Coloured ions

Question 1

How does the colouring of ions occur?

Answer 1

• compounds absorbing energy that corresponds to light in the visible region of the spectrum
• e.g. if soln looks purple, it absorbs all light from beam of white light shone on it except red and blue
  
  • red and blue light passes through so the solution appears purple

Question 2

Why are transition netal complexes coloured?

Answer 2

• due to part-filled d-orbitals
• therefore e- can move from one d-orbital to another
• in isolated TM, all d-orbitals are of exactly the same energy
• in compound, presence of other atoms nearby makes d-orbitals have slightly different energy
• when electrons move to orbital of higher energy (excited state)
  
  • they absorb energy in visible region o fthe spectrum equal to the difference in energy between levels
• colour is therefore missing from spectrum

Question 3

What does the colour of TMs depend on?

Answer 3

• difference in energy
• higher difference, larger energy, larger frequency, shorter wavelength

Question 4

What affect the change in energy and colour change?

Answer 4

• changes of oxd state
• co-ordination number
• ligand

Question 5

What is colorimeter?

Answer 5

• uses ligh source and detector to measure the amount of light of a particular wavelength that passes through a coloured solution?

Question 6

What is colorimetry used for?

Answer 6

• determine conc
• more conc, less light transmitter
• a calibration graph used to measure the conc of solutions of coloured metal compounds

Question 7

What are the colours of the four vanadium species?

Answer 7

• VO₂⁺ (aq)
  
  • yellow
  • oxidation number of 5
• VO²⁺ (aq)
  
  • blue
  • oxidation number of 4
• V³⁺ (aq)
  
  • green
  • oxidation number of 3
• V²⁺ (aq)
  
  • violet
  • oxidation number of 2

Question 8

Why is copper sulfate coloured and zinc sulfate not coloured?

Answer 8

• Cu
  
  • part-filled d-orbital so electrons can move from one d-orbital to another
• Zn has a full d-orbitals

Question 9

State the electron configuration of a Ti(III) ion and that of a Ti(IV) ion. Explain, in terms of electron configurations and electron transitions, why Ti(III) compounds are usually coloured but Ti(IV) compounds are colourless.

Answer 9

• Ti (IV)
  
  • [Ar]
• Ti (III)
  
  • [Ar] 3d¹
• Ti (III) has a d electron that can be excited to a higher level
• absorbs one colour of ligh from white light
  
  • one wavelength
• Ti (IV) has no d electron
  
  • so no electron transition w/ energy equal to that of visible light

Question 10

Why is spectrometer used to determine conc?

Answer 10

• does not interfere with reaction
• rapid determination of conc

Question 11

when using colorimetry why is it important that the container has the same dimensiuon

Answer 11

absorbtion proportional to path length

Question 12

why is a coloured filter used in colorimetry

Answer 12

that particular wavelength is most strongly absored#

Question 13

why does the electron transition cause the metal ion to be coloured

Answer 13

light absorbed of that wavelength, the rest is transmitted

Question 14

what determines the change in energy

Answer 14

• coordination number,
• type of metal,
• type of ligand
• shape