Chemical compounds

Compounds with hydrogen

Germanium, tin, and lead

Ge, Sn and Pb do not react with H₂ directly. The common method of obtaining hydrides:

decomposition of their compounds by diluted acids:

Mg₂Е + 4HCl = ЕH₄ + 2MgCl₂

Hydrides in the series Ge—Sn—Pb elements are low stable. In the series SiH₄ (silane) - GeH₄ (germane) - SnH₄ (stannane) - PbH₄ (plumbane) stability decreases so that the existence of PbH₄ has been confirmed indirectly (increase of fugitivity of Pb in the stream of hydrogen):

GeH₄ = Ge + H₂ (rapidly at 300 ^oC)

SnH₄ = Sn + H₂ (rapidly at 150 ° C)

PbH₄ = Pb + H₂ (immediately at STP)

Ge has the maximal number of compounds with hydrogen, with the homologous series of germanes from GeH₄ to Ge₉H₂₀. All of them have chemical properties that are similar to GeH₄ (gas b.p. = -90 ^oC).

Preparation:

GeO₂ + Na[BH₄] + HCl + H₂O = GeH₄ + H₃BO₃ + NaCl

Germanes are rapidly oxidised:

GeH₄ + O₂ = GeO₂ + 2H₂O

Degree of hydrolysis of germanes is lower compared with silanes. GeH₄ is stable even in 30% solution of alkali, while SiH₄ is easily decomposed even by water.

SnH₄ and Sn₂H₆ are only known in case of tin. SnH₄ is a strong reducing agent that in 15% NaOH and dilute acids reduces HgCl₂ easily:

SnH₄ + 3HgCl₂ = 3Hg + SnCl₂ + 4HCl

There are many derivatives of hydrogen compounds, including relatively stable tetraethyllead, Pb(C₂H₅)₄, used as antidetonator of fuel for internal combustion engines.

Oxygen containing compounds

Carbon

Carbon forms compounds СО, СО₂, С₃О₂, С₅О₂, С₆О₉ and cyclic С₁₂О₁₂ and (C₄O₃)_n.. To inorganic belong mono- and dioxide of carbon. others are derivative of organic substances.

carbon Dioxide (carbon dioxide) CO₂.

It forms at burning of coal and all organic substances.

Preparation. In industry: roasting of limestone:

СаСО₃ = СаО + СО₂ Н = 180 kJ/mol

In a laboratory:

СаСО₃ + 2HCl = CaCl₂ + CO₂ + H₂O (Kipp apparatus)

Structure. CO₂ consists of symmetric linear unpolar molecules (distance of C—O is 0,113 nm): It has sp-hybridization: two -bonds are formed at overlapping of 2 sp-hybride orbitals of C and two 2p_x-orbitals of O.Two other localized -bonds are formed at overlapping of 2p_y and 2p_z-orbitals of C with 2p_y and 2p_z-orbitals of O.

Properties. At ordinary conditions, CO₂ is colourless, uncombustible gas with a weak sourish smell and taste. It is heavier then air - specific density in respect to air makes up 1,529.

Therefore, in the absence of air motion CO₂ is going on the bottom of hollows, where it forms (caves, wells, mines and others. As a result, there is known Dog’s cave near Naples - a man is not hurt, dogs perish. It follows to be very careful at the entrance in such premises, because CO₂ does not support breathing, at concentrations higher then 3% it causes heavy disorder of organism functioning. At concentration 10%, the loss of consciousness and death through the stop of breathing comes quickly.

CO₂ is easily liquefied at 20 and pressure 57 atm. into a colourless mobile liquid with density 0,766 g/cm³_. In the liquid state, it is stored in the balloons painted in a black with yellow inscription carbonic acid.

At rapid evaporation “dry ice” is created, which in the preliminary compressed state slowly evaporates and is used therefore as an effective cooling medium. Sublimation of “dry ice” at ordinary conditions occurs at –78,48 C^.

Chemically, CO₂ is rather inert. It does not support burning, because it is the final product of combustion (it is used as a fire-extinguisher media). In its atmosphere burn only the substances, affinity to oxygen at which are greater than at carbon, for example, magnesium:

CO₂ + Mg = MgO + C H₂₉₈ = –811,7 kJ/mol

At ordinary conditions, CO₂ is well dissolved in water ( 1:1 on a volume). Its saturated water solution has concentration  0,04 M CO₂, pH of solution = 3,7.

At CO₂ dissolution is formed partly H₂CO₃, anhydride of which it is:

CO₂ + Н₂О  Н₂СО₃

This reaction equilibrium is very strongly displaced to the left, therefore greater part of CO₂ is found simply in the dissolved state, and not as H₂CO₃. Taste of this solution is slightly sour.

Carbonic acid

H₂CO₃ is weak dibasic acid:

Н₂СО₃  Н⁺ + НСО₃^- К₁ = 4,2•10^-7

НСО₃^-  Н⁺ + СО₃^2-­ К₂ = 4,8•10^-11

At higher temperatures an equilibrium is displaced toward the practically complete removal of CO₂ from the solution (to the left), and in an alkaline medium as a result of binding of ions H⁺ - to the right.

In the anion СО₃^2- atom of carbon is found in the state of sp²-hybridization:

p-Electrons, which remains by one at every atom, form delocalized _p-p bonds which substantially increase the strength of the structure. This ion is flat in accordance with sp²-hybridization.

To H₂CO₃ correspond two types of salts - neutral, carbonates with an ion СО₃^2- and acidic – hydrogen carbonates which contain an ion HCO₃^-. Most salts of carbonic acid are colourless.

Carbonates. Among them, the water-soluble are only salts of alkali metals (excepting Li₂CO₃) and ammonium. In solution carbonates hydrolyze strongly:

СО₃^2- + Н₂О  НСО₃^- + ОН^- рН > 7

Often this reaction is complicated by the reactions of double exchange between two-charged cations of weak bases and soluble carbonates:

2CuCl₂ + 2Na₂CO₃ + H₂O = (CuOH)₂CO₃ + CO₂ + 4NaCl

In the case of multicharge cations (Fe³⁺, Cr³⁺, Al³⁺, Ti⁴⁺, Zr⁴⁺) hydroxide carbonates are not form even, but as a result of complete hydrolysis are precipitated hydroxides:

2AlCl₃ + 3Na₂CO₃ + 3H₂O = 2Al(OH)₃ + 3CO₂ + 6NaCl

2Al³⁺ + 3CO₃^2- + 3H₂O = 2Al(OH)₃ + 3CO₂

Thermally, the more stable are carbonates, the more active are metals, from which they are formed. So, Na₂CO₃ is melted without decomposition, CoCO₃ decomposes on CoO and CO₂ at 825, and Ag₂CO₃ - at 100. (Analogy with nitrates).

At action of strong acids all carbonates are decomposed with the elimination of CO₂.

Hydrogen carbonates. All known hydrogen carbonates are soluble. In nature, they are slowly formed at interaction of insoluble carbonates with carbonic acid:

СаСО₃ + СО₂ + Н₂О = Са(НСО₃)₂,

therefore, they are always present in natural water. The most used among them is NaHCO₃ - washing soda. Its hydrolysis occurs so:

НСО₃^2- + Н₂О  Н₂СО₃ + ОН^- (К_h)

НСО₃^2-  Н⁺ + СО₃^2- (К_d)

it is needed to confront them, in this case K_h >> K_d, therefore pH ~ 8.

Like carbonates hydrogen carbonates are decomposed by strong acids. At heating higher then 60 they begin to decompose with evolving carbon dioxide:

NaHCO₃ Na₂CO₃ + CO₂ + H₂O

On that thermal decomposition is based one of methods of removal of hardness of water.

carbon Monooxide (charcoal gas).

Forms at interaction of CO₂ with an incandescent coal:

CO₂ + C  CO H^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 Н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 ( = 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).

C

O 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.