Carbon Monooxide ( charcoal gas).

F orms at interaction of CO₂ with an incandescent coal:

CO₂ + C Û CO      DH^о = 171,5 kJ/mol

The reaction is reversible, its shift to the right is predetermined by an entropy factor, and to the left – by enthalpy. Below 400° reaction is practically fully displaced to the left, and higher than 1000°, to the right. At ordinary conditions, CO is fully stable substance.

Preparation.

In industry: in great scale - in the form of generator and aquatic gases.

Generator gas forms at incomplete combustion of anthracite coal or `coke in large furnaces (generators). It is the mixture of gases: 25% CO, 70% H₂, 4% CO₂, other - N₂, CH₄, O₂.

Water-gas forms at passing of water steam above incandescent carbon:

С + Н₂О = СО + Н₂ DН°₂₉₈ = 117,6 kJ/mol

In a laboratory: at decomposition of formic or oxalic acid or their salts at intearaction with hot concentrated H₂SO₄:

НСООН = СО + Н₂О

Н₂С₂О₄ = СО + СО₂ + Н₂О

Structure. A molecule CO is extraordinarily stable (one of most stable among the known molecules). It sustains heating to 6000°. Energy of bond (1069 kJ/mol) is more than in H₂ (938 kJ/mol) and to the triple bond between atoms:

 

 

C                                                          O

             2s             2p                              2s        2p

                C       O                     or                           C   O

 

 

In accordance with a method of Valence bonds two bonds are formed at coupling of two unpaired electrons of carbon and two - of O and both are displaced toward oxygen; another bond forms according donor-acceptor mechanism: to two-electronic orbital of oxygen is overlapped with a lone electron orbital of carbon. This pair is displaced to carbon, and on the whole the molecule of CO is weakly polar (m = 0,37•10^{-29 C.}m).

Properties. Small polarity and capacity for polarizing predetermine low b.p. -191,5° and low solubility in water (3,3 vol. at 100 vol. H₂O).

CO is an extraordinarily toxic gas. Its content in air must not exceed 0,02 mg/l. At slight poisoning, fresh air is an antidote. Qualitatively, the presence of CO in air can be revealed by passing it through a solution of PdCl₂, which darkens:

CO + PdCl₂ + H₂O = Pd¯ + CO₂ + 2HCl

At ordinary conditions, CO is nonsaltforming oxide, it is unreactive with water, acids and alkalis. With alkalis, it reacts only at elevated temperatures, and at the presence of catalysts:

СО + NaOH = HCOONa,

So, CO acts as the anhydride of formic acid.

For CO there are characteristic reactions of addition and oxidation, where it is reduced:

Fe₂O₃ + 3CO = 2Fe + 3CO₂

NiO + CO = Ni + CO₂

CO + Cl₂ = COCl₂ (phosgene)

COCl₂ is slowly decomposed by water, quicker by alkalis and it is a typical chloroanhydride:

COCl₂ + H₂O = CO₂ + 2HCl

Phosgene is very poisonous substance and was used as a chemical weapon.

CO finds wide application in the organic syntheses. It is used as a reagent for preparation of paraffins, petrol, methyl alcohol, aldehydes, a lot of acids (formic, oxalic, vinegar, propionic, adipinic, acrylic), hydrogen cyanide and many other products.

At higher pressures CO reacts with powdered metals with formation of complex metal carbonyls: Fe(CO)₅, Co₂(CO)₈, Ni(CO)₄, Cr(CO)₆ and others. Ligand in these compounds uses unshared electronic pair of carbon. Carbonyls are easily decomposed; they find application for preparation of pure metals. Like CO, all of them are very poisonous.

 

OXYGEN CONTAINING COMPOUNDS

Silicon

Silicon dioxide. Si interacts with O vigorously when heated:

Si + O₂ = SiO₂ DH°₂₉₈ = -908 kJ / mol

SiO₂ structure. Natural SiO₂ occurs in crystalline (quartz) and amorphous forms. Crystalline SiO₂ exists as a set of several polymorphs, which are mutually transformed from one into another in accordance with the following scheme:

 

     β-quartz ^{870 º}        β-tridymite ^{1470 º}      β-cristobalite    ^1723º   liquid                     

 

575 ^º                                            130 ^º                                                   270 ^º                                                         <1650 ^º

 

 

  α-quartz              α-tridymite              α-cristobalite                           glass   

 

Quartz, tridymite, and crystobalite can have mutual transitions from one to another, but they are very slow. Consequently, tridymite, and crystobalite can be stored very long time at room temperature and exist as separate minerals despite their thermodynamic instability. Note that a -modifications are stable at room temperature and b -modifications — at high temperature. At the rapid cooling of the melt, crystallization has not time to occur and amorphous vitreous form, fused silica (quartz glass) appears.

Various modifications of SiO₂ are inorganic polymers with heterochains.

 

These giant molecular entities have strong covalent bonds Si—O—Si.  Each Si atom is surrounded by tetrahedron of four O atoms and O atoms, in turn, connect to each other two such tetrahedrons.

These connected tetrahedrons twisted in a spiral like staircase of the type of screw. Spirals have parallel orientation in space. The crystalline form with rightscrew spirals is called a -quartz, and with leftscrew spirals is called b -quartz.

Properties. Polymeric SiO₂ is not soluble in water at STP. It is stable to attacks of acids. It dissolves only in hydrofluoric acid, HF, due to the formation of volatile SiF₄ with stronger Si—F than Si–O bonds:

SiO₂ + 4HF = SiF₄­ + 2H₂O

SiO₂ dissolves slowly in alkali solutions, and quickly in their melts forming silicates:

SiO₂ + 2NaOH = Na₂SiO₃ + H₂O

SiO₂ is a silicic acid anhydride. As a compound with extremely low volatility (fugitivity), it displaces other acid anhydrides when melting with salts that are used in glassmaking:

SiO₂ + Na₂SO₄ Na₂SiO₃ + SO₃

       

 

Silicic acids. The overwhelming majority of meta-silicic acid, H₂SiO₃, salts are insoluble in water. Na₂SiO₃ is one of the few soluble salts. It is called “soluble glass” and aqueous solutions of the salt – “liquid glass”.

H₂SiO₃ is a very weak acid. Its soluble in water fraction of molecules dissociates in negligible small degree (K₁ = 1 ∙ 10^-10). Liquid glass forms strong alkaline medium and high viscosity after hydrolysis due to the formation of polymorphs of orthosilicic acid (silicate glue):

Na₂SiO₃ + H₂O = Na₂H₂SiO₄

Na₂H₂SiO₄ + 2H₂O = H₄SiO₄ + 2NaOH

Some salt H₂SiO₃ with weak bases completely hydrolyze in a solution. Therefore, they cannot be obtained by the double replacement reaction:

Na₂SiO₃ + 2NH₄Cl + H₂O = H₄SiO₄ + NaCl + 2NH₃­

Metasilicic acid is displaced from its salts in aqueous solutions by other acids, including H₂CO₃. At first, water-soluble orthosilicic acid appears which can exist in very dilute solutions:

Na₂H₂SiO₄ + 2HCl = H₄SiO₄ + 2NaCl

The growth of concentration or just standing leads to polymerization:

 

The elementary block of such a chain polymer is H₂SiO₃, but the further condensation leads to branched, cross-linked, bulk polymers, and, eventually, polymeric SiO₂ due to transformations of OH-groups:

H – O   O – H              H - O   O – H

            Si                +            Si               +          -H₂O

H – O   O – H            H – O   O – H     ^{. . .}

 

                                                                    HO                OH

                                                                                         Si

        OH OH                    H - O    O            O

 

H – O – Si – O – Si – OH -H₂O         Si                               Si

              OH OH                    H – O    O               O

                                                                                         Si

                                                                    HO                 OH

 

Tetrahedral surrounding of silicon by oxygen atoms occurs in any polymeric form of silicic acid. These polymers are insoluble and have nonstoichiometric composition (SiO₂)_x∙(H₂O)_y. When x> 1 they correspond to polysilicic acids, which derivatives is a variety of natural silicates.

Silicate materials are produced artificially in large quantities. The maximal amount among synthetic silicates has glass, which composition expresses the formula Na₂CaSi₆O₁₄ or Na₂O∙CaO∙6SiO₂. The partial replacement of Na, Ca, Si atoms by the atoms of other elements gives a great number of different kinds of special glass depending on the nature of its application. For example, laboratory glassware contains significant content of B₂O₃ and Al₂O₃. It is characterized by high chemical stability (especially, in acidic medium) and low coefficient of thermal incandescences.

 

Silicon halides. Silicon forms halides of general formula SiX₄ with halogens:

SiO₂ + 2C + 2Cl₂  SiCl₄ + 2CO

Tetrahalides are very reactive substances. They hydrolyze easily as halogenanhydrides of ortho-silicic acid:

SiCl_4(aq) + 4H₂O = H₄SiO_4(aq) + 4HCl_(aq)    DG°₂₉₈ = -238 kJ / mol

Hydrolysis is a result of consecutive H₂O molecules addition and HX molecules removal up to H₄SiO₄ formation.

SiX₄ fumes in air due to hydrolysis (e.g. SiCl₄ is used to create artificial fog). SiCl₄ is an active chlorinated agent that interacts with oxides of metals and nonmetals:

3 SiCl₄ + 2Al₂O₃ = 3SiO₂ + 4AlCl₃

SiF₄ interacts with HF acid forming hexafluorosilicic acid, H₂SiF₆:

SiF₄ + 2HF = H₂[SiF₆]

Si atom in H₂SiF₆ is in sp³d²-hybrid state and its coordination number is 6. H₂SiF₆ is not known in the free state, it is a strong acid (» H₂SO₄).

Other silicon tetrahalides do not form hexahalido compounds. Their resistance to various reagents and heating decreases toward SiI₄.

Silicon nitride. Amorphous Si interacts with N₂ only at t>1300 °C. Silicon nitride, Si₃N₄, formed is a solid stable compound that only decomposes slowly by molten alkali or hot concentrated HF:

Si₃N₄ + 12NaOH = 3Na₂SiO₃ + 4NH₃

Si₃N₄ + 16HF = 2(NH₄)₂SiF₆ + SiF₄

Silicon carbide. Si or SiO₂ react with carbon in electric furnaces at t » 2300 °C. SiC is a very hard (the second after diamond) refractory material. It is chemically stable at normal conditions, but decomposes when heated in molten alkali (in the presence of O₂), Cl₂, hot steam:

SiC + 4NaOH + 2O₂ = Na₂SiO₃ + Na₂CO₃ + 2H₂O

              ¯8e                   ­2e×2

SiC + 2Cl₂ SiCl₄ + C

SiC + 2H₂O SiO₂ + CH₄

CHEMICAL COMPOUNDS

Compounds with oxygen

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)₂degree ofhydrolysis 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 ∙( Н ₂ О )_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 H С l, H₂SO₄ , HNO₃ , but it dissolves in a mixture of diluted HNO₃ + H₂O₂ , and concentrated H С l , Н ₂ SO₄ :

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 properties in the series Ge—Sn—Pb.

 

 

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