Silicon

Elemental Si, especially crystalline, is chemically inactive at STP. It is not soluble in acids. More active amorphous Si reacts with the mixture of HF and HNO_3(conc):

3Si + 4HNO₃ + 18HF = 3H₂[SiF₆] + 4NO + 8H₂O

(HNO₃ oxidizes Si to SiO₂, and SiO₂ with HF form complex).

Si dissolves easily even in dilute solutions of strong alkalis:

Si + 2NaOH + H₂O = Na₂SiO₃ + 2H₂

Amorphous Si directly combines with F₂, at t= 400-600  with O₂, Cl₂, Br₂ and S, and at higher temperatures - with N₂ and carbon.

Silicon has strong affinity for oxygen (H  = -908 kJ / mol) that is why silicothermy is used in the extraction of metals from their compounds. Silicon in these reactions plays a role of reducing agent:

2Cr₂O₃ + 3Si + 3CaO 4Cr + 3CaSiO₃

2V₂O₅ + 5Si 4V + SiO₂

CHEMICAL PROPERTIES

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) G^o = 83 kJ/mol

Sn + SnO₂ = 2SnO G^o = 8 kJ/mol

Pb + PbO₂ = 2PbO G^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 - -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.