CD - Colour

Question 1

What is electronic transition

Answer 1

Where electrons become excited and move to higher energy levels when their molecule absorbs energy

Question 2

Describe which wavelengths cause a compound to seem coloured when electrons absorbs specific wavelengths of light

Answer 2

Only the wavelengths not absorbed by the electrons will be transmitted. These wavelengths are what give the compound colour

Question 3

Define the function of a spectrometer

Answer 3

Spectrometers show which wavelengths of visible and UV light have been absorbed by a test sample

Question 4

Explain why transitional metals appear coloured when they are bonded to ligands

Answer 4

• Transition elements form ions with incomplete d subshells
• Without ligands, all the orbitals in the 3d subshells have the same energy
• When ligands bond to the ions, some orbitals gain more energy than others. This splits the 3d orbitals into two different energy levels
• Electrons tend to occupy lower orbitals, but can be promoted to higher levels when they absorb light. The change in energy corresponds to the frequency of light absorbed

Question 5

Give factors that effect the difference in energy levels for the splitting of d orbitals for complex metal ions

Answer 5

• The central metal ion
• The ligands
• The coordination number

Question 6

Explain how the type covalent bonds effects the energy needed to excite electrons

Answer 6

• Single bonds are formed when 2 atomic orbitals, each holding 1 electron each, come together to form 2 molecular orbitals. Since each orbital can hold 2 electrons, only 1 is filled so the energy gap between the orbitals is very large
• Double bonds have 4 atomic orbitals so the energy gap between the highest filled and lowest empty molecular orbital is smaller than a single bond
• Delocalised systems have many molecular orbitals. These have a much smaller energy gap than in a double bond so can absorb visible light

Question 7

Explain in terms of molecular orbitals why adding functional groups to increase the size of a delocalised system changes the colour of the compound

Answer 7

The more delocalisation, the more molecular orbitals form and thus the closer the energy levels between orbitals.
A decrease in the energy gap means less energy is needed to promote the electrons to the higher orbitals so a lower frequency of light is absorbed

Question 8

Explain conjugation

Answer 8

When C=C and C-C alternate, a delocalised system is formed