Compounds e(IV)

The E(+4) oxidation state is unstable in case of Cr. Its stability is increased slightly in the series Cr–Мо–W in spite of the restricted number of Мо(IV) and W(IV) compounds.

Cr(IV) oxide is usually obtained by heating Cr(OH)₃ in oxygen atmosphere under pressure:

4Cr(OH)₃ + O₂ 4CrO₂ + 6H₂O

This oxide displays ferromagnetic properties, and conductivity of metallic type.

Cr(IV) salts with chrome forming an acid residue are obtained:

BaCrO₄ + Ba(OH)₂ + H₂ = Ba₂CrO₄ + 2H₂O

Mo(IV) oxide can be the product of the reaction of metallic Mo with water steam:

Mo + 2H₂O МоО₂ + 2H₂

МоО₂ reacts with hydroxides of alkali-earth metals and form molybdates(IV):

Ba(ОН)₂ + МоО₂ = BaМоО₃ + Н₂О

So, like CrO₂, МоО₂ displays acid properties.

W(IV) oxide can be obtained by the reaction:

WO₃ + 2W 3WO₂

Compounds e(V)

The oxidation state +5 of chrome is very unstable. Compounds Cr(V) are reduced vigorously to Cr(ІІІ) stable state or disproportionate into Cr(ІІІ) and Cr(VІ). Мо(V) compounds are much more stable, although their stability is decreased in case of W(V) as a result of W(VІ) compounds growth of stability.

Single binary compound of Cr (V) is CrF₅ that is obtained by the reaction of metallic chrome with F₂ at elevated temperature and pressure. It is hard substance of red colour that is immediately decomposed by water. Oxohalides CrOF₃ and CrOCl₃ are also obtained. The more stable CrOF₃ can be prepared with the following reagents: CrO₃ and ClF₃ (BrF₃ and BrF₅).

Binary compounds Мо(V) and W(V) are halides only. MoCl₅ appears after interaction of metallic molybdenum with chlorine (t^o). Vapours of this chloride consist of monomers. Building blocks of crystals are dimers where molybdenum atoms form bridges with chlorine atoms. It is hydrolysed by water easily:

MoCl₅ + H₂O  MoOCl₃ + 2HCl

WF₅ has not been prepared yet. However, WCl₅ can be obtained at WCl₆ reduction by hydrogen. WCl₅ hydrolyses like MoCl₅.The product of hydrolysis is WOCl₄.

Мо(V) compounds in aqueous solutions are not so rare and unstable in contrast to W(V) compounds.

Oxidation state + 6

Compounds Е(VI). The stability of the oxidation state (+6) is significantly increased in the series Cr-Mo-W.

Cr(Vl) compounds. The most important chrome(VI) compounds are chrome trioxide, or chromic acid anhydride CrO₃ and the salts of the relevant acids—chromic H₂CrO₄ and dichromic H₂Cr₂O₇.

Chrome trioxide, or chromic anhydride CrO₃ forms dark red needle-like crystals. This acid oxide is well soluble in water forming chromic acids:

CrO₃ + H₂O H₂CrO₄ chromic acid

2CrO₃ + H₂O H₂Cr₂O₇ dichromic acid

Both acids exist only in an aqueous solution. Their salts, however, are quite stable. The salts of chromic acid are called chromates, and of dichromic acid—dichromates or bichromates.

The more complex H₂Cr_nO_3n+1 isopolyacids acids with n = 1 – 4 [(H₂O)^.nCrO₃] are also formed. It is worth mentioning that these acids are also stable in the aqueous medium only. The product of water evaporation from their solutions is CrO₃. This property of Cr(VI) acids is used to obtain CrO₃. Chrome trioxide precipitates when concentrated sulfuric acid is reacted with a saturated solution of dichromate of an alkali metal:

K₂Cr₂O₇ + H₂SO_{4 (conc.)}= K₂SO₄ + H₂Cr₂O₇

Owing to the dessicating properties of concentrated solutions of sulfuric acid the equilibrium of the second step is almost completely shifted to the right:

H₂Cr₂O₇ =2CrO₃ + H₂O.

H₂Cr₂O₇ is a strong and the most stable acid. The first step of dissociation of H₂CrO₄ is practically complete whereas the degree of dissociation of the second step is low:

H₂CrO₄ H⁺ + HCrO₄^- К₁ = 6^.10^-1

HCrO₄^- H⁺ + CrO₄^2- К₂ = 3^.10^-7

Salts of chromic acids (chromates and dichromates) are stable under ordinary conditions.

Aqueous solutions of chromates are weakly alkaline owing to hydrolysis:

+ H₂O + 2ОН^- ; + Н₂О

Aqueous solutions of dichromates are weakly acid:

Cr₂O₇^2- + H₂O 2HCrO₄^- 2H⁺ + CrO₄^2-

If a solution of a chromate, for example potassium chromate K₂CrO₄, is acidified, the purely yellow colour of the solution changes to an orange one owing to the transformation of CrO₄^2- into Cr₂O₇^2- ions. A potassium dichromate K₂Cr₂O₇ is a salt of dichromic acid that can be isolated as orange-red crystals. The chromate - dichromate transformation reaction is shown below:

2CrO₄- + 2H⁺ = Cr₂O₇^2-+ H₂O

The reaction is reversible and the equilibrium is displaced to dichromate ions Cr₂O₇^2- in an acid medium (the growth of concentration of Н⁺ ions) and to chromate ions CrO₄^2- (Н⁺ ions will bind the hydroxide ions in the solution) in an alkaline medium:

2CrO₄^2- + 2H⁺ Cr₂O₇^2- + H₂O

Thus, chromates are transformed to dichromates in the presence of acids:

2K₂CrO₄ + H₂SO₄ = K₂Cr₂O₇ + K₂SO₄ + H₂O

and dichromates form chromates in the presence of alkalis:

K₂Cr₂O₇ + 2KOH = 2K₂CrO₄ + 2H₂O

The equilibrium can be also shifted by the addition of solutions of salts that form virtually insoluble chromates (Ва, Рb) or dichromates:

K₂Cr₂O₇ + 2Pb(NO₃)₂ + H₂O = 2PbCrO₄ + 2KNO₃ + 2HNO₃

At the interaction of dichromates with concentrated H₂SO₄ the double replacement takes place.:

K₂Cr₂O₇ + H₂SO₄ = K₂SO₄ + H₂Cr₂O₇

Н₂Cr₂O₇ obtained loses water forming CrO₃:

H₂Cr₂O₇ =2CrO₃ + H₂O

Dichromates can be decomposed at high temperature:

2K₂Cr₂O₇ 4KCrO₂ + 2O₂

Alkali metal chromates are prepared by acidifying chrome(III) compounds in the presence of an alkali. For instance, when bromine reacts with a potassium chromite solution, potassium chromate is formed:

2K₃[Cr(OH)₆]+ 3Br₂ + 4KOH = 2K₂CrO₄ + 6KBr + 8H₂O

This oxidation confirms transformation of emerald green colour of the chromite solution into a bright yellow colour of chromate-ion.

Chromates can also be prepared by fusing Cr₂O₃ with an alkali in the presence of an oxidizing agent, for instance potassium chlorate:

Cr₂O₃ + 4KOH + KC1O₃ = 2K₂CrO₄, + KCl + 2H₂O

E(VI) oxidising properties.

Cr(VI) oxidising properties. Chromates and dichromates are strong oxidizing agents and are widely used for oxidizing various substances. The oxidising ability of chrome (VI) compounds depends strongly on the acidity of medium in aqueous solutions and becomes significantly stronger in acid medium:

K₂Cr₂O₇ + 3Na₂SO₃ + 4H₂SO₄ = Cr₂(SO₄)₃ + 3Na₂SO₄ + K₂SO₄ + 4H₂O

K₂Cr₂O₇ + 14HCl = 2CrCl₃ + 3Cl₂ + 2KCl + 7H₂O

K₂Cr₂O₇ + 3K₂S + 7H₂SO₄ = Cr₂(SO₄)₃ + 3S + 4K₂SO₄ + 7H₂O

K₂Cr₂O₇ + 6FeSO₄ + 7H₂SO₄ = Cr₂(SO₄)₃ + 3Fe₂(SO₄)₃ + K₂SO₄ + 7H₂O

2K₂Cr₂O₇ + 3N₂H₄ + 8H₂SO₄ = 2Cr₂(SO₄)₃ + 3N₂ + 2K₂SO₄ + 14H₂O

or in a general case:

K₂Cr₂O₇+ reducing agent + H₂SO₄ (acid medium) = Cr₂(SO₄)₃ +...

Oxidation conducted in an acid solution is usually attended by a sharp change in colour.

The equilibrium that sets in an acid solution is:

Cr₂O₇^2- + 14H+ + 6e^- =2Cr³⁺ + 7H₂O

and in an alkaline one:

[Cr(OH)₆]^3- + 2OH^- = CrO₄^2- + 4H₂O + 3e-

In accordance with Le Chatelier's principle, when the acidity of a solution grows, equilibrium shifts in the direction of the reduction of chrome(VI), and when the acidity diminishes, in the direction of oxidation of chrome(III). In other words, the oxidizing properties of chrome(VI) compounds are expressed the greatest in an acid solution, and the reducing properties of chrome(III) compounds—in an alkaline one. It is exactly for this reason, as indicated above, that chromites and chromates are oxidized in the presence of an alkali, while chrome(VI) compounds are used as oxidizing agents in acid solutions.

Cr₂O₇^2- + 6e^- + 14H⁺ = 2Cr³⁺ + 7H₂O, E^o = 1,36 B

The most important dichromates are potassium dichromate K₂Cr₂O₇ and sodium dichromate Na₂Cr₂O₇•2H₂O that form orange-red crystals. Both salts are widely used as oxidizing agents in the production of many organic compounds, in the leather industry in tanning hides, and in the match and textile industries. A mixture of concentrated sulfuric acid with an aqueous solution of potassium or sodium dichromate known as chromic acid mixture is often used for vigorous oxidation and for cleaning chemical utensils. All salts of chromic acids are poisonous.

Chrome trioxide, or chromic anhydride CrO₃ precipitates as dark red needle-like crystals when concentrated sulphuric acid is reacted with a saturated solution of potassium or sodium dichromate:

K₂Cr₂O₇ + H₂SO₄ = 2CrO₃ +K₂SO₄ + H₂O

Chrome trioxide is one of the strongest oxidizing agents. For example, ethyl alcohol ignites when it comes into contact with it. Upon oxidizing a substance, chrome trioxide transforms into Cr₂O₃. Chrome trioxide readily dissolves in water and gives chromic and dichromic acids.

Peroxocompounds. Some peroxocompounds of chrome are known (e.g blue СrO₅) that have pyramidal shape with apical oxygen atom. СrO₅ is unstable and exists only in aqueous solutions.

Preparation:

K₂Cr₂O₇ + 4H₂O₂ + H₂SO₄ = 2CrO₅ + K₂SO₄ + 5H₂O