chemistry

Chromium- Important Transition Metals

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 21st 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 = 3rd     

(vi) Melting point = 2173K or 1900oC                            

(vii) Boiling point = 2963K or 2690oC            

(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:

                        2CrO42-(aq) + 2H+(aq)                             ⇌            Cr2O72-(aq) + H2O(l)

                   (Yellow)                                                                     (orange)

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).

                      Cr2O72- + 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.

                   Cr2O72- + 14H+ + 3Zn → 2Cr3+ + 3Zn2+ + 7H2O

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

                   Cr2O72- + 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.

  Cr2O72-    +              14H+     +              6Fe2+        →            2Cr3+      +              6Fe3+ +  7H2O

Oxidizing agent                                                                  Reducing agent

Orange                                                   Light Green                 Green                     Yellowish Brown

Or       K2Cr2O72-  + 14H+ +  6FeSO4 → Cr2(SO4)3 +  3Fe2(SO4)3 +  7H2O

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

                                                                K2Cr2O7                   FeSO4

                                                                M1V1      =              M2V2

                                                                   n1                            n2

                                                                M1V1      =              M2V2

                                                                   1                              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.

Also Read !!

Transition Elements

Vanadium

Coordination Compounds 

Peculiar Behaviour of Beryllium 

Group 1 Elements alkali Metals Occurrence, Physical and Chemical Properties, Reactivity and Compounds

Trends in the physical properties of period 3 elements

Trends in the chemical properties of period 3 elements

Anomalous Behavior of Lithium

Group IV-A Elements Properties

Group VII-A Elements Properties

Electron affinity trends

Ionization Energy Trends

Electronegativity – Definition, Periodic Trends ,Examples , Importance ,Electronegativity Difference

Ionic Bond

Covalent Bond

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