Explain the following:(a) Out of Sc3+, Co2+ and Cr3+ions, only Sc3+ is

1.	Explain the following:(a) Out of Sc3+, Co2+ and Cr3+ions, only Sc3+ is colourless in aqueous solutions.(Atomic no.: Co = 27; Sc = 21 and Cr = 24)(b) The E° Cu2+/ Cu for copper metal is positive (+0.34), unlike the remaining members of the first transition series(c) La(OH)3 is more basic than Lu(OH)3.
Answer» (a) Co²⁺ : [Ar]3d⁷ Sc³⁺ : [Ar]3d⁰Cr³⁺ : [Ar]3d³ Co²⁺ and Cr³⁺ have unpaired electrons. Thus, they are coloured in aqueous solution.Sc³⁺ has no unpaired electron. Thus it is colourless. (b) Metal copper has high enthalpy of atomisation and enthalpy of ionisation. Therefore the high energy required to convert Cu(s) to Cu²⁺(aq) is not balanced by its hydration enthalpy. (c) Due to lanthanoid contraction the size of lanthanoid ion decreases regularly with increase in atomic size. Thus covalent character between lanthanoid ion and OH^- increases from La³⁺ to Lu³⁺. Thus the basic character of hydroxides decreases from La(OH)₃ to Lu(OH)₃

Explain the following:(a) Out of Sc3+, Co2+ and Cr3+ions, only Sc3+ is colourless in aqueous solutions.(Atomic no.: Co = 27; Sc = 21 and Cr = 24)(b) The E° Cu2+/ Cu for copper metal is positive (+0.34), unlike the remaining members of the first transition series(c) La(OH)3 is more basic than Lu(OH)3.

Answer»

(a) Co²⁺ : [Ar]3d⁷ Sc³⁺ : [Ar]3d⁰Cr³⁺ : [Ar]3d³

Co²⁺ and Cr³⁺ have unpaired electrons. Thus, they are coloured in aqueous solution.Sc³⁺ has no unpaired electron. Thus it is colourless.

(b) Metal copper has high enthalpy of atomisation and enthalpy of ionisation. Therefore the high energy required to convert Cu(s) to Cu²⁺(aq) is not balanced by its hydration enthalpy.

(c) Due to lanthanoid contraction the size of lanthanoid ion decreases regularly with increase in atomic size. Thus covalent character between lanthanoid ion and OH^- increases from La³⁺ to Lu³⁺. Thus the basic character of hydroxides decreases from La(OH)₃ to Lu(OH)₃

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