Transition Metals: Formation of Coloured Ions

Question 1

How does colour arise in transition metals?

Answer 1

• TMs have partially filled d-orbitals and electrons are able to move between d-orbitals
• When ligands bond to ions, d-orbitals split into different energy levels
• For electrons to change from lower orbitals (ground state) to higher orbitals (excited states) they need energy equal to energy gap
• Electrons get this energy from visible light
• Colour of compound is made up of reflected frequencies of visible light

Question 2

How can you measure the amount of energy electrons absorb to become excited?

Answer 2

• △E = h𝝂 = hc/λ

• △E = energy absorbed (J)
• h = Planck’s constant (6.63x10^-34 Js)
• 𝝂 = frequency of light absorbed (Hz/s–1)
• c = speed of light (3.00 x 10^8 ms–1)
• λ = wavelength of light absorbed (m)

Question 3

What is the speed of light in ms-1?

Answer 3

• 3x10^8 ms-1

Question 4

What is the colour of compounds with no 3d electrons or with a full 3d sub-level?

Answer 4

• No electrons will jump, so no energy absorbed

- Compound will look white or colourless because all light will be reflected

Question 5

Which factors can affect the colour of transition metal ions/complexes?

Answer 5

• Changes in oxidation state
• Changes in shape
• Changes in co-ordination number
• Changes in ligand
• These changes alter the △E which affects frequency of light absorbed, therefore affecting the colour

Question 6

What can be used to determine the concentration of solution by measuring how much light is absorbed?

Answer 6

• Spectroscopy

- Involves a colorimeter

Question 7

Describe what happens during spectroscopy

Answer 7

• White light is shone through a filter
• Filter is chosen to only let colour of light through that is absorbed most by sample
• Light passes through sample to colorimeter, which shows how much light was absorbed by sample
• The more concentrated a coloured solution is, the more light it will absorb and the higher the absorbance value
• Produce calibration curve with concentration of ion against relative absorbance

Question 8

Why is spectroscopy a useful method for measuring the concentration of coloured ions?

Answer 8

• Easy to get loads of readings / work out concentration quickly
• Doesn’t use up any substance or interfere with any reactions
• Can measure very low concentrations