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Hydroxides of alkali metals react with

Chemical properties of NaOH. Aqueous solutions.

amphoteric metals and some non-metals:

2NaOH + Zn = Na₂ZnO₂ + H₂

2NaOH + Si + H₂O = Na₂SiO₃ + 2H₂

2NaOH + Cl₂ = NaCl + NaClO + H₂O

Acid and amphoteric oxides, acids and amphoteric hydroxides, salts:

2NaOH + ZnO = Na₂ZnO₂ + H₂O

NaOH + HCl = NaCl + H₂O

NaOH + Al(OH)₃ = Na[Al(OH)₄]

3NaOH + FeCl₃ = Fe(OH)₃ + 3NaCl

Halogenderivatives (halogenanhydrides):

6NaOH + PCl₃ = Na₃PO₃ + 3NaCl + 3H₂O

2. Alkalisinfluence standard electrode potential value, E^o, and can change a character of redox interaction:

in alkaline medium:

I₂ + 2NH₂OH + 2KOH = 2KI + N₂­ + 4H₂O

in acid medium:

3HI + NH₂OH = NH₄I + I₂ + H₂O

When melting with alkalis:

most stable natural materials are destroyed (silicates, alumosilicates):

2NaOH + SiO₂ = Na₂SiO₃ + H₂O

at oxidizing of alkaline melts some anionic compounds of elements are formed:

2Cr₂O₃ + 3O₂ + 8NaOH = 4Na₂CrO₄ + 4H₂O

PEROXIDES, SUPERPEROXIDES, OZONIDES. Best studied are peroxides Na₂O₂, superoxides MO₂ and ozonides МО₃. All of them are crystalline compounds that contain molecular ions О₂^2-, О₂^-, О₃^-in the sites of crystal lattice. First two molecular ions structure was considered by the MO method (see topic “Oxygen”).

Preparation. Ozonidesare obtained according to the reaction:

4О₃ + 4KOH = 4KO₃ + O₂ + 2H₂O

This reaction is the intermolecular redox interaction between O₃ (oxidant) and O^2- (reductant). О₃, while transforming into ozonide, adds one electron from O^2- (coefficients are put to the right), and O₂, forming a molecule, loses 4 electrons. Coefficients are 4 before КО₃ and 1 before O₂.

Properties. Peroxides are strong oxidants, easily decomposed by water and CO₂. Their reaction with water consists of two steps: the strong hydrolysis is explained by active decomposition of Н₂О₂ in an alkaline medium:

Na₂O₂(s) + 2H₂O(l) ¾® 2NaOH(aq) + H₂O₂(aq)

2Н₂О₂ ¾® 2Н₂О + О₂

Peroxides are applied for the regeneration of O₂ and elimination of harmful products of human wastes in the isolated space (submarine boats, space vehicles and others like that):

2Na₂O₂ + 2H₂O = 4NaOH + O₂

2Na₂O₂ + 2CO₂ = 2Na₂CO₃ + O₂

4KO₂ + 2CO₂ = 2К₂СО₃ + 3О₂

The so-called “oxylite” is a mixture of sodium peroxide and potassium superoxide that found practical applications:

Na₂O₂ + 2KO₂ + 2CO₂ = Na₂CO₃ + K₂CO₃ + 2O₂.

There CO₂ is replaced from air and substituted by identical volumes of О₂. This reaction is used in rebreathers (a type of breathing set), the insulating gas-masks and for air-regenerating in submarines. The following reaction allows to eliminate very poisonous hydrogen sulfide:

4Na₂O₂ + H₂S = Na₂SO₄ + 2NaOH + 2Na₂O

Superoxides react with water, e.g.

2KO₂(s) + 2H₂O(l) ¾® 2KOH(aq) + H₂O₂(aq) + O₂(g)

Ozonidesare slowly decomposed at storage already at normal conditions:

2KO₃ = 2KO₂ + О₂

Water decomposes them instantly:

4KO₃ + 2Н₂О = 4КОН + 5О₂

To balance redox reactions with peroxides, superoxides and ozonides it is better to use an ionic method at putting coefficients. For this purpose:

Make general equation of transformation, for example, for the reaction

КО₂ + СО₂ ® К₂СО₃ + О₂

О₂^- ® О^2- + О

Coefficients are put before ions, by evening of charges (2 before О^2- and 1 before О^2-):

2О₂^- ® О^2- + О

The quantities of atoms are evened at the left and at the right (as to the left there are 4 atoms of oxygen, to the right it is required to put coefficient 3 before the atom of oxygen, because before an ion O^2- a putting of a coefficient is impossible – and therefore are evened the charges of ions):

2О₂^- = О^2- + 3О

In order to convert the atoms of oxygen on the right part into molecules all coefficients are doubled:

4О₂^- = 2О^2- + 3О₂

The coefficients got are put onto the final equation of interaction:

4КО₂ + 2СО₂ = 2К₂СО₃ + 3О₂

Hydrides are usually obtained at passing dry H₂ over a heated metal.

Properties. These salt-like (saline, ionic) hydrides are very strong reductants that are oxidized slowly in dry air, and in moist air oxidation results in the self-ignition:

2КН + О₂ ® 2КОН

Hydrides are easily decomposed with water and the CO₂:

КН + Н₂О = КОН + Н₂

КН + СО₂ ® НСООК

Salts of oxygencontaining acids. There is no hydracid for which salts of alkali metals are not known. These salts are crystalline, and stable compounds at STP.

SODA.It is an absolute leader of annual production (hundreds million tons). It is used in glass industry, for softening of water, in production of cleansers and others like that.

Obtaining:

The Leblan method. (1791, France) historically, it was the first method of production. Rock-salt NaCl was treated with concentrated H₂SO₄:

2NaCl + H₂SO₄ = Na₂SO₄ + 2HCl

(here the most important by-product is hydrochloric acid).

Then Na₂SO₄ is mixed up with a limestone (СаСО₃) and coal and roasted to melting in a furnace.

There proceed the reactions:

Na₂SO₄ + 2С = Na₂S + 2СО₂

Na₂S + СаСО₃ = Na₂СО₃ + СаS

Soda is extracted from the melt with water, and insoluble CaS remains as a waste product.

Ammonia-soda process (the Solvay method, 1863,Belgium) is the main method of modern soda production. It gives very pure soda. This method is more effective, than the Leblan method, as less fuel is required.

That method is based on the formation of slightly soluble salt NaHCO₃ , at interaction of NaCl with ammonium hydrogen carbonate NH₄HCO₃ in aqueous solution:

NaCl + NH₄HCO₃ = NaHCO₃¯ + NH₄Cl (I)

In industry gaseous NH₃, and then СО₂ are passed into an almost saturated solution of rock-salt, which interact with the formation of NH₄HCO₃:

NH₃ + CO₂ + H₂O = NH₄HCO₃

СО₂ for the process is obtained by the decomposition of limestone:

СаСО₃ = СаО + СО₂

NaHCO₃ crystallizes and farther becomes separated from the solution by filtration. Partly the product is used as a washing soda, and the bulk is transformed by heating into Na₂CO₃:

2NaHCO₃ Na₂CO₃ + СО₂ + Н₂О

A half of initially taken СО₂ being consumed, it is returned into the process again.

Addition to technological solution of lime water in result of its elimination from NH₃, which was there in a combined form of NH₄Cl and also it is returned into the technological process:

2NH₄Cl + Ca(OH)₂ = CaCl₂ + NH₃ + 3H₂O

Thus, the only waste product of the process (together with unreacted NaCl) is CaCl₂.

Properties. Na₂CO₃ in nonaqueous state is a white powder: density 2.4-2.5 g/cm³, m.p. 850°C. In water it is easily dissolved with a strong warming-up. Its aqueous solutions as a result of hydrolysis (see Carbon) have an alkaline medium.

TESTS FOR alkali and alkaline-earth metals subgroup elements

Читайте також:

1. Heating of Metals
2. Some Alkali Metals Subgroup Trends

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