Oxidation states of chromium
How many oxidation states does chromium have?
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:
Na₃[Cr(OH)₆] = Cr(OH)₃ + 3NaOH (with boiling);
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.
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:
CrO₃ + H₂O = H₂CrO₄;
2CrO₃ + H₂O = H₂Cr₂O₇.
Chromium (VI) oxide enters into typical acid-base and oxidation-reduction reactions:
CrO₃ + CaO = CaCrO₄;
CrO₃ + 2KOH = K₂CrO₄ + H₂O;
- 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.