Water and Polarity

Water and Polarity

1. Bond polarity and strength

   • polar bond if EN difference >1.9
   • van der Waals radius
     • Distance between nucleus of atom and its effective electronic surface
     • Bonds between atoms result from overlap of radii
   • types of ionic interactions
     • ionic bond
     • ion-dipole interaction
     • salt bridge (interaction between two charged functional groups)
   • van der Waals forces
     • noncovalent weak dipole-dipole interactions of ions
     • London dispersion: attraction between transient induced dipoles
       • H2O insoluble
   • van der Waals<hydrogen bond<ion-dipole<ionic<covalent(C-H)<covalent(O-H)

2. Hydrophobicity and Amphipathic Interactions

   • Micelles
     • temporary induced dipoles
     • Hydrophobic parts buried inside, hydrophilic parts outside
   • Hydrophobic effect
     • nonpolar substances spontaneously aggregate in an aqueous solution and exclude water molecules
     • based on entropy changes

3. Hydrogen Bonds

   • Positive end of one dipole is H atom covalently bonded to highly electronegative atom (hydrogen-bond donor)
   • Negative end of other covalently dipole is atom with lone pair of electrons (hydrogen-bond acceptor)
     • hydroxyl, carbonyl, amino
   • Water
     • Each water molecule can accept 2 H bonds and donate 2 H bonds, participating in 4 H bonds at a time and allowing it to be the universal solvent
       • Enabled by 104.3 deg bond angles
     • When many water molecules form hydrogen bonds with each other, they form a strong and flexible lattice. This creates a high surface tension.
     • Why is solid water less dense than the liquid form? H bonds less compact

4. pH and pKa

   • Water
     • Molarity of pure water is 55.5 M
     • Water is a monoprotic weak acid
     • @ 25degC, Kw=10^-14
   • Titration
     • Equivalence point (EP): Point in titration at which enough base added to neutralize acid
     • pKa: Halfway point in acid-base titration; ½ ionized

5. Buffers

   • Examples
     • CH3COOH and CH3COONa
     • H2CO3 and NaHCO3
     • NaH2PO4 and Na2HPO4
     • Note that each acid:conjugate base pair differ by 1 H+
   • Le Chatelier's Principle
     • When H+ added to buffer system, stress added to reaction. To relieve the stress, H+ reacts with A– to maintain equilibrium. Forces equilibrium toward formation of HA