Formation of coloured ions (A-level only) — AQA A-Level Chemistry
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Formation of coloured ions (A-level only) explained
Each combination of metal, oxidation state, ligand and co-ordination number gives a different d orbital splitting and therefore a different colour, making colour a genuine identification tool.
Read the full explanation
Learn the aqueous ions first: [Cu(H₂O)₆]²⁺ is blue, [Fe(H₂O)₆]²⁺ pale green, [Co(H₂O)₆]²⁺ pink, [Cr(H₂O)₆]³⁺ ruby/green and [Mn(H₂O)₆]²⁺ very pale pink. For [Fe(H₂O)₆]³⁺, the pure ion is purple, but it often appears yellow or orange-brown in solution due to hydrolysis; AQA accepts these colours. Then learn the changes: excess ammonia turns copper(II) deep blue as [Cu(NH₃)₄(H₂O)₂]²⁺ forms, concentrated hydrochloric acid turns it yellow as [CuCl₄]²⁻ forms, and cobalt(II) goes from pink to blue. Oxidation state matters too, since MnO₄⁻ is purple while Mn²⁺ is almost colourless.
Colour arises when some of the wavelengths of visible light are absorbed and the remaining wavelengths of light are transmitted or reflected.
White light contains all the visible wavelengths. When it passes through a solution of a transition metal complex, the frequencies whose energy matches the d orbital splitting are absorbed, and the light that reaches your eye is what is left over, transmitted through the solution or reflected from a solid. The colour you see is therefore the complement of the colour absorbed. A solution of [CuCl₄]²⁻ looks yellow because everything except yellow light has been removed, and the mark scheme for exactly that question awards one mark for saying visible light is absorbed and another for saying only yellow light is transmitted or reflected. Note the two traps: the complex does not absorb yellow light, and it does not emit light.
d electrons move from the ground state to an excited state when light is absorbed.
In a free gaseous ion the five d orbitals have the same energy. Ligands approaching the metal repel electrons in the d orbitals pointing towards them more than those pointing between them, so the set splits into a lower group and an upper group separated by an energy gap ΔE. An electron in the lower group can absorb a photon whose energy is exactly ΔE and jump to the upper group; that is the ground state to excited state transition, and the missing photons produce the colour. Two conditions must hold: there must be electrons in the lower set to promote, and spaces in the upper set to promote them into. Sc³⁺ is 3d⁰ and Zn²⁺ is 3d¹⁰, so neither can do it and both are colourless.
The energy difference between the ground state and the excited state of the d electrons is given by: ∆E = hν = hc/λ Changes in oxidation state, co-ordination number and ligand alter ∆E and this leads to a change in colour.
ΔE = hν = hc/λ ties the size of the d orbital gap to the light absorbed. Use h = 6.63 × 10⁻³⁴ J s and c = 3.00 × 10⁸ m s⁻¹, and remember that λ must be in metres, so a wavelength quoted as 600 nm becomes 6.00 × 10⁻⁷ m. A bigger gap means a higher frequency and a shorter wavelength absorbed, which changes the colour you see. Three things change the gap and the mark scheme names exactly these: the oxidation state of the metal, the ligands attached, and the co-ordination number. That is why [Cu(H₂O)₆]²⁺ is pale blue, [Cu(NH₃)₄(H₂O)₂]²⁺ is deep blue and [CuCl₄]²⁻ is yellow: the same metal, but a different gap in each.
The absorption of visible light is used in spectroscopy.
A UV-visible spectrometer passes light of each wavelength in turn through a sample and records how much is absorbed, producing an absorption spectrum of absorbance against wavelength. The peak of that curve, λmax, is the wavelength most strongly absorbed, and it corresponds to the d orbital gap through ΔE = hc/λ. Because every complex has its own λmax, the spectrum identifies the species present, and the same measurement tells you which colour filter to use in a colorimeter: choose the filter that transmits the light the solution absorbs most strongly, which is the complementary colour to the one you see. Absorbance is also proportional to concentration, and that is what makes the technique quantitative.
A simple colorimeter can be used to determine the concentration of coloured ions in solution.
The routine is fixed. Select the filter that transmits the light the solution absorbs most strongly, the complementary colour. Zero the colorimeter with a blank of pure solvent so that the cell and the water contribute nothing. Make up a set of standard solutions of known concentration by accurate dilution of a stock solution, measure the absorbance of each, and plot a calibration curve of absorbance against concentration, which should be a straight line through the origin. Then measure the absorbance of the unknown under identical conditions and read its concentration from the graph. If the ion is too faintly coloured, add a ligand first to convert it into a more intensely coloured complex. Every solution must be measured with the same filter and the same cell.
Your focus
- Recall the colours of the common hexaaqua ions of copper, iron, cobalt, chromium and manganese.
- Identify an unknown transition metal ion from an observed colour and its change on adding a named reagent.
- Explain why two solutions containing the same metal can differ in colour by referring to ligand or oxidation state.
Show all 18 objectives
- Explain the colour of a named complex by stating which light is absorbed and which is transmitted.
- Distinguish absorption from emission when accounting for the colour of a solution.
- Predict how the observed colour relates to the wavelengths removed from white light.
- Describe the splitting of the d sub-level by ligands and label the gap between the two levels.
- Explain absorption of light as promotion of a d electron from the ground state to the excited state.
- Deduce whether a given ion will be coloured from the number of electrons in its d sub-level.
- Calculate ΔE from a given wavelength or frequency, converting units to metres first.
- State the three changes to a complex that alter ΔE and give an example of each.
- Explain why two copper(II) complexes with different ligands are different colours.
- Interpret an absorption spectrum to find λmax for a coloured complex.
- Calculate ΔE for a complex from its λmax using ΔE = hc/λ.
- Justify the choice of colour filter for a colorimetric measurement on a given coloured solution.
- Outline a colorimetric method for finding the concentration of a coloured ion, from filter choice to final reading.
- Plot and use a calibration curve of absorbance against concentration.
- Explain why a blank is used and why the same filter and cell must be used throughout.
Formation of coloured ions (A-level only) exam tips
Marking Points
- one mark for the correct colour of a named aqueous transition metal ion
- one mark for the colour of the product complex after a named reagent has been added
- one mark for identifying the ion present from a described colour and a stated colour change
- one mark for naming the change responsible, such as a change of ligand or of oxidation state
- one mark for stating that visible or white light is absorbed as d electrons are excited
- one mark for stating that the remaining wavelengths are transmitted or reflected to the eye
- one mark for naming the observed colour as the light that is not absorbed
- one mark for referring to light explicitly in at least one of those statements
- one mark for stating that the d orbitals are split into two energy levels by the ligands
- one mark for an electron being promoted from the ground state to the excited state by absorbing light
- one mark for the energy of the absorbed photon being equal to ΔE
- one mark for explaining a colourless ion such as Sc³⁺ or Zn²⁺ by its empty or full d sub-level
- one mark for converting the wavelength into metres before substituting into ΔE = hc/λ
- one mark for correct substitution into ΔE = hν or ΔE = hc/λ
- one mark for the answer in joules with the correct power of ten
- one mark for naming oxidation state, ligand and co-ordination number as the three changes that alter ΔE
- one mark for describing an absorption spectrum as absorbance plotted against wavelength
- one mark for reading λmax from a spectrum as the wavelength of maximum absorbance
- one mark for using λmax in ΔE = hc/λ to find the d orbital splitting energy
- one mark for selecting a filter that transmits the light the solution absorbs most strongly
- one mark for choosing a filter of the colour complementary to the solution
- one mark for zeroing the instrument with a blank containing solvent only
- one mark for preparing standard solutions of known concentration and plotting a calibration curve
- one mark for reading the unknown concentration from the calibration curve using its measured absorbance
Examiner Tips
- 💡State the full formula of the complex alongside the colour, because a colour on its own often scores only half.
- 💡Use colourless for a solution with no colour and reserve white for a solid.
- 💡Learn colours in pairs, starting solution and finishing solution, because questions almost always ask for a change.
- 💡Write two clauses: what is absorbed, then what is transmitted or reflected.
- 💡Never use the word emitted about the colour of a complex.
- 💡Name the observed colour as the transmitted light, matching the colour given in the question.
- 💡Use the words ground state and excited state, because the mark scheme uses them.
- 💡Draw the split as two horizontal levels with the gap labelled ΔE whenever a diagram is allowed.
- 💡Check the d electron count before claiming that a species is coloured.
- 💡Write the conversion from nanometres to metres in your working so the examiner can follow it.
- 💡Quote the answer in joules per photon unless the question asks for kJ mol⁻¹, which needs a further step using the Avogadro constant.
- 💡When asked for three ways to change colour, give the three named in the specification and nothing else.
- 💡Label both axes if asked to sketch a spectrum: absorbance up the side, wavelength across the bottom.
- 💡Say that the filter transmits the light the solution absorbs when justifying a filter choice.
- 💡Use the spectrum for the quantity asked for: λmax gives energy, absorbance gives concentration.
- 💡Name the blank you would use, because distilled water or the solvent is what scores.
- 💡Describe the calibration curve as absorbance against concentration, in that order.
- 💡State that all measurements use the same filter and cell when asked how to make the comparison valid.
Common Mistakes
- describing a solution colour as a precipitate colour, or the other way round
- writing clear instead of colourless, which examiners will not accept as a colour
- assuming every transition metal ion is intensely coloured, when Mn²⁺ is very pale and Zn²⁺ is not a transition metal ion at all
- confusing the colours of iron(II) and iron(III) aqueous ions, or thinking orange-brown is incorrect for iron(III)
- writing that a yellow solution absorbs yellow light, which the mark scheme specifically rejects
- saying the complex emits the colour you see rather than transmitting or reflecting it
- describing the absorption without ever using the word light, which fails the mark scheme requirement
- claiming all wavelengths are absorbed, which would make the solution black rather than coloured
- saying the electron is promoted from the 3d to the 4s sub-level rather than between the split d orbitals
- describing the electron as being emitted from the ion rather than promoted within it
- claiming the absorbed energy can take any value rather than matching the gap exactly
- saying Zn²⁺ is colourless because it has no d electrons, when it has a full 3d¹⁰ sub-level
- substituting a wavelength in nanometres straight into ΔE = hc/λ, giving an answer nine orders of magnitude too small
- multiplying by λ instead of dividing, so producing an energy far too large
- listing concentration or temperature as a way of changing the colour of a complex
- confusing frequency with wavelength and using hλ in place of hc/λ
- reading the wavelength at which absorbance is lowest as λmax
- choosing a colorimeter filter that matches the colour of the solution rather than its complement
- assuming λmax is the same for all complexes of the same metal
- quoting λmax in nanometres and then using it in ΔE = hc/λ without converting to metres
- plotting the calibration graph with concentration on the vertical axis and then reading it incorrectly
- using a different filter or a different cell for the unknown than for the standards
- forgetting to zero with a blank, so that every absorbance reading is offset
- extrapolating far beyond the most concentrated standard, where absorbance is no longer proportional to concentration