AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
The energy difference between the ground state and the excited state of…
Full specification wording
Formation of coloured ions (A-level only): 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.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 4 of 6
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
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.
What earns the marks
- Converting the wavelength into metres before substituting into ΔE = hc/λ.
- Correct substitution into ΔE = hν or ΔE = hc/λ.
- The answer in joules with the correct power of ten.
- Naming oxidation state, ligand and co-ordination number as the three changes that alter ΔE.
Common mistakes and how to put them right
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.
Exam tips
- 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.
Try the lesson
The start of the Formation of coloured ions (A-level only): 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. lesson, made from checked teaching notes for this specification point.
Have a go first
Which equation gives the energy difference ∆E between the ground state and the excited state of d electrons in terms of frequency ν?
Listen
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. A strong answer earns marks for these points. converting the wavelength into metres before substituting into ΔE = hc/λ. correct substitution into ΔE = hν or ΔE = hc/λ. the answer in joules with the correct power of ten. naming oxidation state, ligand and co-ordination number as the three changes that alter ΔE. Exam tip: Write the conversion from nanometres to metres in your working so the examiner can follow it.
See it
Key words
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unit conversions
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size of the d orbital gap
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ligand effects on colour
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See every definition
- unit conversions
- The wavelength must be in metres, so a wavelength quoted as 600 nm becomes 6.00 × 10⁻⁷ m before substituting into ΔE = hc/λ.
- size of the d orbital gap
- The energy difference between the ground state and the excited state of the d electrons, calculated using h = 6.63 × 10⁻³⁴ J s and c = 3.00 × 10⁸ m s⁻¹.
- ligand effects on colour
- Changes in oxidation state, co-ordination number and ligand alter ∆E and this leads to a change in colour.
Label the diagram
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1Box 1
2Box 2
3Box 3
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Test your understanding of Formation of coloured ions (A-level only): 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.
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