General Properties of Transition Elements - The d- and f- Block Elements, Class 12, Chemistry Class 12 Notes | EduRev

Chemistry Class 12

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Transition Elements:

Definition: They are often called "transition elements" because their position in the periodic table is between s-block and p-block elements.

Typically, the transition elements have an incompletely filled d-level. Since Zn group has d10 configuration and are not considered as transition elements but they are d-block elements.

General Characteristics:

(i) Metallic character: They are all metals and good conductors of heat & electricity.

(ii) Electronic configuration: (n – 1)d1– 10ns1–2

General Properties of Transition Elements - The d- and f- Block Elements, Class 12, Chemistry Class 12 Notes | EduRev

(iii) M.P.

General Properties of Transition Elements - The d- and f- Block Elements, Class 12, Chemistry Class 12 Notes | EduRev

(iv) Variation in atomic radius:

General Properties of Transition Elements - The d- and f- Block Elements, Class 12, Chemistry Class 12 Notes | EduRev

(v) Variable oxidation states possible:

General Properties of Transition Elements - The d- and f- Block Elements, Class 12, Chemistry Class 12 Notes | EduRev

Colour : (aquated)

Colour : (aquated)

Sc3+ → colourless

Ti4+ → colourless

Ti3+ → purple

V4+ → blue

V3+ → green

V2+ → violet

Cr2+ → blue

Cr3+ → green

Mn3+ violet

Mn2+ → light pink

Fe2+ → light green

Fe3+   yellow

Co2+ → pink

Ni2+ → green

Cu2+ → blue

Zn2+ → colourless

CHROMATE - DICHROMATE:

Preparation:

General Properties of Transition Elements - The d- and f- Block Elements, Class 12, Chemistry Class 12 Notes | EduRev

[Lime (CaO) added with Na2CO3 which keeps the mass porous so that air has access to all parts and prevents fusion.]
Then, 

General Properties of Transition Elements - The d- and f- Block Elements, Class 12, Chemistry Class 12 Notes | EduRevGeneral Properties of Transition Elements - The d- and f- Block Elements, Class 12, Chemistry Class 12 Notes | EduRev

NaCl crystallises out first and filtered off. Then K2Cr2O7 crystallised out on cooling

  • Other props & test of CrO42– & Cr2O72– : Already discussed
  • Similarities between hexavalent Cr & S–compounds.

General Properties of Transition Elements - The d- and f- Block Elements, Class 12, Chemistry Class 12 Notes | EduRevGeneral Properties of Transition Elements - The d- and f- Block Elements, Class 12, Chemistry Class 12 Notes | EduRevGeneral Properties of Transition Elements - The d- and f- Block Elements, Class 12, Chemistry Class 12 Notes | EduRev

Ques: In laboratory K2Cr2O7 is used mainly not Na2Cr2O7. Why?

Ans: Na2Cr2O7 is deliquescent enough and changes its concentration and cannot be taken as primary standard solution whereas K2Cr2O7 has no water of crystallisation and is not deliquescent.

General Properties of Transition Elements - The d- and f- Block Elements, Class 12, Chemistry Class 12 Notes | EduRevFig: Structure of Na2Cr2O7 

Ques: How to standardise Na2S2O3 solution in iodometry?

Ans: K2Cr2O7 is the primary standard ⇒ strength is known by weighing the salt in chemical balance and dissolving in measured amount of water.

Then in acidic solution, add KI

Cr2O72-  + 14 H+ + 6I- → 2Cr3+ + 3I2  + 7H2O

This I2 is liberated can be estimated with S2O32-.

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