Чамберс К., Холлидей А.К. Современная неорганическая химия, 1975
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This form melts at 290 K, and boils at 318 K. The ft form, obtained when the a form is allowed to stand for a long time at a temperature below 298 K. exists as asbestos-like, silky, felted needles and has a structure consisting of SO4 tetrahedra linked together in long chains.
Solid sulphur trioxide reacts explosively with liquid water : SO3 + H2O -* H2SO4 : A/f = - 88 kJmol"1
and it fumes strongly in moist air. The gas sulphur trioxide does not readily dissolve in water, but it reacts with concentrated sulphuric acid, thus :
H2SO4 + SO3 -> H2S2O7
H2S2O7 + SO3 -» H2S3O10
and so on.
Sulphur trioxide unites exothermically with basic oxides to give sulphates, for example
CaO + SO3 -* CaSO4
Sulphur trioxide is used on an industrial scale for sulphonating organic compounds.
SULPHURIC ACID, H2SO4
Sulphuric acid is probably the most important chemical substance not found naturally. Its manufacture is therefore important ; the total world production is about 25 000 000 tons a year.
Manufacture
The different methods of manufacturing sulphuric acid are essentially the same in principle and consist of three distinct processes :
I . Production of sulphur dioxide.
2.Conversion of sulphur dioxide to sulphur trioxide.
3.Conversion of sulphur trioxide to sulphuric acid.
1.Sulphur dioxide is obtained in the following three ways :
(a)By burning elemental sulphur (imported) :
S + O2 -» SO2
(b) As a by-product of the roasting process in the extraction of certain metals from their sulphide ores, for example
2ZnS + 3O, -> 2ZnO + 2SO2T
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2PbS + 3O2 -> 2PbO + 2SO2t 4FeS2 + 11O2 -> 2Fe2O3 -f 8SO2T
Since arsenic is often found in nature associated with sulphide ores, sulphur dioxide obtained by this method may contain some arsenie(III) oxide as impurity, and in certain processes this is a distinct disadvantage.
(c) From anhydrite, CaSO4 (the only sulphur compound found in large quantities in Great Britain). Anhydrite, shale (SiO2) and coke are finely powdered, intimately mixed and compressed into pellets which are fired in a reverberatory furnace at a temperature of about 1700 K.
The carbon reduces a quarter of the anhydrite to the sulphide :
CaSO4 + 2C -> CaS + 2CO2
The sulphide then reacts with the remaining anhydrite :
CaS + 3CaSO4 -» 4CaO 4- 4SO2
Thus the overall reaction is :
2CaSO4 + C -> 2CaO + CO2 + 2SO2
The gases from the kiln contain about 9% sulphur dioxide. (The calcium oxide combines with the silica to form a silicate slag which, when cool, is crushed and mixed with some anhydrite to give cement, a valuable by-product.)
In all the above methods, the sulphur dioxide obtained is impure. Dust is removed by first allowing the gases to expand, when some dust settles, then by passage through electrostatic precipitators and finally by washing with water. Water is removed by concentrated sulphuric acid which is kept in use until its concentration falls to 94%.
2. The combination of sulphur dioxide and oxygen to form the trioxide is slow and does not proceed to completion :
2SO2 + O2 ^ 2SO3 : AH = - 94 kJ mol"l
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Although the left to right reaction is exothermic, hence giving a better equilibrium yield of sulphur trioxide at low temperatures, the reaction is carried out industrially at about 670-720 K. Furthermore, a better yield would be obtained at high pressure, but extra cost of plant does not apparently justify this. Thus the conditions are based on economic rather than theoretical grounds (cf. Haber process).
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There are two processes, (a) and (b), using different catalysts:
(a)In the Contact process the catalyst now used is vanadium
pentoxide, V2O5, with sodium oxide on an invert carrier. Platinum is a more efficient catalyst than vanadium pentoxide but is far more expensive and rendered inactive or poisoned by the presence of arsenic, which has no inhibiting effect on vanadium pentoxide, and, consequently, platinum is no longer used.
The catalyst is carried on perforated shelves inside cylindrical steel vessels called converters. The gas enters these at 670-720 K at atmospheric pressure.
(b)In the older Lead Chamber process (so called because the chamber is lined with lead, on which cold sulphuric acid has little action), the catalyst is nitrogen oxide. This is a homogeneous catalyst. Sulphur dioxide, oxides of nitrogen, air and steam are passed slowly through mixing chambers and sulphuric acid of strength 60-70% (chamber acid) is formed. As the chambers are fairly cool, it condenses.
Since the catalyst is in the gaseous state, it is being continually removed from the mixing chambers. Its recovery, and the necessity of continual charging of the incoming gases with it, make the lead chamber plant complicated by comparison with that of the Contact process.
The gases coming out of the mixing chambers pass into the Gay- Lussac tower, packed with coke, over which concentrated sulphuric acid trickles. The acid absorbs the nitrous fumes to form "nitrated acid'. This nitrated acid, mixed with some of the weaker chamber acid, is pumped to the top of the Glover tower packed with flints. The mixture of acids passes down the tower and meets the stream of hot gases from the sulphur burners, passing up. The nitrous fumes are extracted from the nitrated acid and the gases now pass into the lead-lined mixing chambers. The incoming hot gases serve also to concentrate the chamber acid to a strength of about 80 %.
The plan of the whole process is shown in Figure 10.5.
The mechanism of the reaction in the lead chamber iscomplicated. The simple representation:
NO + air =± NO2
SO2 + NO2 + H2O -> H2SO4 + NO
is incomplete, for intermediate products, notably nitrosyl hydrogen sulphate, (NO)(HSO4), which are sometimes found in crystalline form and are known as 'chamber crystals', have been identified. The mechanism is now thought to be:
2NO + O -> 2NO
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Figure 10,5. The Lead Chamber process for |
the manufacture of sulphuric acid |
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SO2 + H2O -> H2SO3
H2SO3 + NO2 ~> SO5NH2
'sulphonitronic acid'
This substance can then react in two possible ways :
2SO5NH NO2 -> 2(NO)(HSO4) H2O + NO
or
SO5NH2 ^ H2SO4 + NO
The nitrosyl hydrogensulphateformed can also react in two ways, viz.:
2(NO)(HSO4) + SO2 + 2H2O =± 2SO5NH2 + H2SO4
or
4(NO)(HSO4) -f 2H2O ^ 4H2SO4 + 4NO 4- O
The final products are then sulphuric acid, nitrogen oxide and oxygen: the two latter react and the cycle goes on. Theoretically therefore, the nitrous fumes are never used up. In practice, however, some slight replacement is needed and this is achieved by adding a little concentrated nitric acid to the mixture in the Glover tower:
(NO)(HSO4) + HNO3 ^ H2SO4 + 2NO2
3. The conversion of sulphur trioxide to sulphuric acid arises as a separate reaction only in the Contact process.
Sulphur trioxide is not very soluble in water but dissolves readily in concentrated sulphuric acid.
The sulphur trioxide from the Contact chamber is passed into
300 GROUP VI
concentrated sulphuric acid, to which water is added at the required rate:
SO3 + H2SO4 -> H2S2O7
H2S2O7 4- H2O -» 2H2SO4
The 94 % acid from the sulphur dioxide drying towers (above) is used here and its strength brought up to 98 %. This is "concentrated1 sulphuric acid. Stronger acid up to "106%' may also be made. This concentration is suitable for sulphonating in, for example, the detergent industry.
Uses
The production of 'superphosphate' (calcium hydrogenphosphate + calcium sulphate) for fertilisers is the biggest use of sulphuric acid. Second to this is the manufacture of ammonium sulphate from ammonia (by the Haber process). This is also a fertiliser. Other uses are: conversion of viscose to cellulose in the manufacture of artificial silk, and so on; kpickling' (removal of oxide) of metals before galvanising or electroplating; manufacture of explosives, pigments and dyestuffs, as well as many other chemicals, for example hydrochloric acid; refining of petroleum and sulphonation of oils to make detergents; and in accumulators.
Properties
Pure sulphuric acid is a colourless, viscous and rather heavy liquid (density 1.84 g cm"3). On heating, it decomposes near its boiling point, forming sulphur trioxide and a constant boiling (603 K) mixture of water and sulphuric acid containing 98 % of the latter. This is 'concentrated' sulphuric acid, which is usually used. Further heating gives complete dissociation into water and sulphur trioxide.
Affinity for water
Concentrated sulphuric acid has a strong affinity for water and great heat is evolved on mixing; hence the acid must be added to water to dilute it. Because of this affinity, the acid can be used to dry gases with which it does not react, for example oxygen, chlorine, sulphur dioxide, and is used in desiccators. It will remove water of crystallisation from some compounds, for example
G R O U P VI 301
CuSO4.5H2O -» CuSO4 + 5H2O
and also 'combined' water, for example in sugars and other organic compounds:
C^H^On -» 12C + 11H2O
-» C0| + C02| + H20
HO O
ethanedioic acid (oxalic acid)
Oxidising properties
Concentrated sulphuric acid is an oxidising agent, particularly when hot, but the oxidising power of sulphuric acid decreases rapidly with dilution. The hot concentrated acid will oxidisenonmetals, for example carbon, sulphur and phosphorous to give, respectively, carbon dioxide, sulphur dioxide and phosphoric(V) acid. It also oxidises many metals to give their sulphates; cast iron, however, is not affected. The mechanisms of these reactions are complex and the acid gives a number of reduction products.
Hot concentrated sulphuric acid is a useful reagent for differentiating between chloride, bromide and iodide salts, since it is able to oxidise (a) iodide, giving iodine (purple) and the reduction products, hydrogen sulphide, sulphur and sulphur dioxide together with a little hydrogen iodide; (b) bromide, giving bromine (red-brown) and the reduction product sulphur dioxide together with hydrogen bromide. It is unable to oxidise the chloride ion and steamy fumes of hydrogen chloride are evolved.
Acidic properties
Concentrated sulphuric acid displaces more volatile acids from their salts, for example hydrogen chloride from chlorides (see above) and nitric acid from nitrates. The dilute acid is a good conductor of electricity. It behaves as a strong dibasic acid :
H2SO4 4- H2O ^ H3O+ + HSO4 : Ka = 40 mol T1 at 298 K HSO4 + H2 O^H3 O+ +SO5":K a =1 . 0x 1(T2 mol 1~1 at 289K
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the value of Ka for the first dissociation indicating that this reaction goes virtually to completion in dilute solution. The acid exhibits all the properties of the hydrogen ion, i.e. neutralising bases, giving hydrogen with many metals and so on. Dilute sulphuric acid attacks iron, but lead very soon becomes resistant due to the formation of a superficial layer of insoluble lead sulphate.
FUMING SULPHURIC ACID (OLEUM)
When sulphur trioxide is dissolved in concentrated sulphuric acid the pure 100% acid is first formed; then a further molecule of the trioxide adds on:
H2SO4 + SO3 -> H2S2O7 heptaoxodisulphuric(VI) acid pyrosulphuric acid
or oleum
or fuming sulphuricacid
The formation of other polysulphuric acids H2S3O10 up to H2O(SO3)n, by the addition of more sulphur trioxide, have been reported.
Pure sulphuric acid is a true acid. In dilute aqueous solution, sulphuric acid is an acid because the solvent water has an affinity for the proton:
H2SO4 + H2O ^ H3O+ + HSO,:
In the pure acid the 'dihydrogen sulphate' has a proton affinity, so that
H2SO4 + H2SO4 ^ H3SO^ + HSO;
If some polysulphuric acid is present, this can lose a proton more easily, for example
H2SO4 + H2S2O7 ^ H3SO^ + HS2O7".
Hence the strength of the acid goes up as sulphur trioxide is dissolved in it*. The acidity of pure and fuming sulphuric acids is not so apparent as in ordinary aqueous acids because it is masked by the oxidising and other properties; moreover, the conductivity
* Actually, the pure acid H2SO4 always contains some H2S2O7, because there is an equilibrium:
2H2SO4 ^ H2S2O7 + H2O
Thus water is availableto take the proton, and H2S2O7 to lose it, even in the 'pure' acid H2SO4.
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is very low because the large HaSO^ and HSO^ ions can move only slowly through the viscous acid in an electric field. Only recently has it been possible to find an acid-base indicator sufficiently resistant to the oxidising and sulphonating action of the concentrated acid to be used in it; this indicator shows the acid to be quite strong.
THE SULPHATES AND HYDROGENSULPHATES
The hydrogensulphates (or bisulphates) containing the ion
are only knownin the solid state for the alkali metals and ammonium. Sodium hydrogensulphate is formed when sodium chloride is treated with cold concentrated sulphuric acid:
NaCl + H2SO4 -> NaHSO4 + HClt
It may also be obtained by crystallising sodium sulphate from a dilute sulphuric acid solution:
Na2SO4 + H2SO4 -> 2NaHSO4
The hydrogensulphate ion dissociates into hydrogen and sulphate ions in solution; hence hydrogensulphates behave as acids.
When solid sodium hydrogensulphate is heated, sodium 4pyrosulphate' is formed; further heating gives sodium sulphate and sulphur trioxide:
2NaHSO4 T> Na2S2O7 + H2Ot
Na2S2O7 -» Na2SO4 + SO3T
Electrolysis of the hydrogensulphate of potassium or ammonium can yield a peroxodisulphate and thence hydrogen peroxide.
The sulphates of many metals are soluble in water, but those of barium, lead, mercury(I), calcium and strontium are insoluble or only sparingly soluble. Soluble sulphates often crystallise out as hydrates, for example the vitriols such as, FeSO4.7H2O; NiSO4.7H2O; CuSO4.5H2O and double salts, for example FeSO4.(NH4)2SO4. 6H2O, and the alums, for example KA1(SO4)2.12H2O. In these salts, most of the water molecules are attached to the cation; the remaining water molecules are connected by hydrogen bonds partly to the sulphate ions and partly to the cationic water molecules (for example CuSO4.5H2O, seep.412).
The sulphates of the alkali and alkaline earth metals and manganese(II) are stable to heat; those of heavier metals decompose on heating, evolving sulphur trioxide and leaving the oxide or the metal: