1. Hydrogen Bonding & The Properties of Water

2. Hydrogen bonding
When hydrogen is bonded to a highly electronegative atom (such as nitrogen, oxygen, fluorine), the bonding electron pair is drawn towards the electronegative atom

3. Hydrogen bonding
Hydrogen has no inner shell electron and is very small in size, the positive charge density developed is high
The nucleus of hydrogen atom is exposed to attraction by nearby electron cloud, a lone pair electrons on the electronegative atom

4. Hydrogen Bonding
Essential requirements for formation of hydrogen bond:
A hydrogen atom must be directly bonded to a highly electronegative atom (e.g. F, O and N)
An unbonded pair of electrons (lone pair electrons) is presented on the electronegative atom

5. Hydrogen Bonding in Hydrogen Fluoride..
Fluorine atoms have three electron lone pairs for bonding to other HF molecules

6. Hydrogen bonds tend to be weak
Hydrogen bonds tend to be weak. Covalent bonds are about 20 times stronger than a typical hydrogen bond.
Hydrogen bonds tend to be transient - In liquid water these hydrogen bonds between water molecules tend to last only seconds!
Hydrogen bonds are very numerous which somewhat offsets their weak and transient nature.

7. Ice Both lone pairs are involved in hydrogen bonds
Both hydrogen atoms are involved in hydrogen bonds

8. Hydrogen bonding
Examples

9. Elixir of Life Special properties of water 1. cohesion & adhesion
surface tension, capillary action
2. good solvent
many molecules dissolve in H2O
hydrophilic vs. hydrophobic
3. lower density as a solid
ice floats!
4. high specific heat
water stores heat
5. high heat of vaporization
heats & cools slowly
Ice! I could use more ice!

10. Cohesion & Adhesion Cohesion Adhesion H bonding between H2O molecules
water is “sticky” (clumps together)
surface tension
drinking straw
Adhesion
H bonding between H2O & other substances
capillary action
meniscus
water climbs up paper towel or cloth
Try that with flour… or sugar…

11. Water is the solvent of life
Polarity makes H2O a good solvent
polar H2O molecules surround + & – ions
solvents dissolve solutes creating solutions
What dissolves in water easily?
polar or non-polar molecules?
How about Oxygen? Does that dissolve in H2O?

12. What dissolves in water?
Hydrophilic
substances have attraction to H2O
polar or non-polar?
What dissolves in water easily?
polar or non-polar molecules?
How about Oxygen? Does that dissolve in H2O?

13. What doesn’t dissolve in water?
Hydrophobic
substances that don’t have an attraction to H2O
polar or non-polar?
Oh, look hydrocarbons!
What dissolves in water easily?
polar or non-polar molecules?
How about Oxygen? Does that dissolve in H2O?
fat (triglycerol)

14. Specific Heat
Specific heat is the amount of heat required to raise the temperature of one gram of a substance by one degree Celsius. 
Because of cohesion, water molecules resist increasing their motion.
this is another way of saying that is water molecules resist the net breaking of hydrogen bonds

15. Consequently, water resists heating; water has a very high specific heat.
Absorbs a lot of energy to heat it up
Releases a lot of energy to cool it down

16. This tendency to not want to change temperature both causes resistance to radical temperature swings within beings and bodies of water (e.g., a lake).

17. Specific heat & climate

18. High heat of vaporization
Because it also involves the breaking of hydrogen bonds, water resists vaporizing (evaporating).
Consequently, it takes a lot of heat to evaporate water.
This high heat of vaporization is also utilized by organisms as a cooling process, e.g., sweat or panting.

19. 5. Heat of vaporization Evaporative cooling
Organisms rely on heat of vaporization to remove body heat

20. Ice Floats
At warm temperatures, water molecules have a lot of energy and are able to move past and mix with each other despite the attractions between the hydrogen atoms and unbonded electron pairs.
As water is cooled down, however, the molecules have less energy and hydrogen bonding takes over and form a ordered crystal that spaces the molecules farther apart (decreasing density).

21. In ice…
the molecular motion is restricted due to lack of energy. The molecules are arranged in a way to form the maximum number of hydrogen bonds

22. Ice floats

23. The ice structure has large empty spaces which gives it a lower density than water.

24. In ice…
Each H2O molecule is tetrahedrally bonded to 4 other H2O molecules by hydrogen bonds
Creates an open structure and gives rise to a lower density of ice than water
Insulates the water below and prevents complet solidification  keep fish survive in polar regions

25. Why is “ice floats” important?
Oceans & lakes don’t freeze solid
surface ice insulates water below
allowing life to survive the winter
if ice sank…
ponds, lakes & even oceans would freeze solid
in summer, only upper few inches would thaw
seasonal turnover of lakes
sinking cold H2O cycles nutrients in autumn

26. Youtube video clip
(Why do ice float?)

27. Review
Why Does Water Expand When It Freezes?