….. And the Reasons Water is So Special Hydrogen Bonds ….. And the Reasons Water is So Special

Electronegativity  is a measure of the tendency of an atom to attract a bonding pair of electrons.

Polar bond (oxygen is more electronegative than hydrogen)

Each water molecule can bind with up to 4 other water molecules Hydrogen bond (hydrogen atom of one polar molecule is attracted to the slightly negative charge of another polar molecule) Each water molecule can bind with up to 4 other water molecules

Hydrogen Bonds Results when hydrogen is bonded to 2 highly electronegative atoms at the same time Application: Water molecules bonding to each other and other polar molecules DNA – 2 strands linked together (nitrogenous bases) Secondary, Tertiary, Quaternary structures of proteins

Water is wonderful … and unique! How do the unique chemical and physical properties of water make life on earth possible? Polar molecule Results in hydrogen bonding Cohesion Adhesion Surface Tension Moderates temperature High specific heat Evaporative cooling Oceans don’t freeze solid Solvent

Hydrogen bonding in water results in: Cohesion Transport of water against gravity in plants Water exiting the leaves “tugs” on water molecules leading all the way down to the roots Adhesion Water clings to the walls (cellulose does not dissolve in water … nice quality) of plant vessels to help counter the downward pull of gravity Surface tension Measure of how difficult it is to stretch or break the surface of a liquid Some animals can stand, walk, or run on water

Solid water (ice) is less dense than liquid water When water freezes the hydrogen bonds are “locked” into place and creates an organized crystal structure with air pockets that make it less dense.

Water moderates temperature Absorbs heat from air that is warmer Hydrogen bonds break Releases heat to air that is cooler Hydrogen bonds form High specific heat This absorption and release of of heat from water only slightly changes its own temperature Coastal areas generally have milder climates

Evaporative Cooling Water molecules must have a relatively high kinetic energy to break hydrogen bonds and change states from liquid to gas Prevents land organisms from overheating Prevents tissues in leaves from becoming too warm in sunlight

Water is the solvent of life The oxygen (-) regions of water cling to sodium (Na+) ions while the hydrogen (+) regions of water are attracted to chloride (Cl-) ions. Eventually all ions are dissolved as a “hydration shell” surrounds them.

The Dissociation of Water Molecules

Acid Increases the hydrogen ion concentration Base Reduces the hydrogen ion concentration Increases the hydroxide ion concentration Logarithmic scale pH 1 is 10 X more acidic than pH 2 pH = - log [H+] pH 7; [H+] = [OH-] = 10-7 [H+] x [OH-] = 10-14 If pH = 5 than [H+] = 5 and [OH-] = 9

Biological Application of pH Enzymes have optimal pH’s in which they work best Pepsin is found in the stomach optimal pH is very acidic

HCO3 - (bicarbonate ion (base)) pH Buffers Substances that minimize changes in concentrations of H+ and OH- in solution Human blood pH stability Uses Carbonic Acid – Bicarbonate buffering system Response to rise in pH H2CO3 (carbonic acid) (H+ donor) HCO3 - (bicarbonate ion (base)) (H+ acceptor) H+ (hydrogen ion) + Response to drop in pH