Chromi­um is an el­e­ment of the 4ᵗʰ pe­ri­od of the 6ᵗʰ group (it is lo­cat­ed in the side sub­group). In na­ture it is only found in the form of com­pounds – for ex­am­ple chromite or cro­coite. As a sim­ple sub­stance it is a sil­very-white met­al cov­ered with an ox­ide film.

Ob­tain­ing chromi­um and its chem­i­cal prop­er­ties

In nor­mal con­di­tions, chromi­um is rather in­ert. Heat­ing sig­nif­i­cant­ly in­creas­es its re­ac­tiv­i­ty. The met­al may be ob­tained by the fol­low­ing re­ac­tions:

2Al + Cr₂O₃ = 2Cr + Al₂O₃ (alu­minother­mic method, rel­a­tive­ly pure met­al may be ob­tained).

Chromites can also be re­duced by coal, but fer­rochrome is ob­tained – an al­loy of iron and chromi­um:

Fe­Cr₂O₄ + 4C = Fe + 2Cr + 4CO.

Chromi­um re­acts with many non-met­als and acids, and also with wa­ter, salts and al­ka­line reagents:

• 4Cr + O₂ = 2CrO₃ (heat­ing to 600 ᵒC or 1112 ᵒF re­quired);

• 2Cr + N₂ = 2CrN (with ni­tro­gen the re­ac­tion takes place at 800-900 ᵒC or 1472-1653 ᵒFᵒ);

• 2Cr + 3Br₂ = 2Cr­Br₃ (with bromine the re­ac­tion is car­ried out a tem­per­a­ture close to the tem­per­a­ture of red in­can­des­cence, with oth­er halo­gens such in­tense heat­ing is not re­quired);

• Cr + S = Cr₂S₃, CrS (sul­fides form with heat­ing up to 300 ᵒC (572 ᵒF));

• 2Cr + 3H₂O = Cr₂O₃ + 3H₂ (in in­can­des­cent state);

• Cr + 2HCl = Cr­Cl₂ + H₂;

• 4Cr + 12H­Cl + 3O₂ = 4Cr­Cl₃ + 6H₂O (in the pres­ence of oxy­gen);

• 2Cr + 3Cu­Cl₂ = 2Cr­Cl₃ + 3Cu;

• 2Cr + KClO₃ = Cr₂O₃ + KCl (chromi­um (III) ox­ide and potas­si­um chlo­ride form).

Con­cen­trat­ed ni­tric and sul­fu­ric acids pas­si­vate chromi­um. The re­ac­tion can only take place with in­tense heat­ing:

• 2Cr + 6H₂­SO₄ = Cr₂(SO₄)₃ + 3SO₂ + 6H₂O;

• Cr + 6H­NO₃ = Cr(NO₃)₃ + 3NO₂ + 3H₂O.

Chromi­um does not re­act di­rect­ly with hy­dro­gen.

Ox­i­da­tion states of chromi­um

In com­pounds (which are bright­ly col­ored in the ma­jor­i­ty of cas­es), chromi­um dis­plays sev­er­al pos­si­ble ox­i­da­tion states - +2, +3, +4 (en­coun­tered quite rarely, chromi­um ox­ide CrO₂ is known), +6.

Chromi­um in the ox­i­da­tion state of +2

Chromi­um (II) ox­ide and hy­drox­ide CrO and Cr(OH)₂ are com­pounds which dis­play typ­i­cal base prop­er­ties. The com­pounds are col­ored – chromi­um (II) ox­ide is black, and the hy­drox­ide is yel­low.

Salts of chromi­um (II) have a bluish col­or. They are usu­al­ly ob­tained by ox­i­da­tion-re­duc­tion re­ac­tions from chromi­um (III). Hy­dro­gen re­leased in the course of re­ac­tion re­duces Cr³⁺ to Cr²⁺. The re­duc­tive abil­i­ty of Cr²⁺ salts is very high (in some cas­es these salts can even dis­place hy­dro­gen from wa­ter).

Chromi­um (II) ox­ide

Chromi­um (II) ox­ide is formed in the de­com­po­si­tion of chromi­um car­bonyl (with heat­ing):

Cr(CO)₆ = CrO + 5CO + C.

Chromi­um amal­gam can also be ox­i­dized by oxy­gen in the air:

2Cr/Hg + O₂ = 2CrO + 2Hg.

With di­lut­ed ox­i­diz­ing acids (ni­tric, sul­fu­ric) chromi­um does not re­act. Chromi­um II ox­ide also does not re­act with al­ka­lis. With mild heat­ing (to 100 ᵒC or 212 ᵒF) it may ox­i­dize:

4CrO + O₂ = 2Cr₂O₃.

Chromi­um (II) ox­ide is a good re­duc­er:

CrO + 2HCl = Cr­Cl₂ + H₂O (re­acts typ­i­cal­ly with hy­drochlo­ric acid – with the for­ma­tion of salt and wa­ter).

Chromi­um can be re­duced from ox­ide with coal:

CrO + C= Cr + CO.

Chromi­um (II) hy­drox­ide

Chromi­um (II) hy­drox­ide can be ob­tained by a re­ac­tion of the cor­re­spond­ing salt with an al­ka­li with­out the pres­ence of oxy­gen:

Cr­Cl₂ + 2NaOH = Cr(OH)₂ + 2NaCl.

In air the hy­drox­ide eas­i­ly ox­i­dizes:

4Cr(OH)₂ + O₂ + 2H₂O = 4Cr(OH)₃.

It dis­plays base prop­er­ties, and re­acts with acids (and with acidic ox­ides):

Cr(OH)₂ + H₂­SO₄ = Cr­SO₄ + 2H₂O.

Salts of chromi­um (II)

Salts of chromi­um (II) are ob­tained in the re­duc­tion of chromi­um (III):

2Cr­Cl₃ + 3Zn + 4HCl = 2Cr­Cl₂ + 3Zn­Cl₂ + 2H₂.

In so­lu­tions, these salts are quick­ly ox­i­dized by oxy­gen in the air:

4Cr­Cl₂ + O₂ + 4HCl = 4Cr­Cl₃ + 2H₂O.

So­lu­tions of salts of chromi­um (II) are col­ored blue.

Chromi­um in an ox­i­da­tion state of +3

The most sta­ble ox­i­da­tion state of chromi­um in com­pounds is +3. Chromi­um dis­plays am­pho­ter­ic prop­er­ties (the com­pound can act as a base or acid de­pend­ing on con­di­tions). It does not dis­solve in wa­ter. Chromi­um (III) ox­ide is a rather in­ert sub­stance, so it re­acts with long heat­ing (or smelt­ing).

Chromi­um (III) ox­ide

Chromi­um (III) ox­ide can be ob­tained in the de­com­po­si­tion of am­mo­ni­um dichro­mate and the re­duc­tion of potas­si­um dichro­mate:

• (NH₄)₂Cr₂O₇ = Cr₂O₃ + N₂ + 4H₂O;

Here you can find some ex­per­i­ments with ni­tro­gen com­pounds.

• 2K₂Cr₂O₇ + 3C = 2Cr₂O₃ + 2K₂­CO₃ + CO₂.

As a base, chromi­um (III) ox­ide acts as a base in the fol­low­ing re­ac­tions (smelt­ing):

• Cr₂O₃ + 3SO₃ = Cr₂(SO₄)₃;

• Cr₂O₃ + 3K₂S₂O₇ = Cr₂(SO₄)₃ + 3K₂­SO₄.

Cr₂O₃ de­pend­ing on con­di­tions may dis­play prop­er­ties of both and acidic and al­ka­line ox­ide. For ex­am­ple, it acts as an acidic ox­ide in smelt­ing with al­ka­lis and car­bon­ates of al­ka­line met­als. Chromites form (some­times they are also called meta-chromites):

Cr₂O₃ + 2KOH = 2KCrO₂ + H₂O.

Chromi­um (III) hy­drox­ide

Chromi­um (III) hy­drox­ide is crys­talline or amor­phic de­pend­ing on con­di­tions sub­stance of a gray­ish-green col­or. It dis­plays am­pho­ter­ic prop­er­ties, and it eas­i­ly dis­solves in acids and al­ka­lis:

• 2Cr(OH)₃ + 6HCl = 2Cr­Cl₃ + 6H₂O;

• Cr(OH)₃ + 3NaOH = Na₃[Cr(OH)₆].

Chromi­um (III) hy­drox­ide can be ob­tained from the ac­cord­ing salt and from a com­plex com­pound:

1. Na₃[Cr(OH)₆] = Cr(OH)₃ + 3NaOH (with boil­ing);

2. Cr­Cl₃ + 3NH₃ + 3H₂O = Cr(OH)₃ + 3N­H₄­Cl.

Salts of chromi­um (III)

There are two types of salts in which chromi­um (III) is present:

• with cations Cr³⁺;

• with chromite-an­ions CrO₂⁻.

Chromites are un­sta­ble in an acidic medi­um:

NaCrO₂ + 4HCl = Cr­Cl₃ + NaCl + 2H₂O (in an ex­cess of acid).

Salts where chromi­um at an ox­i­da­tion state of +3 acts as a cation dis­play all typ­i­cal prop­er­ties of salts (most of them are sol­u­ble in wa­ter and hy­drolyze – they de­com­pose in wa­ter with the for­ma­tion of chromi­um hy­drox­ide Cr(OH)₃):

Cr₂(SO₄)₃ + 6H₂O = 2Cr(OH)₃ + 3H₂­SO₄.

Chromi­um salts with an ox­i­da­tion state of +3 can take part in ox­i­da­tion-re­duc­tion re­ac­tions, for ex­am­ple in the fol­low­ing:

2Cr­Cl₃ + 3Zn + 4HCl = 2Cr­Cl₂ + 3Zn­Cl₂ + 2H₂ (in the re­ac­tion be­tween hy­drochlo­ric acid and zinc, atom­ic hy­dro­gen is re­leased, which re­duces the chromi­um cation to the chromi­um cation).

Chromi­um in the ox­i­da­tion state of +6

Com­pounds of chromi­um in which it dis­plays an ox­i­da­tion state of +6 are strong ox­i­diz­ers. 2 acids are ex­am­ined in this case as hy­drox­yls – chrome HCrO₄ and dechrome H₂Cr₂O₇. They can only ex­ist in so­lu­tions and are prac­ti­cal­ly not used. Their salts have great prac­ti­cal sig­nif­i­cance – chro­mates and dichro­mates ac­cord­ing­ly.

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Chromi­um (VI) ox­ide

Chromi­um (VI) ox­ide is an un­sta­ble sub­stance of a dark red col­or. It can be ob­tained by the de­com­po­si­tion of dichro­mates with sul­fu­ric acid at a tem­per­a­ture of around 200 ᵒC or 392 ᵒF:

Na₂CrO₄ + 2H₂­SO₄ = CrO₃ + 2NaH­SO₄ + H₂O.

The com­pound is hy­gro­scop­ic (ab­sorbs wa­ter) and dis­solves well with the for­ma­tion of acids.

In a so­lu­tion they are in the bal­ance:

2H₂CrO₄ = H₂Cr₂O₇ + H₂O:

1. CrO₃ + H₂O = H₂CrO₄;

2. 2CrO₃ + H₂O = H₂Cr₂O₇.

Chromi­um (VI) ox­ide en­ters into typ­i­cal acid-base and ox­i­da­tion-re­duc­tion re­ac­tions:

1. CrO₃ + CaO = CaCrO₄;

2. CrO₃ + 2KOH = K₂CrO₄ + H₂O;

3. 2CrO₃ + H₂S = 2Cr(OH)₃ + 3S.

Chro­mates and dichro­mates

Chro­mates and dichro­mates are salts of re­spec­tive acids (they are not ex­tract­ed in a free state. Dichro­mates (or­ange) are sta­ble in an acidic medi­um, and chro­mates (yel­low) in an al­ka­line medi­um. In a so­lu­tion a bal­ance is es­tab­lished be­tween them:

2CrO₄²⁻ + 2H⁺ = Cr₂O₇²⁻ + H₂O.

With heat­ing, dichro­mates change to chro­mates:

4K₂Cr₂O₇ = 4K₂CrO₄ + 2Cr₂O₃ + 3O₂.

Chro­mates and dichro­mates of al­ka­line met­als can be ob­tained in the smelt­ing of chromic iron with soda at a tem­per­a­ture of 1000 ᵒC (1832 ᵒF):

4Fe­Cr₂O₄ + 8Na₂­CO₃ + 7O₂ = 8Na₂CrO₄ + 2Fe₂O₃ + 8CO₂.

Chro­mates and dichro­mates are strong ox­i­diz­ers (they act in al­ka­line, neu­tral and acidic medi­ums):

• K₂Cr₂O₇ + 3H₂S + 4H₂­SO₄ = Cr₂(SO₄)₃ + K₂­SO₄ + 3S + 7H₂O (acidic medi­um);

• K₂Cr₂O₇ + 3(NH₄)₂S + H₂O = 2Cr(OH)₃ + 3S + 6NH₃ + 2KOH (neu­tral medi­um);

• 2K₂CrO₄ + 3(NH₄)₂S + 2KOH + 2H₂O = 2K₃[Cr(OH)₆] + 3S + 6NH₃ (al­ka­line medi­um).

Chromi­um com­pounds are of­ten used as ox­i­diz­ers, dye­ing ma­te­ri­als and in­hibitors of met­al cor­ro­sion from the im­pact of the en­vi­ron­ment.