Formation of coloured ions

Question 1

What happens to the d subshell when ligands bond to the central metal ion?

Answer 1

The d subshell is split into two creating an energy gap between the ground state and excited state.

Question 2

Why do transition metals form coloured compounds?

Answer 2

Colour arises when some of the wavelengths of visible light are absorbed and the remaining wavelengths of light are transmitted or reflected due to the energy gap.

Question 3

What formula tells you the energy gap?

Answer 3

Energy gap(J) = planks constant x frequency(Hz) = planks constant x speed of light(ms^-1) / wavelength of light absorbed(m)

Question 4

What are the 4 factors that affect colour?

Answer 4

• Identity of metal
• Oxidation state of metal
• Identity of ligands
• Coordination number

Question 5

Is the colour seen the wavelength of light that is absorbed?

Answer 5

No, it is the wavelengths of light that are reflected we see.

Question 6

What does a colorimeter do?

Answer 6

A colorimeter measures the absorbance of light by a coloured sample.

Question 7

Why is a ligand added to intensify the colour in some ions for colorimetry?

Answer 7

The colour will be too weak otherwise (particularly for solutions with very low concentrations of ions).

Question 8

You are provided with a 1.00 mol dm–3 solution of iron(III) ions and a visible-light spectrophotometer (colorimeter). Outline a plan for experiments using this solution and this apparatus which would enable you to determine the concentration of iron(III) ions in a solution of unknown concentration.

Answer 8

Add an appropriate ligand to intensify colour,
Make up solutions of known concentration,
Measure absorption or transmission,
Plot graph of results or calibration curve,
Measure absorption of unknown and compare.