Colour by Design - Ultraviolet and visible spectroscopy
18 important questions on Colour by Design - Ultraviolet and visible spectroscopy
What happens when visible light falls on a coloured solution? What do we see?
Some wavelengths are absorbed and some are transmitted. What we see are the transmitted wavelengths.
What colour do molecules which absorb UV radiation appear? Why?
Colourless as our eyes do not detect UV light.
What does absorption of UV light cause in molecules?
Electronic transitions - electrons move to higher energy levels and the molecule becomes 'excited'
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What type of molecules absorb UV light? What can be said about these molecules' delocalised electrons?
- Unsaturated molecules (those with C=C or benzene rings)
- Those with conjugated systems
The delocalised electrons in these systems require slightly less energy to become excited compared with electrons in single bonds.
How do you identify the colour of a test solution through a UV visible spectrum?
You find the wavelength with the highest absorbance. e.g. if the highest absorbance is at 670nm, this wavelength is red so the colour of the solution will be the complementary colour of red which is blue/green.
What is a reflectance spectra used for?
To analyse the pigments from old paintings.
Why are there many shades of blue?
The shade depends on what other colours (frequencies) are absorbed besides red.
Describe the absorption spectrum of a grey pigment.
Partially absorbs all frequencies of light.
Explain how chlorophyll makes plant leaves appear green in white light.
Chlorophyll absorbs blue and red light, reflecting green light.
What is the difference between pigments and dyes?
Pigments are insoluble whereas dyes are soluble (interact with water molecules through hydrogen bonding - attach themselves to another substance)
What indicates that a compound is very coloured?
- conjugated system
- chromophore of the substance
- involve delocalised electrons that can move
What are transition metals?
Substances which have a vacant d orbital(s) (refer to the process/mechanism of why transition metals are coloured)
Why do molecules with a benzene ring structure tend to be flat?
There is lack of free rotation around the double bonds.
Explain why the emissions in an emission spectrum occur at different frequencies.
There are only a set number of energy levels. The emission spectrum shows the electrons falling down from different energy levels to the ground state. The energy is proportional to the frequency emitted: *delta* E = hv
Explain, in terms of electronic energy levels, why a substance such as benzophenone absorbs in the ultraviolet but is not coloured.
In an absorption spectrum, the electrons are excited and so move to higher energy levels. The gap between the energy levels, *delta*E=hv, is too big to absorb light in the visible spectrum. Even though benzene is present which contains delocalisation, there is not enough to reflect colour.
Apple-interchange
Explain, in terms of electron energy levels, the reasons why Sudan II is orange (Q6 (iv) Chemistry by Design questions)
How could the atomic emission spectrum of a green area of a painting be used to show the presence of copper, tin and lead?
It produces a line spectrum where you can compare the different characteristics of wavelengths of each element and thus see what elements are present.
What is occurring in the atoms of the elements concerned to produce an atomic emission spectrum and why are the spectra of the elements different?
Excited electrons falling down from different energy levels to the ground state. Electrons can only possess different quantities of energy in different elements.
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