Hydrogen Bonding & The Properties of Water

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

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

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

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

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 10-11 seconds! Hydrogen bonds are very numerous which somewhat offsets their weak and transient nature.

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

Hydrogen bonding Examples

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!

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…

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?

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?

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)

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

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

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).

Specific heat & climate

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.

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

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).

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

Ice floats

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

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

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

Youtube video clip (Why do ice float?) http://www.youtube.com/watch?v=ZB-XBX1S15w

Review Why Does Water Expand When It Freezes?