The d- and f- Block Elements Set-6 MCQ Online Practice Test | Class 12

Q1. Among the following metal ions, which will have the largest (most exothermic) molar enthalpy of hydration? (Use approximate relation $\Delta H_{hyd}\propto \dfrac{Z^2}{r}$ , where $Z$ is ionic charge and $r$ the ionic radius.)

(A)

$Sc^{3+}$

(B)

$Mn^{2+}$

(C)

$Fe^{3+}$

(D)

$Cu^{2+}$

Q2. Using the spin‑only formula $\mu_{so}=\sqrt{n(n+2)}$ (where $n$ is number of unpaired electrons), which statement correctly compares the effective magnetic moments of $[Fe(H_2O)_6]^{2+}$ and $[Fe(CN)_6]^{4-}$ ?

(A)

$\mu_{eff}([Fe(H_2O)_6]^{2+})\approx 4.9\ \mathrm{BM},\;\mu_{eff}([Fe(CN)_6]^{4-})\approx 0\ \mathrm{BM}$

(B)

$\mu_{eff}([Fe(H_2O)_6]^{2+})\approx 0\ \mathrm{BM},\;\mu_{eff}([Fe(CN)_6]^{4-})\approx 4.9\ \mathrm{BM}$

(C)

Both complexes are diamagnetic (both $\approx 0\ \mathrm{BM}$ )

(D)

Both complexes are high‑spin with $\approx 4.9\ \mathrm{BM}$

Q3. For an octahedral complex of a 3d metal with electronic configuration $d^7$ , if the octahedral splitting energy is $\Delta_o = 1.6P$ (pairing energy $P$ ), what is the most likely $d$ ‑orbital occupancy and number of unpaired electrons?

(A)

High‑spin: $t_{2g}^5e_g^2$ , 3 unpaired electrons

(B)

Low‑spin: $t_{2g}^6e_g^1$ , 1 unpaired electron

(C)

Low‑spin: $t_{2g}^6e_g^1$ , 0 unpaired electrons

(D)

High‑spin: $t_{2g}^5e_g^2$ , 4 unpaired electrons

Q4. An octahedral complex of a $d^4$ ion has $\Delta_o = 1.7P$ . Predict the spin state, number of unpaired electrons and whether a strong Jahn–Teller distortion is expected.

(A)

High‑spin $t_{2g}^3e_g^1$ , 4 unpaired electrons; strong Jahn–Teller distortion expected

(B)

Low‑spin $t_{2g}^4e_g^0$ , 2 unpaired electrons; strong Jahn–Teller distortion expected

(C)

High‑spin $t_{2g}^3e_g^1$ , 4 unpaired electrons; no Jahn–Teller distortion

(D)

Low‑spin $t_{2g}^4e_g^0$ , 2 unpaired electrons; no strong Jahn–Teller distortion

Q5. Which statement correctly describes $[Fe(H_2O)_6]^{2+}$ and $[Fe(CN)_6]^{4-}$ in terms of spin state, spin-only magnetic moment and relative d–d absorption energy?

(A)

$[Fe(H_2O)_6]^{2+}$ is high-spin ( $d^6$ , four unpaired electrons, $\mu_{so}\approx4.90\ \mathrm{BM}$ ) and shows d–d absorption at lower energy (longer wavelength) than $[Fe(CN)_6]^{4-}$ ; $[Fe(CN)_6]^{4-}$ is low-spin (diamagnetic) because CN $^-$ is a strong-field ligand.

(B)

$[Fe(H_2O)_6]^{2+}$ is high-spin with $\mu_{so}\approx4.90\ \mathrm{BM}$ ; $[Fe(CN)_6]^{4-}$ is low-spin (diamagnetic); however $[Fe(CN)_6]^{4-}$ shows d–d absorption at lower energy (longer wavelength) than the aqua complex.

(C)

$[Fe(H_2O)_6]^{2+}$ is high-spin; $[Fe(CN)_6]^{4-}$ is also high-spin and paramagnetic; hence the CN complex shows d–d absorption at lower energy than the aqua complex.

(D)

$[Fe(H_2O)_6]^{2+}$ is high-spin but with $\mu_{so}\approx2.83\ \mathrm{BM}$ ; $[Fe(CN)_6]^{4-}$ is low-spin (diamagnetic); and the CN complex absorbs at higher energy than the aqua complex.

Q6. Calculate the spin-only magnetic moment (in Bohr magneton) of $Mn^{2+}$ (high-spin). Use $\mu_{so}=\sqrt{n(n+2)}$ where $n$ is the number of unpaired electrons.

(A)

$4.90\ \mathrm{BM}$

(B)

$5.92\ \mathrm{BM}$

(C)

$2.83\ \mathrm{BM}$

(D)

$0\ \mathrm{BM}$

Q7. For an octahedral $d^4$ complex the pairing energy is $P=160\ \mathrm{kJ\,mol^{-1}}$ and the octahedral splitting is $\Delta_o=280\ \mathrm{kJ\,mol^{-1}}$ . Which spin state is energetically favoured and by what energy (in kJ mol $^{-1}$ )? (Use $E_{\mathrm{LS}}-E_{\mathrm{HS}}=-\Delta_o+2P$ .)

(A)

Low-spin favoured by $40\ \mathrm{kJ\,mol^{-1}}$

(B)

Low-spin favoured by $20\ \mathrm{kJ\,mol^{-1}}$

(C)

High-spin favoured by $20\ \mathrm{kJ\,mol^{-1}}$

(D)

High-spin favoured by $40\ \mathrm{kJ\,mol^{-1}}$

Q8. $Cr^{3+}$ and $Mn^{4+}$ are isoelectronic ( $d^3$ ) and form octahedral complexes with identical ligands. Which statement correctly compares crystal field splitting $\Delta_o$ and metal–ligand (M–L) bond lengths?

(A)

The $Cr^{3+}$ complex has larger $\Delta_o$ and shorter M–L bonds.

(B)

Both complexes have the same $\Delta_o$ and identical M–L bond lengths.

(C)

The $Mn^{4+}$ complex has larger $\Delta_o$ and shorter M–L bonds.

(D)

The $Cr^{3+}$ complex has larger $\Delta_o$ but the $Mn^{4+}$ complex has shorter M–L bonds.

Q9. Consider octahedral complexes of $Cr^{3+}$ ( $3d^3$ ) and $Mo^{3+}$ ( $4d^3$ ) with identical ligands. Which of the following best describes their ligand field splitting $\Delta_o$ , covalent character and M–L bond lengths?

(A)

$Cr^{3+}$ shows larger $\Delta_o$ , greater covalency and shorter M–L bonds than $Mo^{3+}$ .

(B)

$Mo^{3+}$ shows larger $\Delta_o$ , greater covalency and shorter M–L bonds than $Cr^{3+}$ .

(C)

Both have identical $\Delta_o$ , but $Cr^{3+}$ bonds are more covalent and shorter.

(D)

$Mo^{3+}$ has larger $\Delta_o$ yet significantly longer M–L bonds due to screening.

Q10. Which statement correctly describes $[Fe(H_2O)_6]^{2+}$ and $[Fe(CN)_6]^{4-}$ in terms of spin state, spin-only magnetic moment and relative d–d absorption energy?

(A)

$[Fe(H_2O)_6]^{2+}$ is high-spin ( $d^6$ , four unpaired electrons, $\mu_{so}\approx4.90\ \mathrm{BM}$ ) and shows d–d absorption at lower energy (longer wavelength) than $[Fe(CN)_6]^{4-}$ ; $[Fe(CN)_6]^{4-}$ is low-spin (diamagnetic) because CN $^-$ is a strong-field ligand.

(B)

$[Fe(H_2O)_6]^{2+}$ is high-spin with $\mu_{so}\approx4.90\ \mathrm{BM}$ ; $[Fe(CN)_6]^{4-}$ is low-spin (diamagnetic); however $[Fe(CN)_6]^{4-}$ shows d–d absorption at lower energy (longer wavelength) than the aqua complex.

(C)

$[Fe(H_2O)_6]^{2+}$ is high-spin; $[Fe(CN)_6]^{4-}$ is also high-spin and paramagnetic; hence the CN complex shows d–d absorption at lower energy than the aqua complex.

(D)

$[Fe(H_2O)_6]^{2+}$ is high-spin but with $\mu_{so}\approx2.83\ \mathrm{BM}$ ; $[Fe(CN)_6]^{4-}$ is low-spin (diamagnetic); and the CN complex absorbs at higher energy than the aqua complex.

Q11. Consider the complexes $[NiCl_4]^{2-}$ (tetrahedral) and $[Ni(CN)_4]^{2-}$ (square planar). Which statement about their magnetic and electronic properties is correct?

(A)

$[NiCl_4]^{2-}$ is diamagnetic (all electrons paired) and $[Ni(CN)_4]^{2-}$ has two unpaired electrons.

(B)

$[NiCl_4]^{2-}$ is paramagnetic with two unpaired electrons; $[Ni(CN)_4]^{2-}$ is diamagnetic (square planar, low-spin).

(C)

$[NiCl_4]^{2-}$ has four unpaired electrons; $[Ni(CN)_4]^{2-}$ has one unpaired electron.

(D)

$[NiCl_4]^{2-}$ is low-spin $d^8$ and $[Ni(CN)_4]^{2-}$ is high-spin $d^8$ .

Q12. For octahedral complexes the crystal field stabilization energy (CFSE) strongly affects ligand substitution lability. Rank the following electronic configurations in order of increasing lability (most inert first): (i) low-spin $d^6$ , (ii) $d^3$ , (iii) $d^9$ , (iv) high-spin $d^6$ .

(A)

(iii) $d^9$ > (ii) $d^3$ > (iv) high-spin $d^6$ > (i) low-spin $d^6$ .

(B)

(ii) $d^3$ > (i) low-spin $d^6$ > (iii) $d^9$ > (iv) high-spin $d^6$ .

(C)

(iv) high-spin $d^6$ > (iii) $d^9$ > (ii) $d^3$ > (i) low-spin $d^6$ .

(D)

(i) low-spin $d^6$ > (ii) $d^3$ > (iii) $d^9$ > (iv) high-spin $d^6$ .

Q13. Three lanthanide ions $La^{3+}$ , $Nd^{3+}$ and $Lu^{3+}$ are to be separated by fractional crystallization of their oxalates. Which pair will be hardest to separate and why?

(A)

$La^{3+}$ and $Nd^{3+}$ — their ionic radii are very similar due to lanthanide contraction, so oxalate solubilities are nearly the same.

(B)

$La^{3+}$ and $Lu^{3+}$ — a large difference in ionic radii makes them co-precipitate and thus hardest to separate.

(C)

$Nd^{3+}$ and $Lu^{3+}$ — because $Lu^{3+}$ is much smaller, it forms very similar oxalate solubility to Nd $^{3+}$ .

(D)

All three pairs are equally difficult to separate by fractional crystallization.

Q14. Which of the following octahedral 3d complexes will exhibit a strong static Jahn–Teller distortion?

(A)

Only $[Cr^{2+}(H_2O)_6]^{2+}$ and $[Mn^{3+}(H_2O)_6]^{3+}$ (both high-spin $d^4$ ) show strong Jahn–Teller distortion.

(B)

Only $[Cu^{2+}(H_2O)_6]^{2+}$ ( $d^9$ ) shows strong Jahn–Teller distortion.

(C)

$[Mn^{3+}(H_2O)_6]^{3+}$ (high-spin $d^4$ ), $[Cr^{2+}(H_2O)_6]^{2+}$ (high-spin $d^4$ ) and $[Cu^{2+}(H_2O)_6]^{2+}$ ( $d^9$ ) all show strong Jahn–Teller distortion.

(D)

Jahn–Teller distortions here are weak/dynamic rather than strong and static.

Q15. Octahedral complexes of 4d and 5d transition metals are observed to be low-spin more often than analogous 3d complexes. Which explanation best accounts for this trend?

(A)

Only the pairing energy $P$ decreases down the series, so electrons pair readily in 4d/5d.

(B)

Both the crystal field splitting $\Delta_o$ increases (better metal–ligand overlap of more diffuse 4d/5d orbitals) and the pairing energy $P$ decreases, so pairing becomes energetically favourable — hence low-spin states are preferred.

(C)

Only the increase in $\Delta_o$ is responsible; pairing energy remains essentially the same.

(D)

Predominantly relativistic effects (contraction of orbitals) alone cause low-spin behaviour in 4d/5d complexes.

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