Chromium:

French chemist Louis Nicolas Vauquelin discovered Chromium in 1797 .

It is given the name as chromium which is from Greek word chroma meaning colour. This name is due to its coloured compounds.

Occurrence:

Chromium is a shiny metal. It is 21^st most abundant element on earth crust. Its important minerals are Chromite or chrome iron stone (FeO.Cr₂O₃), chrome ochre (Cr₂O₃), Crocite or Lead chromate (PbCrO₄).

Characteristics:

(i) Symbol = Cr   

(ii) Atomic Number = 24 

(iii) Luster = Brilliant silvery white 

 (iv) Group = VI-B

(v) Period = 3^rd     

(vi) Melting point = 2173K or 1900^oC                            

(vii) Boiling point = 2963K or 2690^oC            

(viii) Electronic configuration = 1s², 2s², 2p⁶, 3s², 3p⁶, 4s¹, 3d⁵

(ix) it is resistant to corrosion.

(x) Oxidation states:

• The compounds of chromium in which it has +2, +3 and +6 oxidation number are stable.
• Oxidation state +2: Compounds in which oxidation state is +2 (Cr²⁺) are called chromous compounds. e.g. CrCl₂, CrS, CrSO₄.7H₂O, CrCO₃ etc.
• These compounds are unstable and thus they on exposure to air oxidize to corresponding stable Cr³⁺ compounds. These compounds are strong reducing agents.
• Oxidation state +3: Compounds in which oxidation state is +3 (Cr³⁺) are called chromic compounds. e.g. Cr₂O₃, CrCl₃, Cr₂(SO₄)₃.18H₂O etc. These compounds are most stable. These compounds are either produced by the oxidation of Cr²⁺ compounds or by reduction of Cr⁶⁺ compounds.
• Oxidation state +6: Compounds in which oxidation state is +6 (Cr⁶⁺) are strong oxidizing agents. Examples of Cr⁶⁺ compounds are CrO₃, K₂CrO₄, K₂Cr₂O₇ etc.
• The acidic character of oxides increases with increasing oxidation number. Covalent character also increases with increasing oxidation number.

Oxide     

    CrO            Cr₂O₃        CrO₃                      

Oxidation number 

    +2                  +3                     +6

Nature

   Basic          Amphoteric           Acidic

Covalent character

 Ionic        Intermediate  Covalent                              

*Cr₂O₃ partly ionizes.                                             

Chromate – Dichromate equilibrium:

  2CrO₄^2-_(aq) + 2H⁺_(aq)      ⇌  Cr₂O₇^2-_(aq) + H₂O_(l)

This is dynamic equilibrium. According to Le-Chatelier’s principle, on addition of acid equilibrium will shift to the right and on addition of base equilibrium will shift to the left.

Reduction of Dichromate (VI) ions:

 Dichromate (VI) ions reduce to chromium (III) ions when treated with Zinc and dilute acid (HCl or H₂SO₄).

 Cr₂O₇^2- + 14H⁺ + 3Zn → 2Cr³⁺ + 3Zn²⁺ + 7H₂O

 (Orange)                          (Green)

Chromium (III) further reduces by zinc to chromium (II)

            2Cr³⁺ + Zn → 2Cr²⁺ + Zn²⁺

Potassium dichromate (VI) as an oxidizing agent:

Acidified potassium dichromate solution acts as a strong oxidizing agent.

e.g (i) Acidified potassium dichromate solution oxidizes zinc to zinc (II) ion.

Cr₂O₇^2- + 14H⁺ + 3Zn → 2Cr³⁺ + 3Zn²⁺ + 7H₂O

e.g (ii) Acidified potassium dichromate solution oxidizes Fe (II) ion to Fe (III) ion.

Cr₂O₇^2- + 14H⁺ + 6Fe²⁺ → 2Cr³⁺ + 6Fe³⁺ + 7H₂O

e.g (iii) Acidified potassium dichromate solution oxidizes secondary alcohol to corresponding ketone.

e.g (iv) Acidified potassium dichromate solution oxidizes primary alcohol to corresponding aldehyde which in turn oxidizes to corresponding carboxylic acid.

e.g. (v) As an oxidizing agent it redox titration (volumetric analysis):

  Potassium dichromate is used in estimation of Fe²⁺ compounds. Fe²⁺ compounds are reducing agents so they need oxidizing agent for their estimation and potassium dichromate is an oxidizing agent so it is used for their estimation.

Cr₂O₇^2- +  14H⁺     +   6Fe²⁺   →    2Cr³⁺   +   6Fe³⁺ +  7H₂O

Or  

 K₂Cr₂O₇^2-  + 14H⁺ +  6FeSO₄ → Cr₂(SO₄)₃ +  3Fe₂(SO₄)₃ +  7H₂O

Using following formula amount of Iron (II) compound is determined.

                                                                K₂Cr₂O₇                   FeSO₄

 M₁V₁/n1      =       M₂V₂/n2

M₁V₁/1      =        M₂V₂/6

Uses of Chromium and its important compounds:

• Chromium is used in chrome plating. Plating of chromium protects the metal from crossion abd give it attractive luster.
• Uses of K₂Cr₂O₇:
• It is used as oxidizing agent for organic compounds.
• It is used in oxidation reduction titration.
• It is used in tanning of hide called chrome tanning.
• Uses of K₂CrO₄:
• It is used as indicator in argentometeric titration (AgNO₃ titration) as an indicator. It produce pink precipitate of Ag₂CrO₄ at the end point.
• It is used to prepare pigments.

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