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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, Eo, 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 О22-, 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:

3 + 4KOH = 4KO3 + O2 + 2H2O

This reaction is the intermolecular redox interaction between O3 (oxidant) and O2- (reductant). О3, while transforming into ozonide, adds one electron from O2- (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)

2О2 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- ® О2- + О

Coefficients are put before ions, by evening of charges (2 before О2- and 1 before О2-):

2- ® О2- + 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 O2- a putting of a coefficient is impossible – and therefore are evened the charges of ions):

2- = О2- + 3О

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

2- = 2О2- + 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

 

Group I element Flame colour   Group II element Flame colour
Li Na K Rb Cs   Deep red Golden yellow Lilac Bluish red Blue   Ca Sr   Ba Brick red Blood red or crimson Green

 


Читайте також:

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




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