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Copper(II) complexes are coloured - AQA - A-Level Chemistry - Question 7 - 2022 - Paper 1
Question 7
Copper(II) complexes are coloured. The colour is caused by the d electrons of copper moving from their ground state to an excited state. When an electron moves from... show full transcript
Worked Solution & Example Answer:Copper(II) complexes are coloured - AQA - A-Level Chemistry - Question 7 - 2022 - Paper 1
Step 1
Explain why aqueous solutions containing [CuCl₂]²⁻ ions are yellow.
Answer
Aqueous solutions containing [CuCl₂]²⁻ ions appear yellow because they absorb visible light, specifically in the blue region of the spectrum. The d electrons in the complex are excited from a lower energy level to a higher energy state when exposed to light. As a result, only yellow light is transmitted or reflected, leading to this characteristic colour.
Step 2
Calculate the frequency, in s⁻¹, of the light absorbed.
Answer
To find the frequency ( u) of the light absorbed, we can use the formula derived from Planck's equation:
u$$ where: - E is the energy change (3.98 × 10⁻¹⁹ J) - h is the Planck constant (6.63 × 10⁻³⁴ Js) Rearranging the formula gives:u = \frac{E}{h} = \frac{3.98 \times 10^{-19} \text{ J}}{6.63 \times 10^{-34} \text{ Js}} \approx 6.00 \times 10^{14} \text{ s}^{-1}$$
Step 3
State three ways in which a transition metal complex can be changed to alter its colour.
Answer
- Changing the oxidation state of the metal: This affects the energy levels and the absorption spectrum.
- Changing the ligand: Different ligands interact differently with the metal ion, altering the colour.
- Changing the coordination number: Varying the number of ligands around the metal can lead to changes in colour.
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