Lecture 2: The Physicochemical properties of water

1. Water is essential for life because:

a) It can dissolve a wide variety of substances and therefore make these available for chemical reactions in the cellb) Water is a liquid at most temperatures encountered on earth

2. Electronegativity is the tendency of an atom (element) to attract shared electrons in a chemical bond

3. Two common electronegative elements found in biological molecules are N and O

4. Water’s structural characteristics are:

a) It is a polar molecule (contains two polar covalent bonds)b) The oxygen contains a double partial negative chargec) The hydrogens each contain a partial positive charged) Water has a dipole momente) Water is a bent molecule (bonding angle = 104°)

5. Dipole moment is a vector with two components:a) Magnitude – depend on the magnitude of partial chargesb) Direction – if water is put in a strong electric field, it will tend

to align itself with the field. The axis of alignment is the direction of the dipole moment

6. Water in the solid state (ice) is highly organized due to hydrogen bonding between individual water molecules in the crystal lattice (hydrogen bonded network).

7. Water in the liquid state strives to achieve the same organization as ice, but the hydrogen-bonded network of water molecules is constantly breaking and reforming (at a rate of 10-12 times per second)

8. When NaCl dissolves in water:a) the Na+ and Cl- ions dissociate and are hydrated by water.

Water forms a hydration shell around each ion, such that b) the partial negative charges on the oxygens of water are

oriented towards the positive Na+ ions andc) the partial positive charges on the hydrogens of water are

oriented towards the negative Cl- ion.

9. In order for a substance to dissolve in water:a) the hydrogen bonds between water molecules must breakb) a new bond must form between water and the solute such

thatc) the bond energies (of the broken hydrogen bond and the bond

between water and the solute) are similar

10. Common non-covalent interactions:a) charge interactions (also called ionic interactions or salt

bridges) (e.g. Na+--Cl-)b) ion-dipole interactions (e.g. Na+-- H20 or Cl--- H20)c) dipole-dipole interactions (e.g. H20 -- H20)d) dipole-induced dipole interactions (e.g. H20 – CH4)e) induced dipole-induced dipole interactions (e.g. interaction

between the hydrophobic tails of fatty acids)

11. Dipole-dipole interactions (which include the hydrogen bond between water molecules) and ion-dipole interactions are equally strong (their bond energies are similar)

i. therefore the energy needed to break hydrogen bonds between water molecules can be gained through the interaction of water with a solute via dipole-dipole or ion-dipole interactions.

ii. this is the reason why ionic substances and polar substances readily dissolve in water.

(Look at slide nr 10, make sure that you can draw the interactions – NB!)

12. Dipole-induced dipole interactions (e.g. between water and a non-polar compound) are much weaker than dipole-dipole interactions (e.g. the hydrogen bond between water molecules)

i. therefore: non-polar substances DO NOT readily dissolve in water.ii. therefore the energy needed to break hydrogen bonds between water

molecules CAN NOT be gained through the interaction of water with a solute via dipole-induced dipole interactions.

13. Hydrophilic substances dissolve in water (See slide 13 for examples)

14. Hydrophobic substances DO NOT dissolve in water (See slide 13 for examples)

15. Amphipathic substances are substances that contain a hydrophobic AND hydrophilic part, e.g. the long-chain fatty acid sodium palmitate.

16. A long chain fatty acid consists of

i. a ‘polar’ head group (Na+ and carboxylate-)

ii. a hydrophobic tail (CH2 groups)

17. Micelle formation by amphipathic compounds in aqueous (water) solution:

i. Micelle: a spherical arrangement of organic molecules in water solution clustered so that

ii. their hydrophobic parts are buried inside the sphere iii. their hydrophilic parts are on the surface of the sphere and in

contact with the water environmentiv. formation depends on the attraction between temporary induced

dipoles (weak) and an increase in entropy

18. A hydrogen bond is a special type of dipole-dipole interaction and consists of two elements:

i. donor – partially + H bonded to electronegative element (N or O)ii. acceptor – lone electron pair on electronegative element (N or O)

19. Hydrogen bonding potential of water:i. water contains two H-bond donors and two H-bond acceptors

ii. each water molecule can form hydrogen bonds with 4 other water molecules

iii. this is referred to as tetrahedral bonding

20. The tetrahedral hydrogen bonding of water explains its physical properties:i. liquid water is denser than ice because the fully formed hydrogen-

bonded array is less extensively packed, therefore ice floats on waterii. water has a relatively high boiling point compared to similar

molecules

21. Hydrogen bonds that commonly occur in biomolecules include (see slide 24):i. OH – OH

ii. OH – O=Ciii. N-H – OHiv. N-H – O=C

22. A strong hydrogen bond is linear and a weak hydrogen bond is non-linear. (See slide 25 – NB – must be able to draw!)