Chambers, Holliday. Modern inorganic chemistry

416 THE TRANSITION ELEMENTS

Copper(l) chloride, bromide and cyanide were used by Sandmeyer to introduce a chlorine, a bromine atom and a cyanide group respectively into a benzene ring by addition to the phenyl diazonium salt.

Copper(I) sulphate, Cu2SO4, is obtained as a white powder by heating together dimethyl sulphate and copper(I) oxide:

(CH3)2SO4 + Cu2O -> Cu2SO4 + (CH3)2O

dimethyl ether

This copper(I) compound, unlike the above, is soluble in water and therefore in the presence of water liberates copper and forms a copper(H) compound :

Cu2SO4 -> CuSO4 -f Cu|

COMPLEXES

The complexes of copper(I) like those of silver(I) (p. 430), but unlike those of preceding transitions metals, tend to prefer a linear coordination of two ligands, i.e. X—Cu—X; thus copper(I) chloride in aqueous ammonia gives the colourless [Cu(NH3)2]+ (readily oxidised in air to give blue [CuII(NH3)4(H2O)2]2+; copper(I) chloride in hydrochloric acid gives [CuQ2]~, although [CuQ3]2~ is also known.

TESTS FOR COPPER COMPOUNDS

Copper(II) ions in aqueous solution are readily obtained from any copper-contaming material. The reactions with (a) alkali (p. 430),

(b) concentrated ammonia (p. 413) and (c) hydrogen sulphide (p. 413) provide satisfactory tests for aqueous copper(II) ions. A further test is to add a hexacyanoferrate(II) (usually as the potassium salt) when a chocolate-brown precipitate of copper(II) hexacyanoferrate(II) is obtained:

2Cu2+ + [Fe(CN)6]4- -> Cu2[Fe(CN)6]

ZINC

THE ELEMENT

The common ores of zinc are zinc blende, ZnS, and calamine, ZnCO3. The metal is extracted (a) by roasting blende with air or by heating

THE TRANSITION ELEMENTS 417

calamine, to give the oxide ZnO, which is then reduced to the metal by heating with coke, or (b) by dissolving out the zinc content of the ore with sulphuric acid, to give a solution of zinc(II) sulphate, ZnSO^, which is electrolysed with an aluminiumcathode on which the zinc metal is deposited.

The data provided at the beginning of this chapter show that zinc has a melting point and boiling point much lower than the preceding transition metals. This allows zinc to be melted or distilled without difficulty, and distillation may be used to purify zinc from less volatile metals. The low boiling point is an indication of weak metallic bonding, which in turn indicates that the filled 3d electron levels are not extensively involved in forming zinc-zinc bonds in the metal. Moreover, zinc in its chemical behaviour shows few characteristics of a transition element; it exhibits only one oxidaton state, + 2, in either ionic or covalent compounds, indicating the involvement only of the two outer, 4s electrons. Its compounds are commonly colourless, but it does show a somewhat greater tendency to form complexes than the analogous elements (Ca, Sr, Ba) of Group II.

The metal is not attacked by dry air at ordinary temperature; in moist air it tarnishes, forming a basic carbonate which acts as a coating preventing further corrosion. When heated in air, it burns with a greenish-blue flame giving a fibrous deposit of zinc oxide. This was the 'philosopher's wool' of the alchemists. Zinc combines directly with chlorine and sulphur but not with nitrogen (cf. magnesium), although the compound zinc nitride, Zn3N2, can be obtained by passing ammonia over red-hot zinc. The metal does not react with water but steam attacks it at red heat (cf. magnesium):

H2O + Zn -» ZnO 4- H2

Despite its electrode potential (p. 98), very pure zinc has little or no reaction with dilute acids. If impurities are present, local electrochemical 'cells' are set up (cf. the rusting of iron, p, 398)and the zinc reacts readily evolving hydrogen. Amalgamation of zinc with mercury reduces the reactivity by giving uniformity to the surface. Very pure zinc reacts readily with dilute acids if previously coated with copper by adding copper(II) sulphate:

Cu2+ +Zn - +Zn 2 + + Cuj

This zinc-copper couple reacts with methanol, the mixture reducing an alkyl halide to an alkane:

Zn + CH3OH + C2H5I -> Zn2+ + CH3O~ + I" + C2H6

418 THE TRANSITIONELEMENTS

Under no conditions is hydrogen obtained from nitric acid. With the dilute acid, reduction to ammonia occurs:

4Zn + 10HNO3 -» 4Zn(NO3)2 4- NH4NO3 + 3H2O i.e. 4Zn 4- 10H+ + NO3 -> 4Zn2+ + NH^ + 3H2O

With more concentrated nitric acid, oxides of nitrogen are formed. Unlike cadmium and mercury and, in fact, all metals of Group II, zinc dissolves readily in alkalis forming zineates, in which the zinc

atom is contained in a complex hydroxo-anion, for example:

Zn + 2OH" + 4H2O -* [Zn(OH)4(H2O)2]2~ + H2

At ordinary temperatures, zinc forms an addition compound with an alkyl halide (cf.magnesium):

Zn + C2H5I -> C2H5ZnI

The compound breaks up on heating:

2C2H5ZnI -> Zn(C2H5)2 + ZnI2

zinc diethyl

The zinc alkyls, of which this is an example, are vile-smelling inflammable liquids. They were the first organo-metatlic compounds prepared by Frankland in 1849. With water, they decompose giving an alkane:

Zn(C2H5)2 + 2H2O -> Zn(OH)2 + 2C2H6

(Cadmium and mercury also form alkyls.)

Uses

Because of its resistance to corrosion, zinc may be used to coat iron. This may be done by dipping the iron into molten zinc or by spraying zinc on the iron articles, for example iron sheets. This is known as galvanising. Smaller iron articles may be coated by heating with zinc dust, a process known as sherardising, or suspensions of zinc may be used in paints.

Sheets of galvanised iron are used for roofing, guttering and the like. Alloys of zinc, notably brass, are used extensively. The metal is used in wet and dry Leclanche batteries.

Zinc oxide or kzinc white' is used in paints, but more preferable, because of its better covering power, is lithopone (a mixture of zinc sulphide and barium sulphate). Both paints have the advantage over white lead that they do not 'blacken' in air (due to hydrogen sulphide). Zinc dust and also zinc chromate are constituents of

THE TRANSITION ELEMENTS 419

rust-preventing paints. Zinc chromate isa yellowpigment. Lithopone is also used as a filler in linoleum.

Zinc carbonate and zinc oxide are constituents of calamine lotion. Zinc oxide, an antiseptic, ispresent in kzinc' ointment and in cosmetic powders.

Zinc is important biologically; there are many zinc-protein complexes, and the human body contains about 2 g. In the human pancreas, zinc ions appear to play an essential part in the storage of insulin.

CHEMICAL PROPERTIES OF ZINC COMPOUNDS

Oxidation state + 2

Zinc(II) oxide, ZnO, is prepared by heating the hydroxide Zn(OH)2 or the carbonate ZnCO3. It is a white solid, insoluble in water, but readily soluble in acids to give a solution containing the zinc(II) cation, and in alkalis to give a hydroxozincate(II) anion:

ZnO + 2H3O+ -» Zn2 + (aq) + 3H2O

(e.g.) ZnO + 2OH~ + 3H2O -> [Zn(OH)4(H2O)2]2-

Zinc(II) oxide is therefore amphoterie.

On heating, the oxide becomes yellow, reverting to white on cooling. When zinc oxide is heated, a little oxygen is lost reversibly. This leaves a non-stoichiometrie compound. The crystal lattice is disturbed in such a way that electrons from the excess zinc metal remaining can move in the crystal almost as freely as they can in a metal. This makes zinc oxide a semiconductor and gives it a yellow colour, which is lost when oxygen is taken up again on cooling to give zinc oxide.

Zinc(II) hydroxide is a white gelatinous solid obtained when the stoichiometric quantity of alkali hydroxide is added to a solution of a zinc salt:

Zn2+(aq) + 2OH~ -> Zn(OH)2

It is soluble in alkali, and in ammonia (seebelow).

Zinc(ll} chloride, ZnCl2, is the only important halide—it is prepared by standard methods, but cannot be obtained directly by heating the hydrated salt It has a crystal lattice in which each zinc is surrounded tetrahedrally by four chloride ions, but the low melting point and solubility in organic solvents indicate some covalent

420 THE TRANSITION ELEMENTS

character. In the hydrated salt, and in solution, species such as [Zn(H2O)6]2 + exist; the latter is slightly acidic, forming [Zn(H2O)5OH]+ . In presence of excess chloride ion, tetrahedral complexes such as [ZnCl4]2~ may be formed. Other important zinc salts are the hydrated sulphate ZnSO4.7H2O, isomorphous with the corresponding hydrated sulphates of, for example, iron(II) and nickel, and often used as a source of Zn2 *(aq), and the sulphide, ZnS, obtained as a white precipitate when hydrogen sulphide is passed through a solution of a zinc(II) salt in presence of ammonia and ammonium chloride.

COMPLEXES

The aquo-complex [Zn(H2O)6]2+ and the tetrahedral [ZnCl4]2"^ have already been mentioned. Numerous hydroxo-complexes, foi example [Zn(OH)6]4~, [Zn(OH)4]2~, have been described. Addition of excess ammonia to an aqueous Zn(II) solution produces the tetraamminozinc cation [Zn(NH3)4]2+. Hence zinc tends to form 4-coordinate, tetrahedral or (lesscommonly)6-coordinate octahedral complexes.

TESTS FOR ZINC

1.Alkali hydroxidegives a white precipitate soluble in excess. The

white precipitate, Zn(OH)2, gives the oxide when dehydrated; the white ^ yellow reversible colour change observed on heating the oxide is a useful confirmatory test.

2.Addition of sulphide ion to a solution of a zinc saltcontaining

ammonia and ammonium chloride gives a white precipitate of zinc sulphide.

QUESTIONS

1. Explain the following observations, giving equations wherever possible.

Anhydrous cupric sulphate is white but forms a blue hydrate and a blue aqueous solution. The solution turns yellow when treated with concentrated hydrochloric acid, dark blue with ammonia, and gives a white precipitate and brown solution when treated with potassium iodide. A yellow-brown aqueous solution of ferric chloride becomes paler on acidification with sulphuric or nitric

422THE TRANSITION ELEMENTS

4.The transition metals form complexes which are usually different in kind and in stability from those formed by the nontransition elements. Give reasons for these differences.

(Liverpool B.Sc., Part I)

5. A compound of cobalt has the formula Co(NH3)JCClr 0.500 g of it was dissolved in 50.00 cm3 M hydrochloric acid; the excess acid required 40.00 cm3 M sodium hydroxide solution to neutralise it. Another 0.500 g portion of the compound was dissolved in water and allowed to react with excess silver nitrate solution. 0.575 g of silver chloride was precipitated.

(a)Calculate the number of moles of ammonia liberated from 0.500 g of the cobalt compound.

(b)Calculate the number of moles of chloride ion released from 0.500 g of the cobalt compound.

(Atomic weights: Ag = 108, Cl = 35.5).

(c)What values for x and y in the original formula do these results suggest?

(Atomic weights: Co = 60, N = 14,H = 1).

(d)When the compound was decomposed before addition of silver nitrate, the value of y was found to be 50% greater than the value you have calculated. Offer an explanation for the

6. In what ways do the chemical and physical properties of zinc(II) differ from those of iron(II)? Account for these differences. Explain what happens when

(a)copper(I) oxide is treated with dilute sulphuric acid,

(b)cobalt(II) chloride solution is treated with an excess of concentrated ammonia solution and air is bubbled through the mixture,

(c)an excess of a concentrated solution df aqueous ammonia is added dropwise to an aqueous solution of nickel(II)chloride,

(d)an excess of an aqueous solution of potassium cyanide is added dropwise to an aqueous solution of nickel(II) chloride.

(JMB, A)

7.(a) Outline the extraction of manganese from pyrolusite and state one important use of the metal. Suggest a method for the preparation of a solution of potassium permanganate starting from manganese, stating the oxidation state of manganese at each stage in the process.

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Outline how you would determine the concentration of permanganate ions in the product (practical details are not required).

(b)Outline the production of (i) chromium, (ii) potassium dichromate, from chromium(III) oxide, stating the oxidation states of chromium at the various stages in (ii). Outline how you would determine the purity of a sample of potassium dichromate (practical details are not required).

Three crystalline compounds, one violet, one pale green, and one deep green in colour, all have the molecular formula

CrCl3.6H2O. When equal masses of the three compounds are separately treated with an excess of aqueous silver nitrate at room temperature, the masses of white precipitate produced are in the ratio 3:2:1. Suggest an explanation for these results.

(C,A)

8. What do you understand by a complex salt? Give examples, using a different metal in each case, of complex salts that may be formed using the following reagents:

How would you distinguish between the two salts that you have chosen in each of (a)and (b)and how wouldyou convert the examples given in (c) and (d) so that the simple metal ion is obtained in each

(a)the factors that determine the electrode potential of the metal;

(b)the preparation of one compound in a high oxidation state;

(c)the change in the M3 + /M2+ redox potential as a result of complex ion formation;

(d)the determination of the formula of any one complex;

(e)the colour of the compounds of the element;

(f)the electronic structure and physical properties of the element.

(JMB, A)

10.Find the element V (vanadium) in the given Periodic Table.

(a)Write down the electronic configurations of the species (i) V and(ii)V2 + .