UV-visible spectroscopy

1. UV -VIS Worksheet
1. A large number of organic compounds absorb energy in the ultraviolet/visible region of the spectrum.
This occurs as a result of transistions between electronic energy levels in molecules.
a) Give two features in organic molecules which are responsible for absorptions in the
ultraviolet/visible region.[2]
b) Predict which of the molecules shown below will absorb in the ultraviolet/visible region, by
circling the relevant molecules.[2]
c) Compounds D and E are shown below.
Compound D is colorless and absorbs in the ultraviolet region. Compound E is pale yellow,
absorbing in the visible region of the spectrum. Explain why there is a difference in the
absorptions of the two compounds.[3]
d) A hydrocarbon F absorbs in the ultraviolet region of the spectrum. When F reacts with
bromine, a new compound G is formed. The original absorption disappears and a new peak
appears in a different part of the ultraviolet region.
i) Suggest what structural feature is responsible for the absorption in F.[1]
ii) Suggest why compound G shows a new absorption in the ultraviolet region.[2]
2. Organic molecules, such as propanone, (CH3)2CO, absorb energy in the uv/visible region of the
spectrum as a result of electronic transitions.
a)Copy the diagram below and draw arrows to show the electronic transitions which cause
absorptions in propanone.[3]
b) Study each of the molecules drawn below and draw those which show more than one
absorption in the uv/visible region.
c)Diphenylmethanone, shown below, also absorbs in the uv/visible region of the spectrum.

2. i. Predict where, relative to the absorptions shown by propanone, diphenylmethanone will
absorb energy.
ii. Explain your answer to (i) [3]
3. a) Explain why the complex [Cu(H2O)6]2+ is colored and why the complex [Zn(H2O)6]2+ has no
color.[5]
b) For each of the compounds shown below, identify the various electronic transitions which bring
about absorptions in the uv/visible region of the spectrum.
i) C6H5NO2
ii) CH3NH2
iii) CH3CH=CH2
4. When the indicator phenolphthalein is added to alkali, it changes from colorless to bright pink.
Explain what has happen to cause this change.
5. a) Explain, in terms of electron transitions, the difference between the yellow color of a sunflower,
and the yellow color of a sodium street light. [4]
b) The uv/visible spectrum shown below was obtained from a solution containing [M(H2O)6]3+,
where M is a transition metal.
Predict the color of the solution, explaining your reasoning.[2]
6.) One major difference between the properties of compounds of the transition elements and
those of other compounds is that the compounds of the transition elements are often coloured.
a) Explain in detail why many transition element compounds are coloured. [3]
b) The following graph shows the absorption spectrum of two complexes containing copper.

3. (i) State the colours of the following complex ions.
[Cu(H2O)6]2+
[Cu(NH3)4(H2O)2]2+
(ii) Using the spectra above give two reasons why the colour of the [Cu(NH3)4(H2O)2]2+ ion is
deeper (more intense) than that of the [Cu(H2O)6]2+ ion.
(iii) Predict the absorption spectrum of the complex [Cu(NH3)2(H2O)4]2+, and sketch
this spectrum on the above graph. [6]