Germanium, tin, and lead

In the series Ge—Sn—Pb the growth of R_a is observed and metallic properties are strengthened. R_a(Ge) is only slightly larger than R_a(Si), which is above it, therefore, the chemical activity (reducing properties) of simple substances of Ge is low.

Oxidation state (+2) stability grows down the group:

Ge_(cryst) + GeO_2(cryst) = 2GeO_(cryst) DG^o = 83 kJ/mol

Sn + SnO₂ = 2SnO DG^o = 8 kJ/mol

Pb + PbO₂ = 2PbO DG^o = -159 kJ/mol

Ge⁺² ion is a strong reducing agent, and Pb⁴⁺ is a very strong oxidising agent, so, for example, co-existence of ions I^-1(reducing agent) and Pb⁴⁺ is impossible; in other words, compounds PbI₄, and PbBr₄ do not exist, and PbCl₄ is extremely unstable.

The change of oxidation states stability from (+4) to (+2) in the series Ge—Sn—Pb can be seen in the reactions with oxidants:

Interaction with O₂:

Ge + O₂ GeO₂ (strong heating)

Sn + O₂ SnO₂ (slight heating)

2Pb + O₂ = 2PbO (metallic Pb is always covered with protective oxide film)

Interaction with chlorine:

Ge + 2Cl₂ GeCl₄

Sn + 2Cl₂ = SnCl₄ (at STP)

Pb + Cl₂ PbCl₂

Interaction with sulfur:

Ge + 2S GeS₂

Sn + S SnS (but there is also SnS₂)

Pb + S PbS (not PbS₂, because Pb⁴⁺ is a strong oxidising agent).

Ge, Sn, and Pb reactions with acids are significantly different. Ge (is situated after H₂ in the electromotive series of metals) does not replace H₂ from acids. Sn and Pb (both are situated before H₂ in the electromotive series) form soluble salts of divalent elements at the action of strong non-oxidant acids (HCl, diluted H₂SO₄ etc.):

Sn (Pb) + 2HCl = Sn(Pb)Cl₂ + H₂

The low oxidation state (+2, not +4) of metals takes place because H⁺ ion is not a strong oxidant (the reaction 2H⁺ +2e = H₂, E^o = 0 V) and H₂ formed is a reductant. The reaction of Pb with HCl is slow due to the formation of surface film of insoluble PbCl₂. The reaction rate increases in concentrated HCl when soluble chlorocompex is formed:

Pb + 4HCl_(conc) = H₂[PbCl₄] + H₂

SnCl₂ + HCl = H[SnCl₃]

Ge and Sn dissolve in concentrated H₂SO₄:

Ge(Sn) + 4H₂SO_4(conc) = Ge(Sn)(SO₄)₂ + 2SO₂ + 4H₂O

This reaction does not virtually proceed with Pb due to low solubility of PbSO₄. Although when H₂SO₄ concentration exceeds 80%, the reaction proceeds well and soluble acid salt Pb(HSO₄)₂ or complex acid H₂[Pb(SO₄)₂] is formed:

Pb + 3H₂SO_4(conc.) = Pb(HSO₄)₂ + SO₂ + 2H₂O

Reactions with HNO₃:

Ge + 4HNO_3(conc.) = H₂GeO₃ + 4NO₂ + H₂O (germanic acid)

4Sn + 10HNO_{3(very dil., 3%)} = 4Sn(NO₃)₂ + NH₄NO₃ + 3H₂O

3Sn + 4HNO_{3(dil., 30%)} = 3SnO₂ + 4NO + 2H₂O [(SnO₂)_x(H₂O)_y - b-stannic acid]

Sn + 4HNO_{3(conc., >60%)} = SnO₂ + 4NO₂ + 2H₂O

Pb + 4HNO_3(conc.) = Pb(NO₃)₂ + 2NO₂ + 2H₂O

Concentrated HNO₃ passivates Pb, since Pb(NO₃)₂ is insoluble in concentrated HNO₃, although it dissolves well in water, that is why Pb reacts with diluted HNO₃ actively:

3Pb + 8HNO_3(dil.) = 3Pb(NO₃)₂ + 2NO + 4H₂O

[Conclusion: Sn occupies an intermediate position: it reacts with concentrated HNO₃ like Ge, and with diluted as Pb].

Formation of octahedral complexes of these elements is possible as a result of d²sp³-hybridization.

Ge and Sn dissolve well in silicic (HF+HNO₃) and simple (HCl+HNO₃) aqua regua forming H₂ [XF₆] and H₂ [XCl₆] (X = Ge, Sn):

3Ge + 4HNO₃ + 18HF = 2 H₂[GeF₆] + 4NO + 8H₂O

Interaction with alkalis: Ge has no interaction, but at the presence of oxidants:

Ge + 2NaOH + 2H₂O₂ = Na₂[Ge(OH)₆] (hexahydroxogermanate).

Sn and Pb dissolve slowly in strong alkalis when heated:

Sn + 2NaOH + 2H₂O = Na₂[Sn(OH)₄] + H₂

Pb + 2NaOH + 2H₂O = Na₂[Pb(OH)₄] + H₂

This interaction confirms the amphoteric nature of Pb, and, especially, Sn.

COMPOUNDS

Carbon

Carbides. They are binary compounds of carbon with more electropositive elements - metals, silicon, boron.

Carbides are formed only at high temperature interaction of carbon with elements (but technologically more frequent of carbon with oxides). All without exception they are very hard substances. Conventionally they can be divided into three large classes according the structure, character of bond and chemical properties:

· ionic– are formed by active metals (they are easily decomposed by water or acids);

· covalent – have an atomic crystalline lattice, are extraordinarily hard and chemically inert (SiC, B₄C);

· inclusion are metal-like, are conductors of electric current, they are very hard and refractory, extraordinarily chemically inert. The atoms of carbon in them are located between the knots of crystalline lattice of metals. Their composition often does not answer the ordinary valence states of atoms of elements (Mn₄C, Mn₃C, Mn₈C₃; Cr₃C₂, Cr₇C₃; MoC, Mo₂X, WC, W₂C; VC; NbC; Nb₂C; TaC; Ta₂C). Sometimes they have no definite stoichiometry and belong to bertollides, for example, TiC_0,6-1,0, VC_0,58-1,0.

Ionic carbides. This name is only conditional. They are divided on 1) acetylenides (the most widespread class), 2) methanidesand 3) those which are decomposed by acids with formation of mixture of hydrocarbons and hydrogen.

Acetylenides. Are formed by most active metals (for example, Li₂C₂), have in composition bivalent ion C₂^2-. They are easily decomposed by water with exothermic heat effect:

СаС₂ + 2Н₂О = Са(ОН)₂ + С₂Н₂­ DН°₂₉₈ = –125,5 kJ/mol

More frequently, worldwide is used and prepared CaC₂ (~ 5 million of ton annually). It is used for preparation of C₂H₂ in gas welding and as reductant is in metallurgy. The carbide of calcium is preprec in electric furnaces at fusion of calcium oxide with coal:

СаО + 3С = СаС₂ + СО­ DН°₂₉₈ = 464,4 kJ/mol

Methanides. Are known only for the beryllium and aluminium (Be₂C and Al₄C₃). With hot water or diluted acids they are decomposed with the formation of methane:

Al₄C₃ + 12H₂O = 4Al(OH)₃ + 3CH₄

Carbides of the third type. These are Fe₃C, Co₃C and some other. Are decomposed by acids:

Fe₃C + 6HCl = 3FeCl₃ + CH₄ + H₂.

At the same time are formed other gaseous hydrocarbons.

To the separate type belongs carbide Mg₂C₃, which is formed from MgC₂ subject to the condition of breaking of carbon off at 500°. At interaction with water, it gives pure propine:

Mg₂C₃ + H₂O = 2Mg(OH)₂ + CH₃–CºCH

The covalent carbides. Most widespread are SiC, V₄C. They are inorganic polymers, and widely used in industry. A carborundum SiC has high hardness and wearproof, chemically is very inert. V₄C is very hard (~diamond) and chemically stable.

Carbides of inclusion. NbC and TaC are melted at 3500° and 3900° and are most refractory from all known substances. Carbides of tungsten and tantalum are extraordinarily hard and are used for creation of superhard alloys.

Such carbides are decomposed by oxidizing alkaline melts only:

2WC + 5O₂ + 8NaOH 2Na₂WO₄ + 2Na₂CO₃ + 4H₂O

WC is oxidized in this reaction. In this case, it is possible conditionally to adopt the oxidation states of W and C equal to zero (0!). Then W gives 6 electrons, C - 4 electrons, and together WC gives 10 electrons to the molecule O₂, which takes four electrons from WC. Basic coefficients are 2 before WC and 5 before O₂.

HYDROGEN COMPOUNDS

Carbon

C—H bond energy makes up 416 kJ/mol and yields only to the bond C–F. These compounds are named hydrocarbons. Their quantity is approximately 5 million and they make the article of study of organic chemistry. We we will consider CH₄ only.

Methane. CH₄ - at ordinary conditions colourless gas, without a smell and taste. B.p. = -161°C, M.p. = -184 °C. Molecule it is comparative small and unpolar. For this reason it is heavily liquified and badly dissolved in water.

Production.There is no necessity in synthesis of CH₄, although it can be synthesized. Reaction C with H₂ though exothermic, but has a reversible character and at ordinary conditions does not occure:

С + Н₂ Û СН₄ DН^о = -75,3 kJ/mol

Toward forming the CH₄ reaction proceeds at heating of reagents in presence the crushed nickel catalyst at high pressures.

In laboratory. 1.heating of mixture CH₃COOH with a natron lime:

CH₃COOH + NaOH = Na₂CO₃ + CH₄­

2.hydrolysis of aluminium carbide:

Al₄C₃ + 12H₂O = 4Al(OH)₃ + 3CH₄­

Properties.CH₄ easily gets ignited and burns with large emission of heat:

СН₄ + 2О₂ = СО₂ + 2Н₂О DН = -800 kJ^.mol^-1

Its mixture with air is explosive. It is visible from equation, that a mixture in which one volume of CH₄ is mixed with two volumes of O₂ is most explosive. Such correlation is easily achieved and from there follows a necessity to be extremely careful at use of natural gas. The mixture of methane with air at ordinary pressure ignites at ~ 700°C. Methane is toxic gas. For its exposure a little stinking mercaptane is added to natural gas.

CH₄, as well as the other saturated hydrocarbons, at ordinary conditions is very inert. It does is not oxidized by such strong oxidants, as acidified solutions of KMnO₄ K₂Cr₂O₇ are. For inorganic chemistry are important the reactions of its partial oxidization:

CH₄ + 1/2 О₂ = СО + 2Н₂ DН = -27.2 kJ/mol

and conversion:

СН₄ + Н₂О = СО + 3Н₂ DН = 204.6 kJ/mol

In industry these reactions are used for preparation of hydrogen.

COMPOUNDS

Silicon

Silicon compounds

Si has oxidation state (+4) in compounds with non-metals; its oxidation state is (- 4) with the most active metals. Pure ionic compounds of this element are absent because a hypothetical Si ⁺⁴ ion has enormous charge and small size that cannot be stabilized by any chemical environment. Therefore, it is very unstable.

Silicides of metals. Si can form binary compounds with metals. In the molten state, it interacts with Mg, Ca, Fe, Pt, Bi and etc. with the formation of silicides: Li₃Si, CaSi, CaSi₂ and etc.

Silicides of active metals are ionic-covalent compounds that decompose by water or acids:

4CaSi + 9H₂O = CaSiO₃ + 3SiH₄ + 3Ca(OH)₂

Silicon with d-metals forms metal-like, hard, and resistant to chemical attack substances of complex structure: Mn₅Si₃, Cr₃Si, CrSi₂ and etc.

Date: 2016-01-03; view: 1059