40. ELEMENTS OF SUBGROUP OF CHROME.

Atomic number	Name	Electronic configuration	ρ g/cm3	t0pl. 0C	t0kip. 0C	EO	Atomic radius, nanometer	Degree oxidations
24	Cr chrome	[Ar] 3d54s1	7,2	1857	2672	1,56	0,125	+1,+2,+3, +4,+5,+6
42	Mo molybdenum	[Kr] 4d55s1	10,2	2610	2560	1,3	0,135	+1,+2,+3, +4,+5,+6
74	Tungsten W	[Xe] 4f145d46s2	19,3	3410	5660	1,4	0,141	+1,+2,+3, +4,+5,+6

Physical properties

Brilliant, grayish color metals. With increase in atomic number temperatures of melting and boiling grow.
Tungsten - the most refractory of the known metals.

Chemical properties

Among Cr – Mo – W reactivity falls.
To increase in oxidation level of elements their oxides and hydroxides naturally have easing of the main properties and strengthening of acid. To the highest RO3 oxides there correspond _{H2RO4 acids}.
In the same direction there is strengthening of oxidizing properties of connections.

Chrome and its connections

Receiving

Aluminothermy: _Cr2O3 + 2Al _Al2O3 + 2Cr
Recovery of oxides carbon oxide (II).
Electrolysis of aqueous solutions of compounds of chrome.
At usual conditions chrome reacts only with fluorine. At high temperatures ^{(higher than 6000C)} interacts with oxygen, halogens, nitrogen, silicon, boron, sulfur, phosphorus.

4Cr + _{3O2  2Cr2O3}
2Cr + _{3Cl2 2CrCl3}
2Cr + _{N2 2CrN} 
2Cr + 3S _Cr2S3

In the heated state reacts with water vapor:

2Cr + _{3H2O Cr2O3} + _3H2

Chrome is dissolved in the diluted strong acids (HCl, _H2SO4)
In lack of air ^{Cr2+ salts}, and on air – ^{Cr3+ salt} are formed.

Cr + 2HCl _CrCl2 + _H2
2Cr + 6HCl + _O22CrCl3 + _2H2O + _H2

Existence of protective oxide film on the surface of metal explains its passivity in relation to concentrated solutions of acids – oxidizers.

Compounds of chrome

Chromous compounds

Chromium oxide (II) and chromium hydroxide (II) have the main character.

_Cr(OH)2 + 2HCl _CrCl2 + _2H2O

Compounds of chrome (II) - strong reducers; turn into compounds of chrome (III) under the influence of air oxygen.

_2CrCl2 + 2HCl _2CrCl3 + _H2
4Cr _(OH)2 + _O2 + _2H2O 4Cr _(OH)3

Compounds of trivalent chrome

_{Cr2O3 chromium oxide (III)} – green, water-insoluble powder. It can be received when calcinating chromium hydroxide (III) or potassium dichromates and ammonium:

2Cr _{Cr2O3 (OH)3} + _{3H2O
4K2Cr2O7  2Cr2O3} + _4K2CrO4 + _{3O2
(NH4) 2Cr2O7  Cr2O3} + _N2 + _4H2O

Amphoteric oxide. At alloyage _{of Cr2O3} with alkalis, soda and acid salts compounds of chrome with oxidation level turn out (+3):

_Cr2O3 + _2NaOH2NaCrO2 + _H2O
Cr2O3 + _{Na2CO3 2NaCrO2} + _CO2
Cr2O3 + _{6KHSO4Cr2(SO4) 3} + _3K2SO4 + _3H2O

At alloyage with mix of alkali and oxidizer receive compounds of chrome in oxidation level (+6):

_2Cr2O3 + 4KOH + _{KClO3 2K2Cr2O7} (potassium dichromate) + KCl + <sub>2H2O

Cr (OH) 3 chromium hydroxide (III)</sub> - water-insoluble substance of green color.

_Cr2(SO4)3 + 6NaOH 2Cr _(OH)3 + _3Na2SO4

Has amphoteric properties - it is dissolved both in acids, and in alkalis:

 

2Cr _(OH)3 + _{3H2SO4 Cr2(SO4)3} + _{6H2O
Cr(OH)3 + KOHK [Cr(OH)4]}
(or, simplifying, _Cr(OH)3 + KOH KCrO2 (potassium chromate) + _2H2O)

Salts of chrome (III) have violet or dark green coloring. On chemical properties remind colourless salts of aluminum.
The Cr (III) connections can show both oxidizing, and recovery properties:

Zn + _{2Cr+3Cl3 2Cr+2Cl2} + _ZnCl2
^2Cr+3Cl3 + 16NaOH + _3Br2 6NaBr + 6NaCl + _8H2O + _2Na2Cr+6O4

Compounds of hexavalent chromium

 

_{CrO3 chromium oxide (VI)} - bright red crystals, water soluble.
Receive from chromate (or bichromate) potassium and _H2SO4 (конц.).

_K2CrO4 + _H2SO4CrO3 + _K2SO4 + _H2O
K2Cr2O7 + _H2SO42CrO3 + _K2SO4 + _H2O

CrO3 - acid oxide, with alkalis forms yellow ^{CrO42-chromates}:

_CrO3 + _{2KOH K2CrO4} + _H2O

In acidic environment chromates turn into orange bichromates _{of Cr2O72-}:

_2K2CrO4 + _H2SO4K2Cr2O7 + _K2SO4 + _H2O

In alkaline condition this reaction proceeds in the opposite direction:

_K2Cr2O7 + _2KOH2K2CrO4 + _H2O

	acidic environment
2CrO42-+ 2H+		Cr2O72-+ H2O
	alkaline condition

All compounds of chrome (VI) - strong oxidizers.

 

   

 

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