Subshell Energy Levels and the Formation of Colour
- When Ligands come along and bond to ions, some of the orbitals are given more energy than others and this splits the 3d orbital into two different energy levels.
- Electrons tend to occupy the lower orbitals (the ground state). To jump up to the higher orbitals (excited states) they need to absorb energy equal to the energy gap/change, /E.
- They absorb this energy from visible light that hits the transition metal ion but the amount of frequency of light absorbed depends on the size of the energy gap.
- The frequencies of light not absorbed are reflected.These reflected frequencies combine to make the complement of the colour of the absorbed frequencies - this is the colour you see.
The energy absorbed when electrons jump up (excite) can be calculated using:
/E = h x v
- /E = energy absorbed
- h = planck's constant (6.63 x 10^-34 js^-1)
- v = frequency of light absorbed (hertz/Hz)
Identifying Transition Metal Ions
The Colour of a complex can be altered by any of the factors that can affect the size of the energy gap.
1. Changes in oxidation state:
- If the oxidation state of a transition metal in a complex ion changes, then the colour of the complex ion may also change.
- i.e. Pale green [Fe(H2O)6]^2+(aq) complex >>> Yellow [Fe(H2O)6]^3+(aq) complex
2. Changes in Coordination number:
- Changes in coordination number may also result in a colour change.
- This always involves a change of ligand too.
- i.e. Blue [Cu(H2O)6]^2+(aq) + 4Cl- >>> Yellow [CuCl4]^2-(aq) + 6H2O(aq)
3. Changes in ligand:
- Changing the ligand can cause a colour change even if the oxidation state and coordination number remain the same.
- i.e. Pink [Co(H2O)6]^2+(aq) + 6NH3(aq) >>> Straw/Pale Brown [Co(NH3)6]^2+(aq) + 6H2O(aq)
- Spectrometry can used to determine the concentration of a solution by measuring how much light it absorbs.
- White light is shone through a filter, which is chosen to only let the colour of light through that is absorbed by the sample.
- The light then passes through the sample to a colorimeter, which shows how much light was absorbed by the sample.
- The more concentrated a coloured solution is, the more light it will absorb.
- This measurement can be used to work out the concentration of a solution of transition metal ions.
- Before you can find the unknown concentration of a sample, you have to produce a calibration graph.
- This involves measuring the absorbances of known concentrations of solutions and plotting the results on a graph.
- Once you have done this, you can measure the absorbance of your sample and read its concentration off of the graph.