Bic1 b lecture 2

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’