Sulphur

Introduction

Sulphur is element number 16 in the periodic table and has a molar mass of 32 g mol-1.

It takes the form of a yellow solid naturally and can be found in this state near volcanoes. Sulphur is also present in a number of metal ores, for example zinc blende (zinc sulphide, ZnS).

The element exhibits allotropy i.e. it can have more than one molecular crystalline form (cf. carbon). The allotropes of sulphur are called rhombic (diamond-like) and monoclinic (needle-like). The two structures are interchangeable at about 96 °C.

Reactions

(1) Zinc and sulphur :

A mixture of powdered zinc and sulphur, when heated up to a high temperature, will react together to produce an extremely exothermic change. The reaction forms zinc sulphide -

Zn(s) + S(s) ZnS(s)

(2) Combustion :

Sulphur will burn in oxygen gas with a blue flame to produce sulphur dioxide,

S(s) + O2(g) SO2(g)

Sulphur dioxide forms an acidic solution, sulphurous acid, when dissolved in water i.e. it turns damp blue litmus paper red. ( see acid rain )

It will also react with oxidising agents to produce sulphate ions e.g. Orange acidified dichromate(VI) ions are turned green and purple acidified manganate(VII) ions are turned colourless.

Reactions of sulphur dioxide

The main use of sulphur dioxide is in the production of sulphuric acid by the Contact Process.

The key reaction involves the oxidation of sulphur dioxide (SO2) to form sulphur trioxide ( SO3 ) in an equilibrium reaction -

2SO2(g) + O2(g) 2SO3(g)

The yield for the reaction is dependent on a number of factors such as temperature, pressure, ratio of reactants and the use of a catalyst.

A typical set of parameters for the Contact process is

temperature pressure ratio of SO2 : O2 catalyst
400-450 oC 1-2 atm approx. 1.8 : 1 vanadium(V) pentoxide

A high temperature will increase the rate of reaction; however, this causes the product to break down, therefore the yield decreases.

A high pressure will increase the amount of product; however, it costs money to build machinery to withstand high pressures.

The catalyst of vanadium(V) oxide is used to increase the rate of reaction; however, it doesn't change the amount of product formed.

Acid rain

As already mentioned sulphur dioxide forms a mildly acidic solution when dissolved in water, sulphurous acid (H2SO3),

SO2(g) + H2O(l) H2SO3(aq)

Sulphur dioxide is produced by car exhaust engines as well as sulphuric acid plants. When the gas reaches the cloud layer in the atmosphere it dissolves in the water droplets present there and later falls to earth as what is known as acid rain.

Test for sulphate ions

When a solution thought to contain sulphate ions has a few drops of dilute hydrochloric acid added to it followed by a few drops of barium chloride(aq) (BaCl2(aq)), the formation of a white precipitate (barium sulphate, BaSO4) indicates the presence of the sulphate ions.

A general equation for the important reaction occurring is given below,

SO22-(aq) + Ba2+(aq) BaSO4(s)

This shows that it is the sulphate and barium ions that are reacting together to give the white solid and any other ions present are merely "spectators".

Preparation of sulphate salts

(1) Copper(II) sulphate :

This is an example of forming a water soluble salt from an acid and a water insoluble base since all bases containing copper are not soluble in water, e.g. copper(II) oxide and copper(II) carbonate.

Therefore a filtration step is needed.

(2) Sodium sulphate :

This is an example of forming a water soluble salt from an acid and a water soluble base since all bases containing sodium are soluble in water, e.g. sodium hydroxide and sodium carbonate.

Therefore titration is needed in this reaction to ensure that equal amounts of both the acid and the base react with one another.