Chambers, Holliday. Modern inorganic chemistry

196 GROUP IV

are chemically (kinetically) inert and, unlike all other Group IV element tetrahalides, they are not hydrolysed by water. Carbon tetrajluoride is a gas. b.p. 145K. and is made by direct combination of carbon and fluorine ; it is also the main product of burning fluorine in benzene vapour. Carbon tetrachloride (tetrachloromethane) is a liquid, b.p. 350 1, and is prepared by the action of chlorine on carbon disulphide (p. 201) in the presence of a catalyst, usually manganese(II) chloride or iron(III) chloride :

CS2 + 3 C 1 2 C C 1 4 + S2C12

Further reaction then occurs between the disulphur dichloride and the carbon disulphide :

2S2C12 + C S 2 C C 1 4 + 6S

Carbon tetrachloride is an excellent solvent for organic substances. It has been used in dry-cleaning and in fire-extinguishers, but it has now largely been replaced because it is highly toxic, causing damage to liver and kidneys. 1,1,1 trichloroethane is the most commonly used dry-cleaning solvent and fluorocarbons are used in many fireextinguishers.

Silicon

Silicon tetrajluoride is formed when hydrogen fluoride reacts with silica or a silicate :

4HF 4- SiO2 -» SiF4t 4- 2H2O

The hydrogen fluoride is conveniently produced in situ by the action of concentrated sulphuric acid on calcium fluoride :

CaF2 + H2SO4 -> CaSO4 + 2HF

Silicon tetrafluoride is a colourless gas, b.p. 203 K, the molecule having, like the tetrahalides of carbon, a tetrahedral covalent structure. It reacts with water to form hydrated silica (silica gel, see p. 186) and hexafluorosilicic acid, the latter product being obtained by a reaction between the hydrogen fluoride produced and excess silicon tetrafluoride :

SiF4 + 2H2O -» SiO2i + 4HF

SiF4 + 2HF -> H2SiF6

Silicon tetrachloride is a colourless liquid, b.p. 216.2 K, and again the molecule has a covalent structure. Silicon tetrachloride is hydrolysed by water :

G R O U P IV 197

SiCl4 + 2H2O -> 4HC1 + SiO2i

Silica gel is again obtained but silicon does not form the corresponding hexachlorosilicic acid since the small silicon atom is unable to coordinate six chlorine atoms.

Silicon difluoride is obtained as a very reactive gas when silicon tetrafluoride and silicon are heated together. It polymerises rapidly to give (SiF2)n, a solid.

Germanium

Germanium forms divalent compounds with all the halogens. Germanium(ll) chloride can be prepared by passing the vapour of germanium(IV) chloride (seebelow) over heated germanium. The reaction is reversible and disproportionation of germanium(II) chloride is complete at about 720 K at atmospheric pressure:

GeCl4 + Ge ^ 2GeCl2

(Germanium(II) fluoride can be prepared by a similar process using a slightly lower temperature.)

Germanium(II) chloride is hydrolysed by water; the reaction can be represented as

GeCl2 + 2H2O -> Ge(OH)2 + 2HC1

but the product Ge(OH)2 may be a hydrated oxide. With hydrogen chloride gas, the reaction is an addition :

GeCl2 -I- HC1 -> GeCl3H [analogous to trichloromethane, (chloroform) CC13H]

In concentrated hydrochloricacid solution, the reaction is

GeCl + Cl" ->[GeCl3]- and salts of this anion are known.

Germanium(IV) chloride can be prepared by passing chlorine over germanium at a temperature of 37CM50 K :

Ge + 2C12 -> GeCl4

It has a covalently bonded structure and is a colourless liquid at room temperature; it is hydrolysed reversibly by water, all the germanium being recoverable by distilling the product with concentrated hydrochloric acid :GeCl4 4- 2H2O — GeO2 4- 4HC1

198 G R O U P IV

Tin

TIN(ll) CHLORIDE

This chloride is prepared by dissolving tin in concentrated hydrochloric acid; on cooling, the solution deposits crystals of hydrated tin(II) chloride, SnCl2. 2H2O ('tin salt'). The anhydrous chloride is prepared by heating tin in a current of hydrogen chloride:

The hydrated salt is decomposed by heat:

SnCl2.2H2O ^ Sn(OH)Cl + HC1 + H2O

This reaction proceeds slowly in aqueous solution, so that the basic salt, Sn(OH)CL is slowly precipitated. Addition of excess hydrochloric acid gives the acids of formulae HSnCl3 and H2SnCl4. Salts of these acids containing the ions SnClJ and SnCl^ (chlorostannates(II)) are known.

A solution of tin(II) chloride is a reducing agent. Hence it reduces : Sn4+ (aq)4- 2e~ -> Sn2 + (aq): E^ - 0.15V

mercury(II) chloride, first to the white insoluble mercury(I) chloride and then, if in excess, to mercury:

2HgCl2 4- SnCl2 -> SnCl4 + Hg2Cl2|

white

Hg2Cl2 + SnCU -> 2Hg| + SnCl4

It reduces iron(IIl) to iron(IF) salts:

2Fe3+ 4- Sn2+ -» Sn4+ + 2Fe2 +

This provides a method of estimating an iron(III) salt. After reduction the iron(II) salt is titrated with manganate(VH) solution.

It reduces nitrobenzene (in the presence of hydrochloric acid) to phenylammonium hydrochloride:

C6H5NO, + 7HC1 4- BSnCU -> C6H5NH, .HC1 + 2H.O

4- 3SnCl4

It reduces phenyl diazonium chloride to phenylhydrazine hydrochloride :

[C6H5. N,]C1 + 4HC1 + 2SnCU -^ C6 H5 NH . NH, . HC1

4- 2SnCl4

GROUP IV 199

Tin(II) chloride is slowly oxidised in air, but keeping a piece of tin metal in the solution prevents this.

TIN(IV) CHLORIDE, SnCl4

Stannic chloride is prepared by treating metallic tin with chlorine:

Sn + 2C1 -+ SnCl4

(This reaction has been used to recover tin from scrap tinplate.) Tin(IV) chloride is a colourless liquid, which fumes in air due to hydrolysis:

SnCl4 + 2H2O ^--- SnO2 + 4HC1 hyd rated

It is soluble in organic solvents (a characteristic of a covalent compound), but dissolves in water and can form hydrates (a characteristic of an ionic compound). Hence the hydra ted Sn4+ must be formed in water and undergo hydrolysis thus (cf. aluminium):

This process goes on until (if alkali is added) the final product is [Sn(OH)6]2~. (If alkali is not added, hydrolysis ultimately gives the hydrated oxide in accordance with the equation above.) The hydrolysis can be suppressed by addition of hydrochloric acid, and with excess of this, hexachlorostannic(IV) acid is formed:

SnCl4 4- 2HC1 -> H2SnIVCl6

Salts of this acid are known and ammonium hexachlorostannate(IV) (NH4)2SnQ6, is used as a mordant.

Lead

LEAD(II) CHLORIDE

The solid is essentially ionic, made up of Pb2+ and Cl~ ions. The vapour contains bent molecules of PbCl2 (cf.SnCU). Lead chloride is precipitated when hydrochloric acid (or a solution of a chloride) is added to a cold solution of a lead(II) salt. It dissolves in hot water but on cooling, is slowly precipitated in crystalline form. It dissolves in excess of concentrated hydrochloric acid to give the acid H2[Pb"Cl4].

200 G R O U P IV

LEAD(II) IODIDE

The solid has a layer structure(p.434). Lead(II) iodide, like lead(II) chloride, is soluble in hot water but on cooling, appears in the form of glistening golden 'spangles'. This reaction is used as a test for lead(II) ions in solution.

LEAD(IV) CHLORIDE

Unlike solid lead(II) chloride which is ionic and which dissolves in water to form hydrated Pb2+ and Cl~ ions, lead(IV) chloride is an essentially covalent volatile compound which is violentlyhydrolysed by water.

Lead(IV) chloride is formed from cold concentrated hydrochloric acid and lead(IV) oxide as described earlier. It readily evolves chlorine by the reversible reaction:

PbCl4 ^ PbCl2 + C12|

Hence, if chlorine is passed into a cold suspension (in hydrochloric acid) of lead(II) chloride, lead(IV) chloride is formed. Addition of ammonium chloride gives the complex salt ammonium hexachloroplumbate(IV) as a yellow precipitate:

2NH4C1 + PbCl4 -> (NH4)2PbIVCl6i

This is filtered off and cold concentrated sulphuric acid added, when lead(IV) chloride separates as an oily yellow liquid:

(NH4)2PbCl6 + H2SO4 -+(NH4)2SO4 + PbCl4 + 2HC1

OTHER IMPORTANT COMPOUNDS

Carbon

CARBIDES

These can be divided into three groups:

The salt-like carbides: Among these are aluminium tricarbide (methanide) A14C3 (containing essentially C4~ ions) in the crystal lattice and the rather more common dicarbides containing the C\ ~ ion, for example calcium dicarbide CaC2; these carbides are hydrolysed by water yielding methane and ethyne respectively:

A14C3 + 12H2O -> 4A1(OH)3 + 3CHJ

CaC2 + 2H2O -> Ca(OH)2 + C2H2|

GROUP IV 201

The covalent carbides: These include boron carbide B4C and silicon carbide SiC; the latter is made by heating a mixture of silica and coke in an electric furnace to about 2000 K :

SiO2 + 3C -> SiC + 2COT

The process is carried out alongside the similar one for producing graphite. Silicon carbide when pure is colourless, but technical silicon carbide (carborundum) is usually grey. These carbides have a diamond-like structure, i.e. covalent bonds extend throughout their crystals, and they are therefore of high melting point and chemically inert. Both are used as abrasives, and boron carbide is used in radiation shielding.

The interstitial carbides: These are formed by the transition metals (e.g. titanium, iron) and have the general formula MXC. They are often non-stoichiometric—the carbon atoms can occupy some or all of the small spaces between the larger metal atoms, the arrangement of which remains essentially the same as in the pure metal (cf. the interstitial hydrides).

CARBON DISULPHIDE, CS2

This was formerly manufactured by passing sulphur vapour over white hot coal or charcoal. An equilibrium was established and the carbon disulphide vapour was condensed, allowing the reaction to proceed:

C + 2S ^ CS2

Large quantities are now manufactured by the reaction between sulphur vapour and methane at a temperature of 900-1000 K in the presence of a clay catalyst:

CH4 + 4S -> CS2 + 2H2S

The CS2 is then removed, after cooling, by a solvent. The molecule has a covalent linear structure S=C=S.

Carbon disulphide is a volatile, evil-smelling liquid, although if carefully purified, the unpleasant smell is removed, as it is due to impurity. The vapour is inflammable and can form explosive mixtures in air:

2CS2 + 5O2 -> 2CO + 4SO2

It is also decomposed by water above 420 K:

CS2 + 2H2O -» CO2 + 2H2S

GROUP IV 203

of sulphuric acid. The lead plates are usually perforated and one is packed with lead(IV) oxide, the other with spongy lead. An inert porous insulator acts as a separator between the plates. When the cell is producing current, the following reactions occur :

Lead(IV) oxide plate (positive) :

PbO2 4- 4H" + 2e' -+ Pb2r + 2H2O

followed by :

Pb2" + SO -

Spongy lead plate (negative) :

Pb -> Pb2+ + 2e~

followed by :

Pb2+ + SO~.......-> PbSO4

Hence the overall chemical reaction in the cell during discharge is :

PbO2 + Pb + 2H2SO4 -> 2PbSO4 4- 2H2O

Hence sulphuric acid is used up and insoluble lead(II) sulphate deposited on both plates. This process maintains a potential difference between the two plates of about 2 V. If now a larger potential difference than this is applied externally to the cell (making the positive plate the anode) then the above overall reaction is reversed, so that lead dioxide is deposited on the anode, lead is deposited on the cathode, and sulphuric acid is re-formed. Hence in the electrolyte, we have :

, t . ._. discharge sulphuric acid «_. . "> water

charge

The density of the electrolyte, measured by a hydrometer, forms a useful indicator of the state of charge or discharge of the battery.

If the charging process continues after all the lead sulphate has been used up, then the charging voltage rises. Hydrogen is liberated from the lead electrode, and oxygen is liberated from the lead dioxide electrode. The accumulator is then said to be "gassing'.

CHEMICAL TEST FOR GROUP IV ELEMENTS

Carbon

All carbon compounds, ifoxidised by either oxygen or an oxide (such

204 GROUP IV

as copper(II) oxide) yield carbon dioxide, which gives a precipitate of calcium carbonate when passed into aqueous calcium hydroxide.

Silicon

All silicon compounds on oxidation yield silica or silicates; these are difficult to detect but silica (given by silicates after acid treatment) is insoluble in all acids except hydrofluoric acid.

Tin

In presence of hydrochloric acid, tin(II) in aqueous solution (1) is precipitated by hydrogen sulphide as brown SnS, and (2)will reduce mercury(II) chloride first to rnercury(I) chloride (white precipitate) and then to metallic mercury.

Tin(IV) in aqueous acid gives a yellow precipitate with hydrogen sulphide, and no reaction with mercury(II) chloride.

Lead

Lead(II) in aqueous solution gives on addition of the appropriate anion (1)a white precipitate of lead(II) chloride, (2)a yellow precipitate of lead(II) chromate, and (3) a yellow precipitate of lead(II) iodide which dissolves on heating and reappears on cooling in the form of glistening 'spangles'.

QUESTIONS

1. Compare and contrast the chemistry of silicon, germanium, tin and lead by referring to the properties and bond types of their oxides and chlorides.

Give brief experimental details to indicate how you could prepare in the laboratory a sample of either tin(IV) chloride or tin(IV) iodide. How far does the chemistry of the oxides and chlorides of carbon

2. What physical and chemical tests could you apply to the oxides and chlorides of Group IV elements to show the changes in their properties as the atomic number of the element increases? At the

G R O U P IV 205

bottom of Group IV tin and lead exhibit two oxidation states. Why are these elements not classified as "transition' metals?

(N, Phys. Sci, A)

3.(a) State two physical and two chemical properties which clearly illustrate the differences between a typical metal and a typical non-metal.

(b)Tor any given group in the Periodic Table, the metallic character of the element increases with the increase in atomic weight of the element.'

Discuss this statement as it applies to the Group IV elements, C, Si, Ge, Sn, Pb, indicating any properties of carbon which appear anomalous. Illustrate your answer by considering:

(i)the physical properties of the elements,

(ii)the reaction of the oxides with sodium hydroxide,

(iii)the reaction of the chlorides with water,

(iv)the stability of the hydrides to heat,

(v)the changes in the stability of oxidation state (IV) with

4. The chemical properties of the elements in a given group of the Periodic Table change with increasing atomic number.

(a)Explain the main factors responsible for this, illustrating your answer by reference to the Group IVB elements, carbon to lead.

(b)Apply the factors outlined under (a) to predict the main chemical properties and bonding relationships of the last three members of Group V of the Periodic Table containing the

elements nitrogen, phosphorus, arsenic, antimony and bismuth. (L, S)

5. Give an account of the chemical properties of the element tin and describe four of its principal compounds. The element germanium (Mendeleef s ekasilicon) lies in Group IV of the Periodic Table below carbon and silicon and above tin and lead. What properties would you predict for this element, for its oxide GeO2 and for its chloride GeCl4? (O and C.S.)

6. By reference to the elements carbon, silicon, tin and lead, show how the properties of an element and those of its compounds can be related to:

(a) the group in the Periodic Table in which the element occurs,

(b) its position in that group. (A, A)