Germanium, tin, and lead

Compounds +2. There are oxides (GeO, SnO - black, PbO - yellow-red (litharge), hydroxides M(OH)₂ - white. Salts of oxygencontaining acids are not typical for Sn²⁺ (and, especially, Ge²⁺). However, SnSO₄ is used in electrolytic tinning[1].

EO oxides are produced by hydroxides decomposition when heated in a stream of N₂:

Е(OH)₂ = ЕO + H₂O (Е - Ge, Sn)

PbO:

2Pb(NO₃)₂ = 2PbO + 4NO₂ + O₂

2Pb + O₂ = 2PbO

EO are insoluble in water, so E(OH)₂ are obtained by the double replacement reactions of soluble salts with alkalis. EO and E(OH)₂ are amphoteric since dissolve in acids and alkalis:

Sn(OH)₂ + 2HCl = SnCl₂ + 2H₂O

Sn(OH)₂ + 2NaOH = Na₂[Sn(OH)₄]

In the series Ge—Sn—Pb basic properties of oxygen-containing compounds are enhanced, and acid properties are reduced: in case of Ge(OH)₂ predominate acidic, and in Pb(OH)₂ basic properties. For instance, Pb(OH)₂ has K_1basic = 10^-5, and K_1acid = 10^-11.

Hydrolysis. Soluble E²⁺ salts hydrolyse in water. In connection with the strengthening of basic properties of hydroxides in the series Ge(OH)₂ – Sn(OH)₂ – Pb(OH)₂ degree of hydrolysis of salts decreases. The lowest hydrolysis degree is observed in case of soluble salts of Pb: for example, Pb(NO₃)₃ gives acid reaction. Ge²⁺ salts almost completely decomposed by water in dilute solutions with the formation of Ge(OH)₂. Salts of Sn (II) occupy an intermediate position:

SnCl₂ + H₂O Û SnOHCl¯ + HCl

HCl solution is added to avoid formation of basic salt precipitate.

Insoluble in water compounds are: PbSO₄, PbCrO₄, PbBr₂, PbI₂, with low solubility - PbCl₂. The soluble salts are nitrate, acetate, hexafluorosilicate. Pb(CH₃COO)₂ has a sweet taste and is called lead sugar. All salts of lead are poisonous.

Red-Ox. Oxidation state (+2) is an intermediate state between typical for these elements 0 and (+4), so they can both oxidants and reductants. Oxidising properties are expressed very poorly, (Е^о(Ge²⁺/Ge) = 0,2 V, Е^о(Sn²⁺/Sn) = -0,136 V, Е^о(Pb²⁺/Pb) = -0,126 V, which are close to 0). Examples of these E²⁺ properties are given below:

PbO + CO = Pb + CO₂, or

SnCl₂ + Zn = Sn + ZnCl₂

Pb(CH₃COO)₂ + Fe = Pb + Fe(CH₃COO)₂

Reducing properties in the series Ge—Sn—Pb diminish. They are expressed more strongly in alkaline medium than in acidic (compare the standard potentials):

Е°Sn⁴⁺/Sn²⁺ = + 0,15 V; E° Sn(OH)₆^-2/Sn(OH)₄^-2 = -0,9 V

Е°Pb⁴⁺/Pb²⁺ (H⁺)= + 1,68 V; E° PbO₂/Pb(OH)₂(OH^-)= +0,28 V

SnCl₂ is often used in practice of chemists as a chemical reductant:

SnCl₂ + 2FeCl₃ 2FeCl₂ + SnCl₄

SnCl₂ + HgCl₂ Hg + SnCl₄

3SnCl₂ + 2BiCl₃ + 12NaOH = 2Bi¯ + 3Sn(OH)₄¯ + 12NaCl

3GeCl₂ + 2K₂Cr₂O₇ + 16HCl = 3GeCl₄ + 2CrCl₃ + 4KCl + 8H₂O

Pb²⁺ is a very weak reductant. Therefore, it can be oxidised only by the strongest oxidants in alkaline medium:

Pb(OH)₂ + Cl₂ + 2NaOH = PbO₂ + 2NaCl + 2H₂O

Na₂[Pb(OH)₄] + Br₂ + 2NaOH = PbO₂ + 2NaBr + 2H₂O

Compounds (+4). Production. GeO₂ and SnO₂ are produced by the direct combination of elements. PbO₂ is obtained in the laboratory by Pb(CH₃COO)₂ oxidation with chlorinated lime:

Pb(CH₃COO)₂ + CaOCl₂ + H₂O = PbO₂¯ + CaCl₂ + 2CH₃COOH

Properties. GeO₂ is an acid oxide. Unlike SiO₂, SnO₂, and PbO₂, it is slightly soluble in water (4 g/l), forming hydrates of varying composition, called germanic acid. Meta-H₂GeO₃ and ortho-H₂[Ge(OH)₆] are known. This is a weak acid:

H₂GeO₃ Û H⁺ + HGeO₃^- K₁ = 1^.10^-9

HGeO₃^- Û H⁺ + GeO₃^2- K₂ = 2^.10^-13

However, GeO₂ displays amphoteric properties (reacts with alkalis, forming hydroxogermanates)

GeO₂ + NaOH + 2H₂O = Na₂[Ge(OH)₆],

and when heated with concentrated HCl it forms GeCl₄:

GeO₂ + 4HCl = GeCl₄ + H₂O

SnO₂ is a polymeric inactive substance. It does not react with acids and alkali solutions, but reveals amphoteric properties (when heating, Н₂SO_4(conc.) or fusion with alkalis):

SnO₂ + 2Н₂SO_4(conc.) = Sn(SO₄)₂ + 2H₂O

SnO₂ + 2NaOH = Na₂SnO₃ + H₂O

Tin (+4) hydroxide can be prepared by the double replacement reaction of soluble compounds:

SnCl₄ + 4NaOH = Sn(OH)₄¯ + 4H₂O

This white amorphous precipitate is a polymer of varying composition (SnO₂)_х∙(Н₂О)_y. Some simplified formulae can be offered:

H₂SnO₃ or SnO(OH)₂ (x = 1, y = 1);

H₄SnO₄ оr Sn(OH)₄ - ortho-form (x = 1, y = 2).

Freshly prepared precipitate is called a-stannic acid. It has amphoteric nature and reacts with alkalis and acids easily:

Sn(OH)₄ 4HCl = SnCl₄ + 4H₂O

Sn(OH)₄ + 2NaOH = Na₂[Sn(OH)₆]

When standing (sooner when heating), the process of Sn(OH)₄ “aging” takes place, transforming into inactive form - b-stannic acid. The process of aging illustrates the following scheme:

PbO₂. There are two polymorphs: orthorhombic a-PbO₂ and tetragonal b-PbO₂. b-PbO₂ is the most stable at normal conditions. b-PbO₂ ® a-PbO₂ transition occurs at 300 ^oC and pressure > 13 000 atm. At atmospheric pressure b-PbO₂ decomposes at 280 ^oC (a-PbO₂ at 220 ^oC):

3PbO₂ = Pb₃O₄ + O₂

Preparation.

1. Action of HNO₃ on Pb₃O₄:

Pb₃O₄ + 4HNO₃ = Pb(NO₃)₂ + PbO₂ + 2H₂O not H₄PbO₄

2. Anode oxidation of Pb²⁺ salts:

PbSO₄ + 2H₂O -2e ® PbO₂ + H₂SO₄ + 2H⁺

3. the action of strong oxidants on them:

Pb(CH₃COO)₂ + Cl₂ + 2H₂O = PbO₂ + 2CH₃COOH + 2HCl

Properties. PbO₂ is insoluble and does not react with water. This is an amphoteric oxide and, like SnO₂, it reacts with acids and alkalis at rigid conditions. The difference is that the salts formed, for example, Na₂PbO₃ or Pb(SO₄)₂ do not exist in aqueous solutions and hydrolyse completely. PbO₂ is not soluble in dilute HСl, H₂SO₄, HNO₃, but it dissolves in a mixture of diluted HNO₃ + H₂O₂, and concentrated HСl, Н₂SO₄:

PbO₂ + 2HNO₃ + H₂O₂ = Pb(NO₃)₂ + O₂ + 2H₂O

PbO₂ + 4HCl_(conc.) = PbCl₂ + Cl₂ + 2H₂O (like MnO₂)

2PbO₂ + 2H₂SO_4(conc.) = PbSO₄ + O₂ + 2H₂O

Fine precipitate of PbO₂ is soluble in alkali solutions:

PbO₂ + 2NaOH + 2H₂O ® Na₂[Pb(OH)₆]

PbO₂ is a very strong oxidant (in acidic medium):

5PbO₂ + 2MnSO₄ + 3H₂SO₄ = 5PbSO₄ + 2HMnO₄ + 2H₂O

PbO₂ + Cr³⁺ + H₂O Cr₂O₇^2- + 3Pb²⁺ + 4H⁺

Hydrated PbO₂ form does not exist (they are extremely unstable), i.e. Pb(OH)₄ or plumbic acids are not known. However, salts of these acids (plumbates) can be obtained for many metals, including Pb²⁺:

PbO + PbO₂ = PbPbO₃ (Pb₂O₃) Pb(II) meta-plumbate

2PbO + PbO₂ = Pb₂PbO₄ (Pb₃O₄) Pb(II) ortho-plumbate

Pb₃O₄ is called red lead and used as a red-orange pigment. It can be prepared by heating PbO in air:

Pb₃O₄ reacts with strong acids; its oxidation state does not change, confirming that Pb₃O₄ can be attributed to salts:

Pb₂PbO₄ + 4HNO₃ = 2Pb(NO₃)₂ + H₄PbO₄ ® PbO₂ + 2H₂O (immediate decomposition)

Pb₃O₄ dissolves in alkalis:

Pb₂PbO₄ + 6NaOH + 4H₂O = 2Na₂[Pb(OH)₄] + Na₂[Pb(OH)₆]

This is a strong oxidant:

Pb₃O₄ + 8HCl = 3PbCl₂ + Cl₂ + 4H₂O

In summary, Ge subgroup oxides stability can be described by the scheme:

oxidizing properties growth

GeO₂ SnO₂ PbO₂

stability growth

oxidation reduction

GeO SnO PbO

reducing properties growth

Thus, there is an increase of the lowest oxidation state stability in accordance with amplification of metallic propertiesin the series Ge—Sn—Pb.

Date: 2016-01-03; view: 1057