Chapter 3: Outline-1

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Chapter 3 Outline-1

• Molecular Nature of Water
• Noncovalent Bonding
• Ionic interactions Hydrogen Bonds
• van der Waals Forces
• Thermal Properties of Water
• Solvent Properties of Water
• Hydrophilic, hydrophobic, and amphipathic
  molecules
• Osmotic pressure

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Chapter 3 Outline-2

• Ionization of Water
• Acids, bases, and pH
• Buffers
• Physiological buffers

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Water

• Solvent for all chemical reactions.
• Transports chemicals from place to place.
• Helps to maintain constant body temperature.
• Part of digestive fluids.
• Dissolves excretion products.

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3.1 Molecular Structure of Water

• The oxygen in water is sp3 hybridized.
  Hydrogens are bonded to two of the orbitals.
  Consequently the water molecule is bent. The
  H-O-H angle is 104.5o.

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Water

• Water is a polar molecule.
• A polar molecule is one in which one end is
  partially positive and the other partially
  negative.
• This polarity results from unequal sharing of
  electrons in the bonds and the specific geometry
  of the molecule.

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Water

• Water molecule with bond ( ) and net
• ( ) dipoles.

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Water

• Water has an abnormally high boiling point due to
  intermolecular hydrogen bonding.

H bonding is a weak attraction between an
electronegative atom in one molecule and an H (on
an O or N) in another.
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3.2 Noncovalent Bonding

• Ionic interactions
• Hydrogen bonding
• Van der Waals forces
• Dipole-dipole
• Dipole-induced dipole
• Induced dipole-induced dipole

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Typical Bond Strengths
Type kJ/mol
Covalent gt210
Noncovalent
Ionic interactions 4-80
Hydrogen bonds 12-30
van der Waals 0.3-9
Hydrophobic interactions 3-12
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Ionic Interactions

• Ionic interactions occur between charged atoms or
  groups.
• In proteins, side chains sometimes form ionic
  salt bridges, particularly in the absence of
  water which normally hydrates ions.

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Hydrogen Bonding

• Water molecules hydrogen bond with one another.
  Four hydrogen bonding attractions are possible
  per molecule
• two through the
• hydrogens and two
• through the nonbonding
  electron pairs.

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Van der Waals Attractions

• a. Dipole-dipole
• b. Dipole-induced dipole
• c. Induced dipole-induced dipole

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Hydrophobic interactions

• Nonpolar molecules tend to coalesce into droplets
  in water. The repulsions between the water
  molecules and the nonpolar molecules cause this
  phenomenon.
• The water molecules form a cage around the
  small hydrophobic droplets.

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3.3 Thermal Properties

• Hydrogen bonding keeps water in the liquid phase
  between 0 oC and 100 oC.
• Liquid water has a high
• Heat of vaporization-energy to vaporize one
  mole of liquid at 1 atm
• Heat capacity-energy to change the
  temperataure by 1 oC
• Water plays an important role in thermal
  regulation in living organisms.

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3.4 Water-solvent Properties

• Water dissolves chemicals that have an affinity
  for it, ie. hydrophilic (water loving) materials.
• -many ionic compounds
• -polar organic compounds
• These compounds are soluble in water due to three
  kinds of noncovalent interactions
• ion-dipole 2. dipole-dipole
• 3. hydrogen bonding

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Ion-dipole Interactions

• Ions are hydrated by water molecules. The water
  molecules orient so the opposite charge end
  points to the ion to partially neutralize charge.
  The shell of water molecules is a solvation
  sphere.

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Dipole-dipole Interactions

• The polar water molecule interacts with an O or N
  or an H on an O or N on an organic molecule.

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Hydrogen Bonding

• A hydrogen attached to an O or N becomes very
  polarized and highly partial plus. This partial
  positive charge interacts with the nonbonding
  electrons on another O or N giving rise to the
  very powerful hydrogen bond.

hydrogen bond shown in yellow
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Nonpolar Molecules

• Nonpolar molecules have no polar bonds or the
  bond dipoles cancel due to molecular geometry.
• These molecules do not form good attractions with
  the water molecule. They are insoluble and are
  said to be hydrophobic (water hating).
• eg. CH3CH2CH2CH2CH2CH3, hexane

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Nonpolar Molecules-2

• Water forms hydrogen-bonded cagelike structures
  around hydrophobic molecules, forcing them out of
  solution.

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Amphipathic Molecules

• Amphipathic molecules contain both polar and
  nonpolar groups.
• Ionized fatty acids are amphipathic. The
  carboxylate group is water soluble and the long
  carbon chain is not.
• Amphipathic molecules tend to form micelles,
  colloidal aggregates with the charged head
  facing outward to the water and the nonpolar
  tail part inside.

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A Micelle

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Osmotic Pressure-2

• Osmosis is a spontaneous process in which solvent
  molecules pass through a semipermeable membrane
  from a solution of lower solute concentration to
  a solution of higher solute concentration.
• Osmotic pressure is the pressure required to stop
  osmosis.

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Osmotic Pressure-3

• Osmotic pressure (p) is measured in an osmometer.

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Osmotic Pressure-4

• p iMRT
• i vant Hoff factor ( as ions)
• M molarity (mol/L for dilute solns)
• R 0.082 L atm/ mol K
• T Kelvin temperature
• 1 M NaCl is 90 ionized and 10 ion pairs.
• i 0.9 0.9 0.1 1.9
• Osmolarity (osm/L) iM

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Osmotic Pressure-5

• Because cells have a higher ion concentration
  than the surrounding fluids, they tend to pick up
  water through the semipermeable cell membrane.
• The cell is said to be hypertonic relative to the
  surrounding fluid and will burst (hemolyze) if
  osomotic control is not effected.

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Osmotic Pressure-6

• Cells placed in a hypotonic solution will lose
  water and shrink (crenate).
• If cells are placed in an isotonic solution (conc
  same on both sides of membrane) there is no net
  passage of water.

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3.5 Ionization of Water

• Water dissociates. (self-ionizes)
• H2O H2O H3O OH-

Kw Ka H2O2 H3O OH-
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Water Ionization-2

• The conditions for the water dissociation
  equilibrium must hold under all situations at
  25o.
• Kw H3OOH-1 x 10-14
• In neutral water,
• H3O OH- 1 x 10-7 M

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Water A/B Properties

• When external acids or bases are added to water,
  the ion product (H3O OH- ) must equal Kw.
• The effect of added acids or bases is best
  understood using the Lowry-Bronsted theory of
  acids and bases.

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Water A/B Properties-2

• Lowry-Bronsted
• acid proton donor
• HA H2O H3O A-
• A B CA CB
• C conjugate (product) A/B

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Water A/B Properties-3

• Lowry-Bronsted
• base proton acceptor
• RNH2 H2O OH- RNH3
• B A CB CA

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Measuring Acidity

• Added acids increase the concentration of
  hydronium ion and bases the concentration of
  hydroxide ion.
• In acid solutions H3O gt 1 x 10-7 M
• OH- lt 1 x 10-7 M
• In basic solutions OH- gt 1 x 10-7 M
• H3O lt 1 x 10-7
  M
• pH scale measures acidity without using
  exponential numbers.

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pH Scale

• Define pH -log(10)H3O
• 0---------------7---------------14
• acidic basic
• H3O1 x 10-7 M, pH ?
• 7.0

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pH Scale-2

• H3O1 x 10-5 M, pH ?
• 5 (acidic)
• H3O1 x 10-10 M, pH ?
• 10 (basic)
• What if preexponential number is not 1?

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pH Scale-3

• H3O2.6 x 10-5 M, pH ?
• 4.59 (acidic)
• H3O6.3 x 10-9 M, pH ?
• 8.20 (basic)
• H3O7.8 x 10-3 M, pH ?
• 2.11 (acidic)

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pH Scale-4

• pH to H3O?
• inverse log of negative pH
• orange juice, pH 3.5. H3O?
• H3O 3.2 x 10-4 M
• urine, pH 6.2. H3O?
• H3O 6.3 x 10-7 M

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Strength of Acids

• Strength of an acid is measured by the percent
  which reacts with water to form hydronium ions.
• Strong acids (and bases) ionize close to 100.
• eg. HCl, HBr, HNO3, H2SO4

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Strength of Acids-2

• Weak acids (or bases) ionize typically in the
  1-5 range .
• eg. CH3COCOOH, pyruvic acid
• CH3CHOHCOOH, lactic acid
• CH3COOH, acetic acid

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Strength of Acids-3

• Strength of an acid is also measured by its Ka or
  pKa values.
• HA H2O H3O A-

Larger Ka and smaller pKa values indicate
stronger acids.
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Strength of Acids-4

• Ka pKa
• CH3COCOOH 3.2x10-3 2.5
• CH3CHOHCOOH 1.4x10-4 3.9
• CH3COOH 1.8x10-5 4.8
• Larger Ka and smaller pKa values indicate
  stronger acids.