Sulfur is a non-metallic element, which is very
important in the chemical industry.
Sources
It is found as the element in sulphur beds 200m
below the ground in volcanic areas and also in metal ores such as copper
pyrites (iron sulphides) and zinc blende and also in volcanic areas.
Extraction
Sulfur is extracted using the Frasch Process.
Superheated water at 170°C
and
hot compressed air are pumped into layers of sulphur through pipes. This forces
water and molten sulfur to the surface. Sulfur is insoluble in water and so the
two substances emerging from the pipes are easily separated. The sulfur is
about 99.5% pure and can be used directly.
Properties
The Frasch Process, as a result of some of the following properties, extracts sulfur:
- brittle yellow solid
- melting point of 119°C
- non-conductor of heat and electricity
- burns with blue flame when lit
- insoluble in water
Uses of Sulfur
The main use of sulfur is to manufacture sulfuric acid, quite possibly the most important industrial chemical. Sulfur is also used for:
- vulcanising rubber (making it stronger) in tyres, etc.
- sulfur concrete (used in acid factories) which is resistant to acids
- smaller quantities in matches, fireworks and fungicides, as a sterilising agent and in medicines
Sulfur Dioxide
Sulfur dioxide is a colourless gas, extremely poisonous and has a choking smell. It is produced when sulfur or substances containing sulfur, such as crude oil or coal, are burned in oxygen gas.
Sulfur dioxide is considered a ‘problem pollutant’
in two main ways
it is an acidic oxide that dissolves in water to
form sulphurous acid, which leads to the problem of acidic rain.
SO2(g) + H2O(l) = H2SO3
it causes bronchospasm in asthmatics, therefore
considered a respiratory irritant.
Despite this sulfur dioxide is also very useful for
the following reasons:
- as a bleaching agent especially in the manufacture of wood pulp for paper
- as a food preservative (by killing bacteria) for meats, etc.
- manufacturing of sulphuric acid (as is elemental sulphur)
Sources
- combustion of fossil fuels with sulfur impurities
- volcanic eruptions
- rotting vegetation
Contact Process (Industrial Manufacturing of Sulfuric Acid)
The process has the following stages:
Sulfur dioxide is first produced, primarily by the
reaction (through combustion) of sulfur with oxygen.
S(s) + O2(g) = SO2(g)
Any dust and impurities are removed resulting in
‘clean gases’ consisting of unreacted oxygen and sulfur dioxide. These gases
are heated to a temperature of approx. 450°C and fed
into a reaction chamber of pressure 1-10 atmospheres.
In the chamber the clean
gases undergo ‘roasting’ where they are passed over beds of catalyst, namely
vanadium(V) oxide (V2O5). This catalyses the reaction between sulfur dioxide
and oxygen to produce sulfur trioxide (SO3)
**2SO2(g)
+ O2(g) = 2SO3(g)**
If the
sulfur trioxide is added directly to water sulfuric acid is produced. The
reaction however produces undesirable effects. It is very violent and very
exothermic and produces a thick mist of sulfuric acid = very dangerous.
S03 +
H2O = H2SO4
The acid
produced is very difficult to deal with and so a different route to sulfuric
acid is employed. Instead, the sulfur trioxide is dissolved in concentrated
sulfur acid to give a substance called oleum (H2S2O7)
SO3 +
H2SO4 = H2S2O7
The
oleum formed in then added to the correct amount of water to produce sulfuric
acid of the require concentration.
H2S2O7 +
H20 = 2H2SO4
** The
reaction between sulfur dioxide and oxygen to form sulfur trioxide is
reversible. So the ideas of Le Chatelier can be used to increase the proportion
of sulfur trioxide in the equilibrium mixture. The forward reaction is
exothermic an1d so would be favoured by low temperatures. The temperature of
450°C used is an optimum temperature which produces sufficient amounts of
sulfur trioxide at an economical rates.
Since
the back reaction is also accompanied by a decrease in the number of molecules
of gas, a high pressure will favour it.
**
Properties of Sulfuric Acid
Dilute
sulfuric acid acts as a typical acid by:
- reacting with reactive metals to form a salt and hydrogen gas
- reacting with carbonates to form a salt, carbon dioxide and water
- reacting with bases such as metal oxides to form a salt and water
Concentrated
sulfuric acid acts as a powerful dehydrating agent – it will take water from a
variety of substances.
Examples of these substances include:
sucrose
(cane sugar)
white
crystals become black solid
C12H22O11(s)
= 12C(s) + 11H2O(l)
water of
crystallisation substances
blue
hydrate crystals = white anhydrous powder
CuSO4.5H2O(s)
= CuSO4(s) + 5H2O(l)
Uses of Sulfuric Acid
As said before sulfuric acid is arguably the most
important industrial chemical.
It is mainly used for the raw material for the
manufacturing of many substances:
- fertilisers
- chemicals
- steel manufacture
- soaps and detergents
- fibres
- paints, pigments and dyestuffs
- car batteries