Chambers, Holliday. Modern inorganic chemistry

THE ELEMENTS OF GROUPS IB AND MB 427

Silver has little tendency to formally lose more than one electron; its chemistry is therefore almost entirely restricted to the + 1 oxidation state. Silver itself is resistant to chemical attack, though aqueous cyanide ion slowly attacks it, as does sulphur or a sulphide (to give black Ag2S), hence the tarnishing of silver by the atmosphere or other sulphur-containing materials. It dissolves in concentrated nitric acid to give a solution of silver(I) nitrate, AgNO3.

Oxidation state + 2

The only important compound is the paramagnetic silver(II) fluoride, AgF2, prepared by fluorination of the metal; it is used as a convenient fluorinatmg agent.

Oxidation state +1

Addition of an alkali hydroxide to a solution of a silver(I) salt gives a brown solid, silver(I) oxide, Ag2O; when wet this behaves as "silver hydroxide' AgOH, for example

^AgOH' + C2H5I -» Agl + C2H5OH iodethane ethanol

The oxide is soluble in ammonia to give the complex [Ag(NH3)2]+ (linear). On heating, silver(I) oxide loses oxygen to give the metal (all the coinage metal oxides have low thermal stability and this falls in the order Cu > Ag > Au).

SILVER(I) HALIDES

While the chloride, bromide and iodide are insoluble in water, the fluoride, AgF, is very soluble.

The insoluble halides can be prepared by adding the respective halide ion to silver ions:

Ag+ +X - ^AgX i

(halide)

The chloride is white, the bromide pale yellow and the iodide deeper yellow. These are examples (uncommon) of a coloured compound being obtained from colourless ions. The silver(I) ion intensifies colour in other cases, for example silver chromate(VI), Ag2CrO4, is brick-red while potassium chromate(VI). K2CrO4. is yellow.

428 THE ELEMENTS OF GROUPS IB AND IIB

Silver chloride is readily soluble in ammonia, the bromide less readily and the iodide only slightly, forming the complex cation [Ag(NH3)2]+. These halides also dissolve in potassium cyanide, forming the linear complex anion [Ag(CN)2]~ and in sodium thiosulphate forming another complex anion, [Ag(S2O3)2]3~.

All the silver halides are sensitive to light, decomposing eventually to silver. In sunlight, silver chloride turns first violet and finally black. The use of these compounds in photography depends on this (see below). (Allsilver salts are, in fact photosensitive—the neck of a silver nitrate bottle is black owing to a deposit of silver.)

Silver chloride is reduced to the metal by zinc. One of the methods of recovering silver from "silver residues' depends on this. The residue is first treated with concentrated hydrochloric acid and then sulphuric acid and zinc added:

2AgCl + Zn -> 2Ag + 2C1" + Zn2 +

Photography

It was known in the sixteenth century that silver salts were photosensitive, but it was not until the beginning of the nineteenth century, when Herschel found that silver chloride was soluble in sodium thiosulphate, that photography became possible.

The plate or film of celluloid is coated with a colloidal gelatinised solution when the unchanged bromide is dissolved to form a chloride because of its greater sensitivity). During photographic exposure, decomposition of the bromide occurs to form minute particles of silver. These particles are too small to be seen by the naked eye and are only detectable with the electron-microscope. The number of such nuclei of decomposition in a given area of plate or film depends on the intensity of light falling on the area.

When the film is developed (the developer being a reducing agent), the unchanged silver bromide immediately surrounding these nuclei is reduced to give a visible blackening of the film.

The film is now fixed by washing in sodium thiosulphate ('hypo') solution when the unchanged bromide is dissolved to form the complex ion

AgBr + 2S2Or - [Ag(S203)2]3- + Br~

The fixed plate is now a ^negative', for those patches on which most light fell are black. The process is reversed in printing to make the 'positive'—the printing paper having a covering of silver chloride or bromide or a mixtureof the two.This, in turn, is developed and fixed as was the plate or film.

430 THE ELEMENTS OF GROUPS IB AND MB

In the former, it gives precipitates with halides (except the fluoride), cyanides, thiocyanates, chromates(VI), phosphate(V), and most ions of organic acids. The silver salts of organic acids are obtained as white precipitates on adding silver nitrate to a neutral solution of the acid. These silver salts on ignition leave silver. When this reaction is carried out quantitatively, it provides a means of determining the basicity of the acid.

Gravimetrically, silver nitrate is used to determine the chloride ion.

Silver nitrate is used volumetricallyto estimate chloride, bromide, cyanide and thiocyanate ions. Potassium chromate or fluorescein is used as an indicator.

In neutral solution, the indicator is potassium chromate(VI). In acid solution the CrO^' ion changes to Cr2O7~ (p. 378), and since silver dichromate(VI) is soluble, chromate(VI) is not a suitable indicator; other methods can be used under these conditions. (In alkaline solution, silver(I) oxide precipitates, so silver(I) nitrate cannot be used under these conditions.)

COMPLEXES OF SILVER(I)

Some of these have already been noted as 2-coordinate and linear, for example [Ag(CN)2]-, [Ag(NH3)2]+, [Ag(S2O3)]3-. Silver(I) halides dissolve in concentrated aqueous halide solutions to give complexes [AgX2]~, [AgX3]2~, for example [AgCl3]2~.

TESTS FOR SILVER

1.Hydrochloric acid or any soluble chloride gives a white precipitate, soluble in ammonia.

2.Hydrogen sulphide gives a black precipitate,

3.Potassium chromate(VI) gives a brick-red precipitate of silver chromate(VI) in neutral solution.

GOLD

THE ELEMENT

Metallic gold, which is found free in nature, has always been valued for its nobility, i.e. its resistance to chemical attack. This property is to be expected from its position in the electrochemical series. It

432 THE ELEMENTS OF GROUPS IB AND MB

Gold(III) chloride dissolves in hydrochloric acid to form tetrachlorauric acid, HAuCl4. Here again, the gold(III) is 4-co-ordinate in the ion [AuQ4]~. If alkali is added to this acid, successive replacement of chlorine atoms by hydroxyl groups occurs, forming finally the unstable tetrahydroxoaurate{III) ion, [Au(OH)4] ~ ~ :

[AuCl4] ~ -+ [AuCl3OH] ~ -> [Au(OH)4] "

This ion is very easily reduced to gold, and hence alkaline solutions of chloraurates(III) (often wrongly called kgold chloride') are used with a reducing agent to prepare colloidal gold.

Other than the fluoride, no compounds of gold(III) are known in which gold acts as a metal ion, i.e. there are no gold(III) salts. There are, however, numerous complexes of gold(III) which are 4-co-ordin- ate, for example the compound diethyl gold(III) sulphate [(C2H5)2Au]2SO4.4H2O, which has the structure:

Au

H5C2

TESTS FOR GOLD COMPOUNDS

Gold compounds are all easily reduced in alkaline solution to metallic gold which may occur in colloidal form and so be red, blue or intermediate colours. Reduction to gold, followed by weighing of the metal precipitated, may be used in quantitative analysis.

II (ZINC)f CADMIUM, MERCURY

These elements formed Group IIB of Mendeleef 's original periodic table. As we have seen in Chapter 13, zinc does not show very marked 'transition-metal' characteristics. The other two elements in this group, cadmium and mercury, lie at the ends of the second and third transition series (Y-Cd, La-Hg) and, although they resemble zinc in some respects in showing a predominantly 4- 2 oxidation state, they also show rather more transition-metal characteristics. Additionally, mercury has characteristics, some of which relate it quite closely to its immediate predecessors in the third transition series, platinum and gold, and some of which are decidedly peculiar to mercury.

Table 142

SELECTED OFTHE Zn, Cd, Hg

' Metallic radius,

434 THE ELEMENTS OF GROUPS IB AND IIB

Table 14.2 shows that all three elements have remarkably low melting points and boiling points—an indication of the weak metallic ^bonding, especially notable in mercury. The low heat of atomisation of the latter element compensates to some extent its higher ionisation energies, so that, in practice, all the elements of this group can form cations M2 + in aqueous solution or in hydrated salts; anhydrous mercury(II)compounds are generally covalent.

CADMIUM

THE ELEMENT

Cadmium is usually found in zinc ores and is extracted from them along with zinc (p. 416); it may be separated from the zinc by distillation (cadmium is more volatile than zinc, Table 14.2)or by electrolytic deposition.

Cadmium is a soft metal, which forms a protective coating in air, and bums only on strong heating to give the brown oxide CdO. It dissolves in acids with evolution of hydrogen :

Cd2+(aq) + 2e~ -> Cd(s): E^ = -0.40 V

It is used as a protective agent, particularly for iron, and is more resistant to corrosion by sea water than, for example, zinc or nickel.

In its chemistry, cadmium exhibits exclusively the oxidation state -f 2 in both ionic and covalent compounds. The hydroxide is soluble in acids to give cadmium(II) salts, and slightly soluble in concentrated alkali where hydroxocadmiates are probably formed; it is therefore slightly amphoteric. It is also soluble in ammonia to give ammines, for example [Cd(NH3)4]2+. Of the halides, cadmiumill) chloride is soluble in water, but besides [Cd(H2O)J2+ ions, complex species [CdCl]*, [CdQ3]~ and the undissociated chloride [CdCl2] exist in the solution, and addition of chloride ion increases the concentrations of these chloro-complexes at the expense of Cd2+(aq) ions.

Solid cadmium(II) iodide CdI2 has a layer lattice'—a structure intermediate between one containing Cd2* and I~ ions and one containing CdI2 molecules—and this on vaporisation gives linear, covalent I—Cd—I molecules. In solution, iodo-complexes exist, for example

Cadmium(ll} sulphide, CdS, is a canary-yellow solid, precipitated by addition of hydrogen sulphide (or sulphide ion) to an acid solution

THE ELEMENTS OF GROUPS IB AND (IB 435

of a cadmium(II) salt; presence of chloride ion may reduce the concentration of Cd2+(aq) sufficiently to prevent precipitation.

Complexes of cadmium include, besides those already mentioned, a tetracyanocadmiate [Cd(CN)4]2~ which in neutral solution is sufficiently unstable to allow precipitation of cadmium(II) sulphide by hydrogen sulphide. Octahedral [CdCl6]4" ions are known in the solid state, as, for example, K4CdCl6.

TESTS FOR CADMIUM

The reaction of Cd2 +(aq) with sulphide ion, to give yellow CdS, and with hydroxide ion to give the white Cd(OH)2, soluble in ammonia, provide two useful tests.

MERCURY

THE ELEMENT

Mercury has been known for many centuries, perhaps because its extraction is easy; it has an almost unique appearance, it readily displaces gold from its ores and it forms amalgams with many other metals—all properties which caused the alchemists to regard it as one of the "fundamental' substances.

It occurs chiefly as cinnabar, the red sulphide HgS, from which it is readily extracted either by roasting (to give the metal and sulphur dioxide) or by heating with calcium oxide; the metal distils off and can be purified by vacuum distillation.

Mercury has a large relative atomic mass, but, like zinc and cadmium, the bonds in the metal are not strong. These two factors together may account for the very low melting point and boiling point of mercury. The low boiling point means that mercury has an appreciable vapour pressure at room temperature; 1m3 of air in equilibrium with the metal contains 14 mg of vapour, and the latter is highly toxic. Exposure of mercury metal to any reagent which produces volatile mercury compounds enhances the toxicity.

The metal is slowly oxidised by air at its boiling point, to give red mercury(II) oxide; it is attacked by the halogens (which cannot therefore be collected over mercury) and by nitric acid. (The reactivity of mercury towards acids is further considered on pp. 436, 438.) It forms amalgams—liquid or solid—with many other metals; these find uses as reducing agents (for example with sodium, zinc) and as dental fillings (for example with silver, tin or copper).