7.8.1 Sulphur Chemistry

The S₈ unit; rhombic, monoclinic and plastic forms. Combustion. Occurrence as an impurity in fuels.

Sulphur

Sulphur occurs as deposits of free sulphur in America, Japan and Sicily. Sulphur exists as a covalently bonded, molecular solid at room temperature with the formula S₈.

S S S

S S

S

S S

These large molecules each have many electrons , so the Van der Waals forces are quite strong and the melting point is quite high (119^oC). There are two ways of packing the sulphur rings, so solid sulphur exists in two crystalline forms, called rhombic and monoclinic.

Allotropes of sulphur

Rhombic sulphur Monoclinic sulphur

Yellow transparent crystals Amber coloured needles

Melt pt. 113 ^oC Melt pt. 119 ^oC

Obtained when sulphur crystallises Obtained when sulphur

from solution. solidifies above 95.6 ^oC.

At atmospheric pressure the rhombic form is stable below 95.6 ^oC and the monoclinic form above this temperature. Only at the transition temperature can the two allotropes exist in equilibrium with each other.

When sulphur is heated it melts and undergoes a series of changes as the temperature rises.

SOLID SULPHUR S₈ rings

TRANSPARENT YELLOW LIQUID Rings open forming S₈ chains.

COLOUR DARKENS. LIQUID Long chains of about 10⁵ sulphur atoms

REACHES MAXIMUM VISCOSITY become entangled and make the liquid

AT 200 ^oC. viscous.

LIQUID MOBILE AT 400 ^oC. Chains break up to form smaller units and

The liquid becomes mobile.

LIQUID SULPHUR BOILS AT 444 ^oC.

SULPHUR VAPOUR Vapour contains S₈, S₄ and S₂ molecules.

SOLIDIFIES AS S₈(s) S₈ molecules.

If liquid sulphur is cooled suddenly the element solidifies before all the atoms can rearrange themselves into rings again. The product is called plastic sulphur and consists of a mixture of ring and chain molecules. Its elasticity arises, as does that of rubber, from its capacity to revert to a tangled mass of zigzag chains after stretching. When left to stand at room temperature, the atoms slowly reform the ring molecules and the elasticity disappears.

Combustion of sulphur

When heated in air, sulphur burns with a blue flame forming the colourless gas sulphur dioxide.

S (s) + O₂ (g) SO₂ (g)

Sulphur is present as an impurity in petroleum and natural gas and contributes to the production of acid rain when these fuels are burnt.

[See Topic 21.14 ACID RAIN – Ramsden A Level Chemistry 3^rd Edition p 435 – 439]

Detection of sulphur dioxide and sulphurous acid using acidified MnO₄^- and Cr₂O₇^2-. Sulphites and their reactions with Fe³⁺, Ba²⁺ and halogens.

Sulphur Dioxide

Moist sulphur dioxide (or sulphurous acid) is a reducing agent.

SO₃^2- + H₂O � SO₄^2- + 2H⁺ + 2e^- E^θ = -0.17 V

This fact is used as a test for the detection of sulphur dioxide

1. There is a colour change from purple (pink in dilute solution) to colourless on the addition of the gas to a solution of potassium manganate (VII) (permanganate)

2MnO₄^- + 5SO₂ + 2H₂O 2Mn²⁺ + 5SO₄^2- + 4H⁺

2. There is a colour change from orange to blue on adding the gas to a solution of potassium dichromate (VI).

Cr₂O₇^2- + 3SO₂ +2H⁺ 2Cr³⁺ + 3SO₄^2- + H₂O

Sulphurous acid and Sulphites

Sulphur dioxide dissolves in water forming sulphuric (IV) acid (sulphurous acid).

SO₂ (aq) + H₂O (l) � H₂SO₃ (aq)

This is a weak dibasic acid and ionises producing hydrogensulphite ionsHSO₃^- and sulphite ions SO₃^2- ions.

H₂SO₃ � H⁺ + HSO₃^- � 2H⁺ + SO₃^2-

1. Sulphites give sulphur dioxide on heating with dilute acids.

Na₂SO₃ + 2HCl NaCl + SO₂ + H₂O

2. With barium chloride they give a white precipitate of barium sulphite which is soluble in dilute hydrochloric acid.

Ba²⁺ (aq) + SO₃^2- (aq) BaSO₃ (s)

white precipitate

BaSO₃ (s) + 2HCl BaCl₂ + SO₂ (g) + H₂O

This reaction is used as a test for sulphite ions in solution.

3. Sulphite ions (reducing agent) are oxidised to sulphate by air, chlorine, iron (III), dichromate (VI) and manganese (VII).

Write balanced equations for the reactions between

(a) SO₃^2- (aq) + Fe³⁺ (aq)

(b) SO₃^2- (aq) + Cl₂ (aq)

Industrial manufacture of sulphuric acid from sulphur. Consideration should be given to how kinetic and equilibrium principles influence the choice of reaction conditions. The chemistry of the use of sulphuric acid in making phosphate fertilisers and ammonium sulphate fertilisers.

Research the industrial manufacture of sulphuric acid from sulphur by the Contact process including the considerations as outlined above.

REFERENCES : A Level Chemistry (3^rd edition) E. N. Ramsden (Stanley Thornes Ltd.)

21.11 sulphuric acid p429-430

Chemistry In Context (4^th edition) Graham Hill & John Holman

(Nelson)

22.5 applying the principles of reaction rates and equilibria to

industrial processes p348-351

Catalytic properties p297-298

The Contact process

A mixture of sulphur dioxide and air (approximately 8.5% SO₂ and 12.5% O₂ by volume) is passed over a vanadium (V) oxide catalyst at a temperature of about 430 ^oC. The reaction is exothermic and the temperature rises to about 600 ^oC. The mixture is cooled to about 430 ^oC by passage through a heat exchanger (the heat extracted being used to heat the initial sulphur dioxide/air mixture), and at this stage the conversion of sulphur dioxide to sulphur trioxide is about 66% complete.

V₂O₅

2SO₂ (g) + O₂ (g) � 2SO₃ (g) ΔH^θ (298K) = -196kJ mol^-1

430 ^oC

The mixture is then passed through three more converters, at each stage the emerging gas stream being cooled to 430 ^oC as described above before passing from one converter to another. The final gas stream (overall conversion of sulphur dioxide to sulphur trioxide being about 98% complete) is then cooled, and the sulphur trioxide absorber in 98% sulphuric acid to give fuming sulphuric acid or oleum, H₂S₂O₇. Dilution of oleum with water gives sulphuric acid of any desired concentration.

SO₃ (g) + H₂SO₄ (l) H₂S₂O₇ (l)

oleum

H₂S₂O₇ (l) + H₂O (l) 2H₂SO₄ (l)

In order to reduce pollution from the discharge of unreacted sulphur dioxide into the atmosphere, modern Contact plants are designed to absorb the sulphur trioxide from the gas stream in 98% sulphuric acid after it emerges from the third converter. The unchanged sulphur dioxide (with air) is then allowed to pass through the fourth converter and the extra amount of sulphur trioxide similarly absorbed. In this modified process only about 0.05% of the sulphur dioxide remains unconverted and is allowed to pass into the atmosphere.

Since the catalytic oxidation of sulphur dioxide to sulphur trioxide is an exothermic reaction, a high yield of the trioxide will be favoured by a reasonably low temperature (Le Chatilier’s principle) , but the temperature must not be too low or the rate of reaction will become too slow. A temperature of about 430 ^oC is the optimum one.

The reaction takes place with a decrease in volume, hence an increase in pressure should increase the equilibrium yield of sulphur trioxide and also the rate of reaction. Since both yield and reaction rate are quite satisfactory at a pressure slightly greater than atmospheric, the extra expense of making pressure equipment is not justified.

Dilute sulphuric acid as a typical strong acid illustrated by displacement of weaker acids from their salts, reactions with metals; the concentrated acid as a dehydrating agent (sugar and hydrated copper sulphate), oxidising agent with copper, bromides and iodides, role in nitration.

Chemical properties of sulphuric acid

Dilute H₂SO₄

In aqueous solution dilute sulphuric acid functions as a typical strong acid. The ionisation

H₂SO₄ H⁺ + HSO₄^- is virtually complete

The ionisation HSO₄^- � H⁺ + SO₄^2- is about 10% complete

· It displaces weak acids from their salts.

2CH₃COONa + H₂SO₄ 2CH₃COOH + Na₂SO₄

· It reacts with metals displacing hydrogen.

Zn + H₂SO₄ ZnSO₄ + H₂

· It also reacts with oxides, hydroxides and carbonates in typical acid fashion.

2H⁺ + O^2- H₂O

H⁺ + OH^- H₂O

2H⁺ + CO₃^2- H₂O + CO₂

Concentrated H₂SO₄

When concentrated sulphuric acid removes water from a mixture it is termed a drying agent. When it removes the elements of water from a compound and forming a new compound it is described as a dehydrating agent.

1 As a dehydrating agent

It removes the elements of water from a compound forming a new compound. Some dehydrating reactions of the concentrated acid are;

· Sugar

When the concentrated acid is added to sugar the white crystals darken turning brown and then black. Heat is evolved and the black mass swells to a large volume.

conc. H₂SO₄

C₆H₁₂O₆ 6C + 6H₂O

glucose sugar charcoal

· Hydrated copper sulphate crystals

When added to hydrated copper sulphate crystals the colour changes from blue to white showing the formation of the anhydrous salt.

conc. H₂SO₄

CuSO₄.5H₂O CuSO₄ + 5H₂O

blue white

2 As an oxidising agent

Hot concentrated sulphuric acid is an oxidising agent.

SO₄^2- (aq) + 4H⁺ (aq) + 2e^- � SO₂ (aq) + 2H₂O (l) E^θ = +0.17 V

· Metals are oxidised to sulphates

Cu (s) + 2H₂SO₄ (l) CuSO4 (aq) + SO2 (g) + 2H2O (l)

· Compounds are oxidised with the formation of SO₂ or H₂S

Addition of concentrated sulphuric acid to solid halides first produces the hydrogen halide.

X^- (s) + H₂SO₄ (l) HX (g) + HSO₄^- (s)

The acid is able to oxidise HBr to Br₂ and HI to I₂

2HBr (g) + H₂SO₄ (l) Br₂ (l) + SO₂ (g) + 2H₂O (l)

A similar reaction occurs with HI.

3 Role in nitration

Concentrated sulphuric acid is used in the nitration of benzene, along with concentrated nitric acid to produce the nitryl cation NO₂⁺.

HNO₃ + 2H₂SO₄ � NO₂⁺ + H₃O⁺ + 2HSO₄^-

Here nitric acid acts as a base in the presence of the stronger sulphuric acid.

Exercise 1

1 Concentrated sulphuric acid reacts with sodium iodide in a two-stage reaction to give iodine. Explain how sulphuric acid is reacting, and write equations for the two steps.

2 Concentrated sulphuric acid reacts with potassium bromide to form a solid and three gases. Name these products, and explain how they are formed.

3 What would you expect to see when concentrated sulphuric acid is added to copper(II) sulphate crystals?

4 Phosphorus(V) oxide, P₂O₅, is a more powerful dehydrating agent than sulphuric acid. What is the fuming gas that is formed in a reaction between the two chemicals?

5 Give examples, with equations, of reactions in which sulphuric acid acts as (a) an acid, (b) a dehydrating agent, (c) an oxidising agent, (d) a sulphonating agent.

6 The Contact process achieves 95% conversion of sulphur dioxide. What happens to the rest?

Laboratory preparation of sodium thiosulphate from sulphur and sodium sulphite. Supersaturation of sodium thiosulphate. The reactions of thiosulphate ion with acid, halogens and silver ions. The usefulness of the thiosulphate ion as an antichlor in photography.

Sodium thiosulphate Na₂S₂O₃

This is a sulphate in which an oxygen atom has been replaced by a sulphur atom giving the ion S²O₃^2-. Crystalline sodium thiosulphate Na₂S₂O₃. 5H₂O is commonly known as ‘hypo’.

It is made by boiling a solution of sodium sulphite with sulphur for several hours. After filtration and evaporation , crystals of Na₂S₂O₃. 5H₂O are obtained.

SO₃^2- (aq) + S (s) Na₂S₂O₃ (aq)

These crystals melt at 48^oC in their own water of crystallisation and the melt shows supersaturation on cooling.

Preparation of sodium thiosulphate

Boiling sodium sulphite solution will gradually dissolve sulphur.

Na₂SO₃ (aq) + S (s) Na₂S₂O₃ (aq)

Set up a 100 cm³ round bottomed flask with a reflux condenser and put into it 10 g (0.04 mole) of sodium sulphite heptahydrate, 50 cm³ of water and 1.5 g (slight excess) of crushed rhombic sulphur crystals.

Boil gently for about 2 hours, or until no more sulphur dissolves.

Filter off the excess sulphur, then evaporate the filtrate to about 10 cm³. It is unlikely that crystals will form from the cooled solution until a crystal is added to ‘seed’ the supersaturated solution.

Dry the crystals, weigh, and work out the percentage yield.

Chemical properties of thiosulphates

· With acid.

Dilute acids decompose thiosulphates, evolving sulphur dioxide gas and slowly depositing a yellow precipitate of sulphur.

Na₂S₂O₃ + 2HCl 2NaCl + SO₂ (g) + H₂O + S (s)

Ionically yellow ppt.

S₂O₃^2- (aq) + 2H⁺ (aq) SO₂ (g) + H₂O + S (s)

This is an example of disproportionation

S₂O₃^2- SO₂ (g) + S (s)

Ox. No. (S) +2 +4 0

· With halogens

With iodine sodium thiosulphate is oxidised to the tetrathionate ion

2Na₂S₂O₃ + I₂ NaI + Na₂S₂O₄

brown colourless

Ox. No. (S) +2 +2.5

The reaction is rapid, even at room temperature, and is used in volumetric analysis for the estimation of solutions of iodine. The addition of starch near the end point (when the solution has a straw-yellow colour) makes the end point easier to see (blue/black to colourless).

With chlorine (a stronger oxidising agent) sodium thiosulphate is oxidised to the sulphate ion.

Na₂S₂O₃ + 4Cl₂ + 5H₂O 8HCl + Na₂SO₄ + H₂SO₄

Ox. No. (S) +2 +6 +6

This reaction is used as an ‘antichlor’ to remove any residual chlorine from bleached textile fibres.

· With silver ions Ag⁺

It is widely used in photography to ‘fix’ the negative (i.e. to render the film insensitive to light by removing the silver halide that has remained unchanged during exposure).

It does this by forming a soluble complex ion.

Ag⁺ (aq) + 2S₂O₃^2- (aq) [Ag(S₂O₃)₂]^3- (aq)