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Unusual water properties
- high boiling points
- melting point
- heat of vaporization
- surface tension
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Properties of water
- high intermolecular forces
- hydrogen bonds
- polar
- non-tetrahedral bond angles
- H-bond donor and acceptor
- potential to form four H-bonds per water
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In liquid water
the molecules are held by a random three dimensional network.
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Water serves as an excellent solvent
for ionic and polar substances
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Polar compounds
- sugars
- simple alcohols
- amines
- aldehydes
- ketones
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Measure the attraction forces by determining
the dielectric constant F= e1e2/Dr^2
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Water forms hydrogen bonds with the functional groups of these compounds
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Water molecules “reorganize” to around the non-polar compound
and form an H-bonded water network around the compound
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Hydrophobic interaction leads to a local cagelike structure known
as “clathrate” that surrounds each solute molecule
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Amphipatic
refers to molecules that possess both, a strongly polar and strongly nonpolar groups
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Examples of amphipathic
Salts and fatty acids
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The ionic carboxylate head ( polar)
hydrates, while the long hydrophobic tail remains insoluble
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Colligative properties include
- freezing point depression
- boiling point elevation
- vapor pressure lowering
- osmotic pressure effects
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When a solution is separated from water by a semipermeable membrane
the water moves from a range of higher concentration (pure water) of water to a region of lower concentration (solution)
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Water has a small tendency
to form ions
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From the ionization of water
results a H+ and a OH- group
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The acid dissociation constant is given by
Ka = [ H + ] [ A -] /[HA]
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We can refer to Keq as Kw for water. Kw is the
ionization constant of water
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Concentration of water is constant
(55.5M)
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The more acid the solution is
the more [ H + ] and less [ OH -]
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The more basic the solution is
the more [ OH -] and less [ H +]
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Kw=
[ H + ][ OH -]= 1X10-14
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pH
- is defined as the negative logarithm of hydrogen ion concentration
- pH = -log([H+])
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Electrolytes are
substances that form ions and, therefore, cause an increase in electrical conductivity
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Strong electrolytes
completely dissociate in water, to form ions
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Some strong electrolytes are
salts (NaCl and K2SO4) , and strong acids (HCl)
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Weak electrolytes
don’t completely dissociate in water, to form ions
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Some weak electrolytes are
weak acids (HAc, H2CO3)
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The Henderson-Hasselbalch Equation
- For any acid HA, the relationship between the pKa, the concentrations existing at equilibrium and the solution pH is given by
- pH = pKa + log10 [A¯ ] /[HA]
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Titration
is the analytical method used to determine the amount of acid in a solution
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The point at which the concentration of the conjugate acid and conjugate base remain constant is
the pKa
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Buffers
Maintenance of pH is essential to all cells, because changes in pH would be disruptive for to metabolism
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Buffer systems
- intracellular pH: H2PO4 and histidine systems
- extracellular: H2CO3(bicarbonate system)
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The bicarbonate buffer system is important to maintain
the blood plasma pH
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