrogen sulphide.
2H2S (g) + 3O2 (g) → 2H2O (g) + 2SO2 (g)
2H2S (g) + SO2 (g) → 2H2O (l) + 3S (s)
3. As metal sulphate, e.g., gypsum, CaSO4.2H2O, and epsom salt, MgSO4.7H2O.
The allotropy of sulphur
Sulphur exists in a number of allotropic varieties depending on the method of preparation and temperature (fig.2).
The differences between Rhombic and monoclinic sulphur
| Rhombic sulphur | Monoclinic sulphur |
| Stable below 960C | Stable below 960C |
| Octahedral crystals | Needle-shaped crystal |
| Bright yellow | Pale yellow |
| M.pt 1130C | M.pt 1190C |
| Density higher (2.06g/cm3) | Density lower (1.98 g/cm3) |
Facts to prove that Rhombic and monoclinic sulphur are allotropes of sulphur
- One gram of monoclinic slowly changes at room temperature into one gram of rhombic sulphur.
- One gram of either form will burn in oxygen to yield the same mass (2g) of sulphur dioxide.
Uses of sulphur
- Manufacture of sulphuric acid
- For dusting wines to prevent the growth fungus
- For the vulcanization of rubber
- For manufacture of dye,
- Sulphur dioxide bleaches sugar
Chemical properties
- Sulphur burns in air to form sulphur dioxide.
S(s) + O2 (g) → SO2(g)
- It combines with metals and non-metal when heated to give sulphides.
Mg (s) + S (s) → MgS (s)
Fe (s) + S(s) → FeS(s)
S (s) + 3F2 (g) → SF6 (g)
2S (s) + Cl2 (g) → S2Cl2 (g)
C (s) + 2S (s) → CS2 (l)
S (s) + O2 (g) → SO2 (g)
H2 (g) + S (l) ↔ H2S (g)
- Sulphur is oxidized by concentrated nitric and sulphuric acid to sulphuric acid and sulphur dioxide respectively.
S (s) + 6HNO3 (l) → 2H2O (l) + H2SO4 (aq) + 6NO2 (g)
S (s) + 2H2SO4 (l) → 2H2O (l) + 3SO2 (g)
Compounds of sulphur
Hydrogen sulphide
It is a bad smelling covalently bonded gas.
Preparation
By reaction between metal sulphides with aqueous hydrochloric acid.
FeS (s) + 2HCl (aq) →FeCl2 (aq) + H2S (g)
Or FeS(s) + H2SO4(s) → FeSO4(aq) + H2S (g)
Testing for hydrogen sulpide
Turns a strip of paper dipped in lead ethanoate solution brown or black
Pb2+ (aq) + S2- (aq) → PbS(s)
Properties of hydrogen sulphide
- Has rotten egg smell
Sulphur dioxide or sulphur (IV) oxide
Preparation
By reacting copper with hot concentrated sulphuric acid
Cu(s) + 2H2SO4 (aq) → CuSO4(aq) + 2H2O (l) + SO2 (g)
Or
By action of concentrated sulphuric or hydrochloric acid on sodium sulphite or hydrogen sulphite
Na2SO3 (s) + H2SO4 (aq) → Na2SO4(aq) + H2O(l) + SO2(g)
Setup of apparatus
- It turns the colour of acidified potassium dichromate from orange to green
- Decolorizes acidified potassium permanganate
Uses
1. For the manufacture of sulphuric acid.
2. For bleaching silk, straw and sponges.
3. As a preservative for fruits, fruit juices and grains.
4. As an antichlor for removing chlorine from fabrics after bleaching.
5. For the preparation of NaHSO3 and Ca(HSO3)2.
6. For refining sugar and petroleum.
Sulphuric acid, H2SO4
Sulphuric acid is mainly manufactured by the contact process, in which sulphur dioxide is oxidised to sulphur trioxide, which is then reacted with water to give sulphuric acid.
2SO2 (g) + O2 (g) ↔ 2SO3 (g)
SO3 (g) + H2O (l) → H2SO4 (l)
Sulphur dioxide is obtained by burning sulphur in the air. Alternatively, it is obtained as a by-product in the extraction of metals from sulphide ores.
2ZnS (s) + 3O2 (g) → 2ZnO (s) + 2SO2 (g) (During roasting)
The contact process
In sulphur dioxide is reacted with oxygen in the presence of Vanadium (V) oxide (catalyst) to form sulphur trioxide. The conversion of sulphur dioxide and oxygen into sulphur trioxide (sulphur (VI) oxide) is an exothermic reaction with a decrease in gas volume.
2SO2 (g) + O2 (g) ↔ 2SO3 (g) ΔHθm (298 K) = -197.6 kJ mol-1.
Condition for production of sulphur trioxide
Temperature: 4200C
Catalyst: vanadium (V) Oxide
Preparation of sulphuric acid from sulphur trioxide
Sulphur trioxide is dissolved in 98% sulphuric acid to form oleum.
H2SO4 (aq) + SO3 (g) ↔ H2S2O7 (l) (oleum)
The oleum is later diluted with water to produce sulphuric acid.
H2S2O7 (l) + H2O (l) → 2H2SO4 (l)
Note that sulphur trioxide is not dissolved in water directly because this produces a lot of heat and corrosive fumes.
Physical properties of sulphuric acid
Pure sulphuric acid is a viscous liquid with a density of 1.85 g cm-3 and a freezing point of 10.50C. In the absence of water, it does not turn litmus red nor does it react with metals to form hydrogen. It decomposes on boiling (b.pt. 270 0C) to form sulphur trioxide and water, and a constant boiling mixture (azeotrope) is formed, containing 98.3% of acid.
Its high viscosity and boiling point are due to strong intermolecular hydrogen bonding.
Fig. 5 Extensive intermolecular hydrogen bonds in sulphuric acid
Chemical properties
a) Acidity
Sulphuric acid ionises in water to form a strong dibasic acid.
H2SO4 (l) + H2O (l) ↔ H3O+ (aq) + HSO4– (aq)
HSO4– (aq) + H2O (l) ↔ H3O+ (aq) + SO42- (aq)
It neutralizes bases to form hydrogen sulphate and sulphate. It displaces carbon dioxide from carbonates and reacts with strong electropositive metals to form salts and hydrogen.
H2SO4 (l) + OH– (aq) → H2O (l) + HSO4– (aq)
HSO4– (aq) + OH– (aq) → H2O (l) + SO42- (aq)
CO32- (aq) + 2H+ (aq) → CO2 (g) + H2O (l)
Ca (s) + 2H+ (aq) → Ca2+ (aq) + H2 (g)
b) As a dehydrating agent
Concentrated sulphuric acid has such an affinity for water that is, it removes water from mixtures and compounds with the evolution of much heat and because of this, the acid should always be diluted by pouring it into water while stirring and not the other way round.
The concentrated acid absorbs water vapour from moist air and other moist substances (i.e., it is hygroscopic). It is used as a drying agent for gases (except those that react with the acid) and in desiccators. When left exposed to air, it absorbs water and increases in volume
It dehydrates glucose and sugar to a black mass of carbon
It dehydrates ethanol to ethene
Being hygroscopic, it absorbs water from atmosphere and increases in volume.
(c) Oxidizing properties.
Hot sulphuric acid oxidizes hydrogen sulphide to sulphur.
3H2S (g) + H2SO4 (l) → 4H2O (l) + 4S (s)
It oxidizes carbon and sulphur to carbon dioxide and sulphur dioxide respectively
C (s) + 2H2SO4 (l) → 2SO2 (g) + CO2 (g) + 2H2O (l)
S (s) + 2H2SO4 (l) → 3SO2 (g) + 2H2O (l)
It oxidizes copper to blue copper (II) sulphate.
Cu (s) + 2H2SO4 (l) → CuSO4 (aq) + SO2 (g) + 2H2O (l)
Zn (s) + 2H2SO4 (l) → ZnSO4 (aq) + SO2 (g) + 2H2O (l)
(98% acid)
d) Displacement reaction.
Because sulphuric acid is a strong acid and not easily vaporized, it displaces other more volatile acids on warming with their salts.
NaNO3 (s) + H2SO4 (l) → NaHSO4 (aq) + HNO3 (g)
NaCl (s) + H2SO4 (l) → NaHSO4 (aq) + HCl (g)
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