7.8.1 Sulphur Chemistry Notes

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

Sulphur Sulphur occurs as deposits of free sulphur in America, Jap an and Sicily. Sulphur exists as a covalently bonded, molecular solid at room temperature with the formula S 8. 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 (119oC). 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 Yellow transparent crystals Melt pt. 113 oC Obtained when sulphur crystallises from solution.

Monoclinic sulphur Amber coloured needles Melt pt. 119 oC Obtained when sulphur 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 e quilibrium 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 REACHES MA MAXIMUM VISCOSITY o AT 200 C.

Long chains of about 105 sulphur atoms become eenntangled an and ma make th the lliiquid 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.

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Detection of sulphur dioxide and sulphurous acid using acidified MnO 4- and Cr2O72-. Sulphites and their reactions with Fe 3+, Ba2+ and halogens.

Sulphur Dioxide Moist sulphur dioxide (or sulphurous acid) is a reducing agent. SO32- + H2O SO42- + 2H+ + 2e- Eθ = -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+ + 5SO42- + 4H+ 2. There is a colour change from orange to blue on adding the gas to a solution of potassium dichromate (VI). Cr2O72- + 3SO2 +2H+ 2Cr3+ + 3SO42- + 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 ionsHSO 3- and sulphite ions SO 32- ions. H2SO3 H+ + HSO3- → 2H+ + SO321.

Sulphites give sulphur dioxide on heating with dilute acids. Na2SO3 + 2HCl NaCl + SO 2 + H2O

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With barium chloride they give a white precipitate of barium sulphite which is soluble in dilute hydrochloric acid. Ba2+ (aq) + SO32- (aq) BaSO3 (s) white precipitate BaSO3 (s) + 2HCl BaCl2 + SO2 (g) + H2O This reaction is used as a test for sulphite ions in solution.

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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) SO32- (aq) + Fe3+ (aq) (b) SO32- (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. The Contact process A mixture of sulphur dioxide and air (approximately 8.5% SO 2 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) ΔHθ (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, H 2S2O7. Dilution of oleum with water gives sulphuric acid of any desired concentration. SO3 (g) + H2SO4 (l) H2S2O7 (l) + H2O (l)

H2S2O7 (l) oleum 2H2SO4 (l)

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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 reac tion 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 HSO 4 → H + SO42- 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 + O2H2O H+ + OHH2O + 22H + CO3 H2O + CO2 •

+

Concentrated H 2SO4 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 •

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2 As an oxidising agent Hot concentrated sulphuric acid is an oxidising agent. SO42- (aq) + 4H+ (aq) + 2e- → SO2 (aq) + 2H2O (l) Metals are oxidised to sulphates Cu (s) + 2H2SO4 (l)

Eθ = +0.17 V

•

CuSO4 (aq) + SO2 (g) + 2H2O (l)

Compounds are oxidised with the formation of SO 2 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 Br 2 and HI to I 2 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 p roduce the nitryl cation NO 2+. HNO3 + 2H2SO4 → NO2+ + H3O+ + 2HSO4Here 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.

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

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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?

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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.

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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 Na 2S2O3 This is a sulphate in which an oxygen atom has been replaced by a sulphur atom giving the ion S 2O32-. Crystalline sodium thiosulphate Na 2S2O3. 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 Na 2S2O3. 5H2O are obtained. SO32- (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 cm 3 of water and 1.5 g (slight e xcess) of crushed rhombic sulphur crystals. Boil gently for about 2 hours, or until no more sulphur dissolves.

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Filter off the excess sulphur, then evaporate the filtrate to about 10 cm 3. 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. 2+ S2O3 (aq) + 2H (aq) SO2 (g) + H2O + S (s) This is an example of disproportionation S2O32SO2 (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) •

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 (w hen 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) 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) + 2S2O32- (aq) [Ag(S2O3)2]3- (aq) •

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Sulphur Chemistry Test 1 (a) Give the atomic number of sulphur and the electronic configuration of its isolated atom. [1] (b) For sulphur dioxide (SO2) and hydrogen sulphide (H 2S) molecules and sulphate ions (SO 42-) respectively, state the oxidation number of the sulphur. [3] (c)

Describe briefly, giving reagents, reaction conditions, and equations, how substances containing sulphur of  oxidation number  (i) +4, (ii) +6, and (iii) 0, respectively, could be obtained from sulphur dioxide. [9] (d)

In the reaction: Na2H10S2O8 + 4Br 2 2H2SO4 + 2NaBr + 6HBr  only the oxidation numbers of the sulphur and bromine alter. Calculate from the equation the oxidation number of the sulphur in Na 2H10S2O8. [2]

2 (a) Briefly explain what is meant by the term allotropy in relation to sulphur. [4] Draw a diagram to illustrate the molecular structure of a s olid allotrope of sulphur  [2] (b) When radioactive sulphur ( 35S) slowly dissolves in hot aqueous sodium sulphite to give a solution of compound A, and the resulting solution is acidified, a precipitate B is formed which is the only radioactive product, no radioactivity remaining in solution. Explain these observations and indicate what information may be deduced regarding the structure of  A. [6] 3. (a) Give the formula of each of the following compounds, and identify the oxidation number of the sulphur in that compound. (i) sulphuric acid, (ii) sulphurous acid, (iii) barium sulphide, (iv)

sodium thiosulphate,

(v) sulphur hexafluoride. [10] (b)

With reference to the manufacture of sulphuric acid from sulphur dioxide by the Contact Process, state

(i) the equation for the oxidation reaction involved in the process, (ii) whether this reaction is exothermic or endothermic, (iii) the effect of temperature on the yield of reaction product, (iv) the effect of temperature on the rate of reaction, (v) the experimental conditions used in the actual process as carried out industrially. [8] (c) Give two major applications of sulphuric acid in industry, stating clearly the function of the acid in each application. [4] (d) Describe a simple test for sulphate ions in solution. Write an equation. [3]

(Total 52 marks)

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