Sulfuric acid

Sulfuric acid is probably the most important chemical substance which is not found in Nature. The total world consumption is about 25 000 000 tons per year.

Production in industry:

1. Contact method. Catalytic oxidation of SO₂ by air oxygen (440 ^oC, in the presence of V₂O₅ catalyst):

2SO₂ + O₂ 2SO₃; Н⁰₂₉₈ =-184.4 kJ/mol

SO₃ is absorbed by concentrated H₂SO₄. Usage of water is not efficient because gaseous SO₃ reacts first of all with water steam and then predominately H₂SO₄. Fog is the product of this interaction. The solution of SO₃ in H₂SO₄ has a technical name oleum that emphasizes its high viscosity (oleum - Lat. Oil).

H₂SO₄ dissolves SO₃ in any proportions, so the composition of oleum is H₂SO₄·nSO₃. Oleum contains several polysulfuric acids:

The content of SO₃ in oleum is 20-65%. To produce acid, oleum is mixed with H₂SO₄ having the necessary amount of water. Under the influence of water S—O—S bonds of polysulfuric acid are destroyed and converted into H₂SO₄ acid of required concentration.

H₂SO₄ produced by this method has high purity and can be of any concentration.

2. Lead Chamber process. It was called so because the chamber is lined with lead, which resists the action of cold sulfuric acid; the homogeneous catalyst is nitrogen oxide. The method was applied for the first time in the middle of 18^th century.

It was the most important method of industrial production of H₂SO₄ before the modern contact method was developed. Its essence is SO₂ oxidation by nitrogen oxide, NO_2, in the presence of water:

SO₂ + NO₂ + H₂O = Н₂SO₄ + NO

Nitrogen(II) oxide (NO) is oxidised again into the initial NO₂ and returned to the reaction: 2NO + О₂ = 2NO₂.

The process is carried out in special towers (see scheme below). This method gives about 76% H₂SO₄ used in the manufacture of fertilizers:

Structure. Н₂SO₄ and НSO₄^- -ion are the distorted tetrahedrons, with sulfur atom in the centre of a polyhedron. SO₄^2--ion has the shape of regular tetrahedron where all distances and angles are equivalent. Orbitals of sulfur have the state of sp³-hybridisation:

S—O bonds are very strong because of the additional -bonding: displacement of unshared electron pairs of O atoms to the vacant 3d-orbital of S. The equal S—O bonds lengths is a consequence of delocalisation of -electron density and the negative ion charge.

Properties. Anhydrous (100%) Н₂SO₄ under the normal conditions is a colourless oily liquid (m.p. = 10.4 ^oC). Н₂SO₄ molecules have intermolecular H-bonds with one another in the liquid and solid state either.

Н₂SO₄ azeotropeⁱ contains Н₂SO₄ 98.3 % wt. and 1.7% by weight of water, b.p. = 338.8 ^oC.

Н₂SO₄ is a strong dibasic acid in aqueous solutions:

Н₂SO₄ Н⁺ + НSO₄^-, K₁= 1·10³

НSO₄^- Н⁺ + SO₄^2-, K₂= 1,2·10^-2

Due to the large differences in dissociation constants, Н₂SO₄ forms sulfates and hydrogen sulfates. The scheme of their mutual conversion is shown below:

H₂SO₄

K₂SO₄ KHSO₄

KOH

Being dissolved Н₂SO₄ in water, a lot of heat liberates owing to hydrates formation. Therefore, concentrated Н₂SO₄ and water must be mixed with caution. To prevent spray of liquid, pour Н₂SO₄ (as a heavier component) into water, not vice versa. Formation of very stable hydrates explains Н₂SO₄ water absorption properties.

Sulfuric acid is a strong oxidising agent. Its reaction with metals depends on the concentration and activity of metal:

1. Diluted acid (oxidant is hydrogen ion H⁺). Therefore, it dissolves (oxidises) only metals that are situated before H in the electromotive series of metals:

Zn⁰ + Н₂SO_4(dil) = Zn⁺²SO₄ + Н₂⁰

Zn + 2H⁺ = Zn²⁺ + H₂

2. Concentrated Н₂SO₄ (oxidant is sulfur(VI)):

• Sulfur is reduced to SO₂ at the interaction with nonactive metals (Cu, Ag, Hg):

Cu⁰ + 2Н₂SO_4(conc.) = CuSO₄ + SO₂ + 2H₂O

• Sulfur is reduced to the elemental S or H₂S (a mixture of reduction products often appears)at the interaction with active metals:

3Zn + 4Н₂SO_4(conc.) = 3ZnSO₄ + S + 4H₂O

4Zn + 5Н₂SO_4(conc.) = 4ZnSO₄ + H₂S + 2H₂O

It can oxidise other simple substances and compounds:

2HBr + Н₂SO_4(conc.) = Br₂ + SO₂ + 2H₂O

8HI + Н₂SO_4(conc.) = 4I₂ + H₂S + 4H₂O

C + 2Н₂SO_4(conc.) = CO₂ + 2SO₂ + 2H₂O

S + 2Н₂SO_4(conc.) = 3SO₂ + 2H₂O

Н₂SO₄ is a very acidic solvent. CH₃COOH and HNO₃ behave as bases in Н₂SO₄. HClO₄ is one of the strongest acids, but it is a weak one in Н₂SO₄ medium:

HClO₄ + Н₂SO₄ = Н₃SO₄⁺ + ClO₄^-

The only acid that is stronger than Н₂SO₄ in its medium is hydrogen tetra(hydrogen sulfato) borate, H[B(HSO₄)₄]. It has not been obtained in the free state, but can be isolated from its solutions in Н₂SO₄:

B(OH)₃ + 6Н₂SO₄ = [B(HSO₄)₄]^- + 3H₃O⁺ + 2HSO₄^-

Н₂SO₄ salts. Sulfates are very abundant. Most of them are well-soluble in water. The low soluble sulfates are CaSO₄, SrSO₄, BaSO₄, PbSO₄.

Solid hydrogen sulfates are obtained only for the most active metals (NaHSO₄, KHSO₄ etc.). Disulfates (pyrosulfates) are salts of disulfuric (pyrosulfuric) acid obtained at heating of hydrogen sulfates:

2NaНSO₄ = Na₂S₂O₇ + Н₂O,

in turn, when heated stronger, the following reaction proceeds:

Na₂S₂O₇ = Na₂SO₄+ SO₃

Under the influence of water, disulfates are transformed again into hydrogen sulfates.

Chemical properties of sulfuric acid can be summarised in the scheme:

Uses

The production of 'superphosphate' (calcium hydrogen phosphate + calcium sulfate) for fertilisers is the biggest use of sulfuric acid. Second to this is the manufacture of ammonium sulfate 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; manufacture of explosives, pigments and dyestuffs, as well as many other chemicals, for example hydrochloric acid; refining of petroleum and sulfonation of oils to make detergents; and in accumulators.

PEROXOACIDS. There are the following peroxoacids of sulfur: Н₂S₂O₈ is called peroxidisulfuric acid (supersulfuric acid), and Н₂SO₅ is peroxomonosulfuric acid (Caro acid).

Production. Н₂S₂O₈ is obtained electrolytically at the anode in 50% Н₂SO₄ aqueous solution:

2HSO₄^- = Н₂S₂O₈ + 2e

Н₂S₂O₈ can form peroxomonosulfuric acid (Caro acid) under the influence of concentrated Н₂O₂ (Heinrich Caro was a German organic chemist, who obtained this acid in 1898):

Н₂S₂O₈ + Н₂O₂ = 2Н₂SO₅

Structure. These acids are regarded as Н₂O₂ derivatives because groups —O—O— are present in their structure

Properties. Н₂S₂O₈ and Н₂SO₅ are crystalline substances. Their melting points make up 65^o and 47^o, respectively.

Н₂S₂O₈ is a strong dibasic acid. The O—H bond of the Caro acid dissociates strongly. At the same time, —O—O—H group (like hydrogen peroxide, K = 4∙10^-10) dissociates weakly. Therefore, it is a monobasic acid that is stable also in the free state. Salts exist only in solutions.

Peroxoacids and their salts are very strong oxidising agents due to the presence of —O—O— bonds:

2Mn⁺²SO₄ + 5K₂S₂O₈ + 8Н₂O = 2KMn⁺⁷O₄ + 4K₂SO₄ + 8Н₂SO₄

When heated with water Н₂S₂O₈ and Н₂SO₅ hydrolyse:

Н₂S₂O₈ + H₂O = Н₂SO₄ + H₂SO₅

H₂SO₅ + H₂O = Н₂SO₄ + H₂O₂ (industrial method of H₂O₂ production)

THIOSULFURIC ACID AND ITS SALTS

Production in industry: 2Na₂S₂ + 3O₂ = 2Na₂S₂O₃.

In the laboratory: 1. boiling of sulfites aqueous solution with sulfur powder:

Na₂SO₃ + S = Na₂S₂O₃

2. 4SO₂ + 2H₂S + 6NaOH = 3Na₂S₂O₃ + 5H₂O (strong stirring)

Structure. Thiosulfate, salt of thiosulfuric acid, is a distorted tetrahedron as S—S bond is longer than S—O one. Thiosulfuric acid has two tautomeric forms:

Properties. The strength of thiosulfuric acid is close to H₂SO₄ (K₂ = 2∙10^-2), although it is unstable under normal conditions. Therefore, thiosulfates decompose in acidic medium by the reaction:

Na₂S₂O₃ + H₂SO₄ = Na₂SO₄ + Na₂S₂O₃ = S + Н₂SO₃ = SO₂ + H₂O.

The presence of sulfur (-2) predetermines the reducing properties of thiosulfate. Strong oxidising agents oxidise thiosulfates to elementary S and S (VI) compounds:

Na₂SO₃S^-2 + 4Cl₂⁰ + 5H₂O = 2H₂⁺⁶SO₄ + 2NaCl^-1 + 6HCl.

Na₂SO₃S^-2 + Br₂⁰ + H₂O = S⁰ + 2NaBr^-1 + H₂SO₄.

"Soft" oxidants (such as I₂) form tetrathionates:

2Na₂SO₃S + I₂ = Na₂S₄O₆ + 2NaI.

This reaction is widely used in iodometric methods of analysis in analytical chemistry.

PolythioNIC acids. Na₂S₄O₆ is salt of tetrathionic acid.

Preparation. When gaseous Н₂S passes through dilute solution of SО₂ Backenroder liquid is obtained. Its composition includes colloidal sulfur and polythionic acids Н₂S_nO₆, where n = 3-20.

Structure. A chain of elemental sulfur atoms between sp³-hybridized sulfur is a structure feature of polythionic acids. For example, pentathionic acid:

.

Properties. Unknown in the free state, polythionic acids are strong acids. Relatively stable are their salts with alkali metals. Acid salts are unidentified.

Important. Н₂S₂O₆ is not polythionic acid, since two pyramidal [SO₃]-groups are bound directly through the sulfur atoms. Manganese dithionate can be prepared by passing gaseous SO₂ through MnO₂ suspension:

2SO₂ + MnO₂ = MnS₂O₆

HALIDES AND OXOHALIDES OF sulfur

Sulfur reacts directly with all halogens except for iodine.

Thus:

S₂F₂ (b.p. -38⁰)

SF₂ (-35⁰)

SF₄ (-40⁰)

SF₆ (-64⁰, sublimates)

S₂Cl₂ (13.7⁰)

SCl₂ (59⁰, decomposes)

SСl₄ (-30⁰, decomposes)

S₂Br₂ (54⁰ at 0.2 mm Hg)

In the series of compounds of sulfur with halogens from fluorine to bromine stability of these compounds sharply drops. The most stable halide of fluorine is SF₆, of chlorine is S₂Cl₂. The latter compound is decomposed by water:

2S₂Cl₂ + 2H₂O = SO₂ + 3S + 4HCl

Sulfur forms oxohalides:

SOCl₂ is thionyl chloride

SO₂Cl₂ is sulfuryl chloride

SOCl₂ is an extraordinarily active compound.

Preparation:

PCl₅ + SO₂ = SOCl₂ + POCl₃

It is hydrolysed readily:

SOCl₂ + H₂O = SO₂ + 2HCl

SO₂Cl₂ is an extremely active liquid.

Preparation:

SO₂ + Cl₂ = SO₂Cl₂

CuCl₂ + 2SO₃ = CuSO₄ + SO₂Cl₂

Hydrolysis of SO₂Cl₂ takes place:

SO₂Cl₂ + 2H₂O = H₂SO₄ + 2HCl

Therefore, SO₂Cl₂ is a mixed anhydride or halogenanhydride.

Oxygen compounds of Se(VI), Te(VI), and Po(VI). The most important of them are SeO₃ and TeO₃ oxides, relevant acids and their salts.

OXIDES. EO₃ are produced indirectly, whereas EO₂ are the products of direct synthesis from simple substances.

SeO₃ is separated during boiling of selenates with liquid SO₃:

K₂SeO₄ + SO₃ = K₂SO₄ + SeO₃,

or at dehydration H₂SeO₄ by P₂O₅.

Molecules of SeO₃ exist only in the gaseous state. Tetramers (SeO₃)₄ are condensed when cooling. The solid tetramer (SeO₃)₄ has two polymorphic modifications: vitreous and asbestos-like. It is difficult to obtain (SeO₃)₄ without impurities (i.e. to use recrystallisation) because it explodes while interacting with any solvent. SeO₃ is a crystalline substance (m.p. = 121 C) under normal conditions, which decomposes at t > 180 C as follows:

2SeO₃ = 2SeO₂ + O₂,

but sublimes in vacuum without decomposition. SeO₃ is an acid oxide (acid anhydride of selenic acid):

SeO₃ + H₂O = H₂SeO₄.

In concentrated solutions, it forms a mixture of polyselenic acids similar to polysulfuric acids (H₂Se₂O₇, m.p. = 19 C, H₂Se₃O₁₀, m.p. = 39 C). When diluting, polyselenic acids solutions are hydrolysed to final product H₂SeO₄.

TeO₃ is obtained by thermal dehydration of orthotelluric acid:

Н₆ТeO₆ ТeO₃ + 3Н₂O

TeO₃ forms two polymorphs in the solid state. The reaction above leads to the -TeO₃, amorphous phase of yellow colour (density 5.1 g/cm³), which is low soluble in cold water (0.5 g /L), and is gradually transforming again into orthotelluric acid in hot water. The crystalline - TeO₃ phase of gray colour (density 6.2 g/cm³), which no longer reacts with acids and alkali solutions even when heated, can be prepared at a long-term heating of -TeO₃ in a sealed tube (300 C).

In general, SeO₃ and TeO₃ oxides are soluble in alkalis forming the corresponding selenates and tellurates:

SeO₃ + NaOH = Na₂SeO₄

SeO₃ oxide’s oxidising ability is so high that oxidises HCl to Cl₂ even at cooling. TeO₃ oxidises HCl only when heated. Therefore, oxidising properties are weakened at the transition from SeO₃ to TeO₃.

ACIDS. Preparation. The highest acids of chalcogens are mostly produced by the action of strong oxidising agents:

H₂Se⁺⁴O₃ + H₂O^-1₂ = H₂Se⁺⁶O^-2₄ + H₂O^-2

Te + 3H₂O₂ = H₆TeO₆

Te + HClO₃ + 3H₂O = H₆TeO₆ + HCl.

H₂SeO₄ is also prepared by electrochemical oxidation of selenious acid.

H₂SeO₄. A convenient laboratory method of preparation.

H₂SeO₄ is obtained by processing Ag₂SeO₃ suspension with bromine water:

Ag₂SeO₃ + Br₂ + H₂O = 2AgBr + H₂SeO₄

Selenic acid exists in the form of meta-acid, H₂SeO₄, telluric is the ortho-acid, H₆TeO₆. Both acids are colourless crystalline substances.

Structure. H₂SeO₄ (m.p. = 62.4 C) can be mixed with water at any proportion; its concentrated solutions are viscous and similar to H₂SO₄. It has strong affinity to water and will remove 'combined' water in sugars and other organic compounds through the formation of solid hydrates H₂SeO₄∙H₂O (m.p. = 26 C), H₂SeO₄∙2H₂O (m.p. = -24 C), H₂SeO₄∙4H₂O (m.p. = -52 C) like H₂SO₄. When heated over 260 C, selenic acid is transformed into SeO₂:

2H₂SeO₄ = 2SeO₂ + O₂ + 2H₂O.

The H₂SeO₄ molecule structure (sp³-hybridisation) and the acid strength are similar to H₂SO₄ (H₂SeO₄, K₂ = 1∙10^-2; H₂SO₄, K₂ = 1,2∙10^-2; both acids have K₁  10³). The significant strength is due to the presence of two very electronegative atoms of oxygen in acid residues. They shift electron density causing the growth of H—O bonds polarity and their ability to dissociate. Moreover, EO₄^2- anions (the bond length Se—O is 0.161 nm), which appear after dissociation, are stabilised in a solution due to the negative charge uniform delocalisation between four oxygen atoms:

H₆TeO₆. Orthotelluric acid molecule has an octahedral structure (sp³d²-hybridisation of tellurium orbitals):

Unlike H₂SO₄ and H₂SeO₄, covalent bonds Te=O are absent in H₆TeO₆, which would be favourable to H—O bonds polarisation and facilitate their dissociation. Therefore, H₆TeO₆ is a very weak acid (K₁ = 2∙10^-8; K₂ = 9∙10^-12; K₃ = 3∙10^-15), which is well soluble in hot water, but its solubility is limited under normal conditions (25% at 20 C).

If heated, H₆TeO₆ is transformed into metatelluric acid, H₂TeO₄. It is much stronger than ortho-H₆TeO₆ form and H₂TeO₄ gradually turns again into orthoacid in a solution. H₆TeO₆ is an illustration of the well-known rule: the more a p-element atom forming an acid the more the OH-groups number associated with it, i.e. the ability of more hydrated forms formation. Tellurium (VI), in addition to other p-elements of the fifth period, has the stable coordination number 6.

When neutralising H₆TeO₆ by alkalis, acid salts are formed. The most abundant among them are the low soluble Na₂H₄TeO₆ and well soluble K₂H₄TeO₆∙3H₂O. Orthotelluric acid H₆TeO₆ can be replaced by metals all six hydrogen atoms. For instance, Na₆TeO₆ neutral salt can be obtained at H₆TeO₆ fusion with NaOH. It gradually transforms into Na₂H₄TeO₆∙3H₂O in the humid atmosphere. There are also salts Ag₆TeO₆ and Hg₃TeO₆.

Properties. Selenic and telluric acids are powerful oxidising agents. In aqueous solutions, they are considerably stronger than H₂SO₄, as evidenced by E comparison:

SeO₄^2-+ 4H⁺ + 2e^-= H₂SeO₃ + H₂O; E  = 1,15 V

H₆TeO₆ + 2H⁺ + 2e^-= TeO₂ + 4H₂O; E  = 1,02 V

SO₄^2-+ 4H⁺ + 2e^-= H₂SO₃ + H₂O; E  = 0,17 V

Concentrated H₂SeO₄ and H₆TeO₆ unlike H₂SO₄ are able to oxidise not only I^‑ and Br^-, but also Cl^-, turning into the more stable oxidation state of these elements (+4):

2HCl + H₂SeO₄ = Cl₂ + H₂SeO₃ + H₂O.

Especially strong is the oxidising ability of H₂SeO₄. The hot anhydrous H₂SeO₄ dissolves well not only Ag (like H₂SO₄) but Au either:

2Au + 6H₂SeO₄ = Au₂(SeO₄)₃ + 3SeO₂ + 6H₂O.

Like aqua regia (HCl + HNO₃) the mixture of H₂SeO₄ + HCl oxidises even Pt:

Pt + 4HCl + 2H₂SeO₄ = PtCl₄ + 2SeO₂ + 4H₂O

Halides. They can be obtained by direct synthesis reaction from simple substances.

Composition, colour, state of halides of
Se	Te
SeF6 colourless gas	TeF6 colourless gas
SeF4 colourless	TeF4 colourless solid
SeCl4 colourless solid	TeCl4 colourless solid
SeCl2 liquid brown	TeCl2 solid green
SeBr4 solid yellow	TeBr4 solid orange
Se2Br2 liquid red	TeBr2 solid brown
	TeI4 gray-black

Molecules SeF₆ (m.p. = -46.6 C) and TeF₆ (m.p. = -38.6 ), like SF₆, have octahedral structure, which shows sp³d²-hybridisation (Se—F and Te—F bond lengths are 0.170 nm and 0.184 nm, respectively). The most abundant are SeX₄ and TeX₄. By its nature, selenium halides are close to the corresponding derivatives of sulfur. For instance, Se₂Cl₂ and Se₂Br₂ are decomposed even at careful heating:

2Se₂Cl₂ = 3Se + Se₂Cl₄

Tellurium halides are significantly different from those of sulfur compounds. They have salt-like behaviour. Unlike hexafluorides of S and Se, TeF₆ hydrolyses by water easily:

TeF₆ + 6H₂O = H₆TeO₆ + 6HF (it is a halogenanhydride).

Thus, in the series S—Se—Te—Po nonmetallic properties are weakened, metallic ones are increased, and ionicity in molecules grow. The chemical nature of halides changes starting with covalent (Se halides) through ionic-covalent (saltlike TeX_n) to the ionic (salts PoX_n).

TESTS FOR SULFUR

Oxidation of a sulfur compound with concentrated nitric acid yields sulfuric acid or a sulfate, which can be tested with barium chloride.

Oxidising agent

Dehydration agent

CH₂=CH₂ or C₂H₅OC₂H₅

NO₂

Br_r

I₂

CuSO₄

Zn SO₄

SO₂

CO₂

HCl

HF

CO

C

CO+CO₂

For drying of gases

C₆H₁₂O₆

Zn

Cu

HI

HBr

The reaction of replacement

Drying agent

Nitration

Sulfurization

Addition to CH₃CH=CH₂ with subsequent hydrolysis

C₂H₅OH

(CO₂H)₂

HCO₂H

C

S

NaCl

CaF₂

(CH₃)₂CHOH

SO₃H

SELENIUM. STANDARD ELECTRODE POTENTIALS, E^O, V

VI		IV		0		-II
Acid medium
SeO	1 ,1	H SeO	0,74	Se	-0,11	H Se
Alkaline medium
SeO	0,03	SeO	-0,36	Se	-0,67	Se

i An azeotrope (pronounced /əˈzi.ətroʊp/ ə-ZEE-ə-trope) is a mixture of two or more liquids in such a ratio that its composition cannot be changed by simple distillation. This occurs because, when an azeotrope is boiled, the resulting vapor has the same ratio of constituents as the original mixture.

Because their composition is unchanged by distillation, azeotropes are also called (especially in older texts) constant boiling mixtures. The word azeotrope is derived from the Greek words ζέειν (boil) and τρόπος (change) combined with the prefix α- (no) to give the overall meaning, “no change on boiling.”

Over 9,000 azeotropic mixtures of pairs of compounds have been documented. Many azeotropes of three or more compounds are also known

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