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BIC 1B01 Lecture 2
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Lecture 2: The physicochemical properties of water
(Campbell and Farrell: Chapter 2.1 – 2.2)
Water is essential for life because:
• It can dissolve a wide variety of substances and therefore make
these available for chemical reactions in the cell
• Water is a liquid at most temperatures encountered on earth
‘Electronegativity’• The tendency of an atom (element) to attract
shared electrons in a chemical bond
Structure of Water
Ice (solid state)
Hydrogen bond
Water (liquid)
Hydrogen bond
Solubility in Water
Hydration shell
Solubility in Water
• Hydrogen bonds between water molecules must break and water must bond with solute
Hydrogen bond
(dipole-dipole
interaction)
Ion-dipole interaction
Dipole-dipole interaction
Similar energyH20 ----- H20 H20 ----- solute
Water readily dissolves• Ionic compounds
• Polar compounds (dipoles)
What about non-polar compounds?
dipole – induced dipole interactions are much WEAKER than ion-dipole or dipole-dipole
interactions
Therefore: water will not readily dissolve non-polar compounds
Hydrophilic (‘loves water’)
• polar compounds (dipoles)
• ionic compounds
Hydrophobic (‘fears water’)
• non-polar compounds
Amphipathic compounds: Compounds that contain a
hydrophilic and hydrophobic part
An Amphipathic long-chain fatty acid
Micelle formation by amphipathic molecules
• Micelle:Micelle: a spherical arrangement of organic molecules in water solution clustered so that– their hydrophobic parts are buried inside the sphere – their hydrophilic parts are on the surface of the sphere and
in contact with the water environment– formation depends on the attraction between temporary
induced dipoles (weak) and an increase in entropy
Hydrogen bonds
H+
O(or N)
donor
O (or N)
acceptor
Special type of dipole – dipole bond
Each water molecule is bound to 4 others in an ice crystal
4 others: ’Tetrahedral bonding’
The tetrahedral hydrogen bonding in water explains its physical properties
Why does ice float on water?
• Liquid water is denser than ice because the fully hydrogen-bonded array is less extensively packed
100 °C
-33.4 °C
Water has a relatively high boiling point
‘STRONG’ ‘WEAK’
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