Formation of coloured ionsAQA A-Level Chemistry: Flashcards

What these 14 flashcards ask

Why does a transition metal solution look coloured?
What happens to d electrons when light is absorbed?
What is the equation for the energy difference ΔE?
What do h, c and λ stand for?
What does a larger ΔE mean for the light absorbed?
Why is Zn²⁺ colourless?
Why is Sc³⁺ colourless?
Name three changes that alter ΔE and so change colour.
Give an example of a colour change caused by a change in oxidation state.
Give an example of a colour change caused by ligand change.
What is a colorimeter used for?
How do you find the concentration of an unknown using a colorimeter?
Why is a filter used in a colorimeter?
Calculate ΔE in J for light of wavelength 600 nm (h = 6.63 × 10⁻³⁴ J s, c = 3.00 × 10⁸ m s⁻¹).

Exam questions on Formation of coloured ions

Aqueous copper(II) sulfate is blue and aqueous zinc sulfate is colourless. The copper(II) ion has the electron configuration [Ar]3d⁹ and the zinc ion has the configuration [Ar]3d¹⁰.
Explain why aqueous copper(II) sulfate is coloured but aqueous zinc sulfate is colourless.2 marks
A solution of [Ti(H₂O)₆]³⁺ is purple. A student is investigating how the colour of a complex ion is related to the light it absorbs. Use h = 6.63 × 10⁻³⁴ J s and c = 3.00 × 10⁸ m s⁻¹.
The purple solution absorbs green light with a wavelength of 510 nm. Calculate the energy difference, ΔE, in J for one ion.2 marks
A student uses a colorimeter to find the concentration of copper(II) ions in a waste solution. Standard solutions of copper(II) sulfate with concentrations of 0.020, 0.040, 0.060 and 0.080 mol dm⁻³ are each treated with excess ammonia solution, and their absorbances at the chosen wavelength are 0.14, 0.28, 0.42 and 0.56 respectively.
Explain why the student adds excess ammonia solution to each solution, and why a coloured filter is used in the colorimeter.3 marks

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).