2.5.4 formation of coloured ions

how can transition metals be identified?

by their colour

why are complexes formed from transition metal ions coloured?

• some wavelengths of visible light are absorbed

• d orbitals have different energy

• d electrons move from the ground state to an excited state when light is absorbed

• the remaining wavelengths of light that are transmitted (in solutions) or reflected (in solids) to give colour seen

∆E?

units?

• the energy difference between the ground state and the excited state of d electrons

• Joules

why does X transition metal have the colour Y?

• X has an incomplete d sublevel

• visible light is absorbed and d electrons are excited

• only Y light is transmitted/reflected

why does X metal form a white solid / a colourless solution ?

• full or empty d sublevel (e.g. Zn or Sc)

• so cannot absorb visible light

why do different transition metals have different colours?

• d electrons move from the ground state to an excited state when light is absorbed

• ∆E is different for different metals

• so different wavelengths of light will be absorbed by different metals

• so appear different colours

why does X transition metal have different colours when part of different complexes?

• changes in:

  • oxidation state

  • coordination number

  • ligand

• alters ∆E

• so different wavelengths of light will be absorbed

• so leads to change in colour

example of colour change for Cu complex

reasons for colour change

• [Cu(NH₃)₄(H₂O)₂]²⁺ → [CuCl₄]^2-

• deep blue solution → yellow-green solution

• change in coordination number, change in ligand

example of colour change for Fe complex

reasons for colour change

• [Fe(H₂O)₆]²⁺ → [Fe(H₂O)₆]³⁺

• green solution to yellow solution

• change in oxidation state

∆E = ?

∆E = hv

∆E = hc / λ

h = ?

v = ?

c = ?

λ = ?

• h = Planck’s constant = 6.63 × 10^-34 Js

• v = frequency, Hz or s^-1

• c = wave speed, ms^-1

• λ = wave length, m

longer wavelength = ______ ∆E

smaller

(so lower energy absorbed)

how is the absorption of visible light used in spectroscopy?

used in a colorimeter

explain how a simple colorimeter can be used to determine the concentration of coloured ions in a solution

• measure absorbance / transmittance of a range of known concentrations

• plot a graph of absorbance against concentration to produce a calibration curve

  • curve must pass through origin

  • use distilled water to set absorbance to zero (conc of ions in the water will be 0)

• measure absorbance of solution with unknown concentration and read value of concentration for the measured absorbance from your graph

what should be kept constant when using a colorimeter?

• width of sample container

  • as path length is directly proportional to absorbance

    • wider container = more light absorbed

benefits of using colorimetry over titration to determine the concentraiton of coloured ions in solution

• quicker to analyse extracted samples

• uses smaller volumes of solution