Describe, giving the reason, which one of the following pairs has the property indicated :
(I) Fe or Cu – higher melting point
(II) $\mathrm{Ti^{3+}}$ or $\mathrm{Sc^{3+}}$ – coloured in aqueous solution
(III) Cr or Zn – higher third ionisation enthalpy
(I) Fe or Cu – higher melting point
(II) $\mathrm{Ti^{3+}}$ or $\mathrm{Sc^{3+}}$ – coloured in aqueous solution
(III) Cr or Zn – higher third ionisation enthalpy
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Solution and Explanation
(II) answer: $\mathrm{Ti^{3+}}$ is coloured in solution, while $\mathrm{Sc^{3+}}$ is colourless. Colour in these ions comes from d-electrons jumping between d-orbitals (a d to d transition), and for that you need at least one d-electron. $\mathrm{Ti^{3+}}$ has a $\mathrm{3d^1}$ arrangement, so it has one d-electron that can make this jump and absorb light, giving colour. $\mathrm{Sc^{3+}}$ has a $\mathrm{3d^0}$ arrangement with no d-electrons, so no such jump is possible and it stays colourless.
(III) answer: Zinc (Zn) has the higher third ionisation enthalpy. The third electron in zinc has to be pulled out of a full, very stable $\mathrm{3d^{10}}$ shell, which takes a lot of energy. For chromium, removing the third electron leaves $\mathrm{Cr^{3+}}$ with a $\mathrm{3d^3}$ arrangement, which is a fairly stable and easy to reach state, so less energy is needed. That is why zinc needs more energy for its third ionisation.
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