Question 25 of 44intermediate🔧 ApplyLong Answer5 marks

Predict which of the following will be coloured in aqueous solution? Ti3+Ti^{3+}, V3+V^{3+}, Cu+Cu^{+}, Sc3+Sc^{3+}, Mn2+Mn^{2+}, Fe3+Fe^{3+} and Co2+Co^{2+}. Give reasons for each.

Correct Answer

Coloured ions: Ti³⁺, V³⁺, Mn²⁺, Fe³⁺ and Co²⁺

Colourless ions: Cu⁺ and Sc³⁺

Reason: When an electron from a lower energy d orbital is excited to a higher energy d orbital, the energy of excitation corresponds to the frequency of light absorbed. This frequency generally lies in the visible region. The colour observed corresponds to the complementary colour of the light absorbed.

Ti³⁺ (3d¹): Has one electron in d-orbital. d-d transition possible. Purple coloured.

V³⁺ (3d²): Has two electrons in d-orbitals. d-d transition possible. Green coloured.

Cu⁺ (3d¹⁰): Has completely filled d-orbitals. No d-d transition possible. Colourless.

Sc³⁺ (3d⁰): Has empty d-orbitals. No electron available for d-d transition. Colourless.

Mn²⁺ (3d⁵): Has five electrons in d-orbitals. d-d transition possible. Pink coloured.

Fe³⁺ (3d⁵): Has five electrons in d-orbitals. d-d transition possible. Yellow coloured.

Co²⁺ (3d⁷): Has seven electrons in d-orbitals. d-d transition possible. Pink coloured.

Exercise: EXERCISES | Q: 4.18 | (Page 28)
For More Understanding

Explanation

This question tests understanding of why transition metal ions are coloured. The key concept from section 4.3.10 is that coloured ions result from d-d transitions when electrons are excited from lower to higher energy d orbitals. Ions with 3d⁰ (empty) or 3d¹⁰ (completely filled) configurations cannot undergo d-d transitions and are colourless. Table 4.8 confirms the colours: Ti³⁺ (purple, 3d¹), V³⁺ (green, 3d²), Mn²⁺ (pink, 3d⁵), Fe³⁺ (yellow, 3d⁵), Co²⁺ (pink, 3d⁷) are coloured, while Sc³⁺ (3d⁰) is colourless. Cu⁺ has 3d¹⁰ configuration (completely filled), similar to Zn²⁺ listed as colourless in Table 4.8.

Solution Steps

  1. Step 1: Identify electronic configuration of each ion.

  2. Step 2: Check if d-d transition is possible (partially filled d-orbitals).

  3. Step 3: Refer to Table 4.8 for colours of ions with partially filled d-orbitals.

  4. Step 4: Ions with 3d⁰ or 3d¹⁰ configuration are colourless as no d-d transition occurs.

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