NATS 1870 Lecture 11: SC NATS 1870 LECTURE 11

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Tuesday, November 3, 2015
SC NATS 1870 LECTURE 11
Chemistry and Colour
Colour as an information giver
Colour can be a temperature indicator (not chemistry)
Can indicate pH level (acidity) in a solution
Fruit ripens, fruit goes bad
Metals can oxidize
Colour change as information
Acidity
Levels of oxidation
Food cooking changes colours
Change of seasons (leaves change colour)
Colour change can be a visible probe of change at the microscopic level
Chemical reactions can happen
a) New molecules can be created = visible colour change
b) Molecules can have their structure change = visible colour change
Key for Colour:
The way a molecule it put together
Spacing of energy levels ( structure, bonds determine spacing of energy levels/
orbits)
Types of bonds:
Ionic-> One atom donates e- to neighbour
-This is a strong bond
-Need UV photo to raise e- to higher level
-Materials are often colourless
!1
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Tuesday, November 3, 2015
Covalent bonds-> equally shared
-These bonds tend to be present in colorants (dyes, pigments)
Molecules have 2 ways electrons can change internal energies
1. Electron transitions -> An electron jumps from one energy level to another
2. Vibrational transitions-> Bonds between atoms can vibrate, Range of energies
possible
3. Rotational transitions-> 3D molecule can rotate in various orientations-each has
different energy
There is a range of rotational+vibrational energies within each (electronic) energy level.
Common Molecular structures in colorants:
1. Double Covalent
-1st e- pair that is shared is tightly bound
-2nd e- pair can readily absorb visible wavelengths
2. Auxochromes- addition of extra atoms to molecular structures charges (lowers)
energy level spacing making invisible molecules (benzene) changed into a substance
with colour (MARITUS YELLOW)
3. Conjugated bonds (chains of alternating single and double covalent bonds):
CAROTENES
4. Porphyrins:
-Chlorophyll- Contains Magnesium
-Phthalo blue- Contains Cooper
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Changing leaf colour
-Summer: green- from chlorophyll
-Fall: Cooler days, shorter nights no more chlorophyll production
-Can reveal yellow leaves-> from carotenes already present
!2
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Tuesday, November 3, 2015
-Can change to red-> from anthocyanin - a chemical made in a leaf, from a reaction
between sugar+sunlight
!3
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