Solved: CHEMISTRY Question for NEET

NEET CHEMISTRYMedium

Correct increasing order for the wavelengths of absorption in the visible region for the complexes of $Co^{3+}$ is:

$[Co(en)_3]^{3+} < [Co(NH_3)_6]^{3+} < [Co(H_2O)_6]^{3+}$

$[Co(H_2O)_6]^{3+} < [Co(en)_3]^{3+} < [Co(NH_3)_6]^{3+}$

$[Co(H_2O)_6]^{3+} < [Co(NH_3)_6]^{3+} < [Co(en)_3]^{3+}$

$[Co(NH_3)_6]^{3+} < [Co(en)_3]^{3+} < [Co(H_2O)_6]^{3+}$

Relation between Energy and Wavelength: The energy of the light absorbed corresponds to the crystal field splitting energy ( $\Delta_o$ ). According to the Planck equation, energy ( $E$ ) is inversely proportional to wavelength ( $\lambda$ ): $E = hc/\lambda$ . Therefore, a higher splitting energy results in the absorption of a shorter wavelength .
Spectrochemical Series: The magnitude of the crystal field splitting energy ( $\Delta_o$ ) depends on the field strength of the ligands. The spectrochemical series arranges ligands in order of increasing field strength. From the series provided in the text: $H_2O < NH_3 < en$ .
Order of $\Delta_o$ : Based on ligand strength, the crystal field splitting energy for the $Co^{3+}$ complexes follows the order: $[Co(H_2O)_6]^{3+} < [Co(NH_3)_6]^{3+} < [Co(en)_3]^{3+}$
Order of Wavelength ( $\lambda$ ): Since wavelength is inversely proportional to energy, the order of absorbed wavelengths will be the reverse of the energy order: $[Co(en)_3]^{3+}$ (strongest field, highest energy, shortest $\lambda$ ) < $[Co(NH_3)_6]^{3+}$ < $[Co(H_2O)_6]^{3+}$ (weakest field, lowest energy, longest $\lambda$ ).

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