CD 5 - Energy and Matter

Question 1

Why do some organic molecules have colour?

Answer 1

• When a substance absorbs light energy, the electrons become excited and are raised to a higher energy level from their ground states
• The difference in energy between the higher energy level and the ground state is equal to the energy absorbed
• For coloured substances, the corresponding wavelength/frequency for this energy will be found in the visible region (using E = hf)

Question 2

What is the relationship between delocalisation and energy absorbed?

Answer 2

• As the amount of delocalisation in a molecule increases, the maximum wavelength absorbed increases
• Therefore energies absorbed are smaller (E = hf) (c = f/)
• Therefore as the amount of delocalisation increases, the difference in energy between (bonding and non-bonding) orbitals must be smaller