Question

Describe giving reason which one of the following pairs has the property indicated : (I) Fe or Cu -- higher melting point (II) (Ti^3+) or (Sc^3+) -- coloured in aqueous solution (III) Cr or Zn -- higher third ionisation enthalpy

Hint:

Colour in transition metal ions is generally due to partially filled d-orbitals. Completely filled or completely empty d-orbitals usually produce colourless ions.

Answer 1

Concept: The elements of the (3d)-series exhibit characteristic physical and chemical properties due to the presence of partially filled (d)-orbitals. Properties such as melting point, colour and ionisation enthalpy are greatly influenced by the number of unpaired electrons, electronic configuration and metallic bonding.

(I) Fe or Cu -- Higher melting point

Step 1: Electronic configurations [ Fe=[Ar],3d^6,4s^2 ] [ Cu=[Ar],3d^10,4s^1 ]

Step 2: Nature of metallic bonding The strength of metallic bonding depends upon the number of unpaired electrons available for delocalisation. Iron possesses a greater number of unpaired electrons than copper. As a result, stronger metallic bonds are formed in iron.

Step 3: Effect on melting point Stronger metallic bonding requires more energy to break. Therefore iron possesses a higher melting point than copper. [ Fe has the higher melting point ] (II) (Ti^3+) or (Sc^3+) -- Coloured in aqueous solution

Step 1: Electronic configurations [ Ti=[Ar],3d^2,4s^2 ] [ Ti^3+=[Ar],3d^1 ] [ Sc=[Ar],3d^1,4s^2 ] [ Sc^3+=[Ar] ]

Step 2: Reason for colour Transition metal ions are coloured when they contain partially filled (d)-orbitals. The colour arises due to (d-d) electronic transitions. [ Ti^3+ ] contains one electron in the (d)-subshell. Therefore (d-d) transitions are possible. Hence it is coloured. [ Sc^3+ ] contains no (d)-electrons. Therefore (d-d) transitions are not possible. Hence it is colourless. [ Ti^3+ is coloured ]

(III) Cr or Zn -- Higher third ionisation enthalpy

Step 1: Electronic configurations [ Cr=[Ar],3d^5,4s^1 ] [ Zn=[Ar],3d^10,4s^2 ]

Step 2: Third ionisation process For chromium: [ Cr^2+=[Ar],3d^4 ] Removal of the third electron gives: [ Cr^3+=[Ar],3d^3 ] For zinc: [ Zn^2+=[Ar],3d^10 ] Removal of the third electron requires breaking the completely filled and highly stable (3d^10) configuration.

Step 3: Comparison Since the (3d^10) configuration of (Zn^2+) is exceptionally stable, a very large amount of energy is required to remove the third electron. Therefore: [ Zn has the higher third ionisation enthalpy ]

Final Answers [ (I) Fe ] [ (II) Ti^3+ ] [ (III) Zn ]