Please enable JavaScript.

Coggle requires JavaScript to display documents.

Chapter 2: Water - Coggle Diagram

- - - - hydrogen bonds are relatively weak
        
        bond dissociation energy = about 23 kJ/mol in liquidH2O
        
        10% covalent, 90% electrostatic
      - hydrogen bonds are fleeting
        
        lifetime of each hydrogen bond is just 1 to 20 picoseconds in liquid
        
        when one hydrogen bond breaks, another forms
    - - in liquid, each H2O molecule forms hydrogen bonds with an average of 3.4 other molecules
      - in ice, each H2O molecule forms 4 hydrogen bonds
    - - during melting or evaporation, heat is taken up by the system and the entropy of the aqueous system increases
        
        solid --> liquid deltaH = +5.9 kJ/mol
        
        liquid --> gas deltaH = +44.0 kJ/mol
      - at room temperature, melting and evaporation occur spontaneously
        
        free-energy change (deltaG) must be negative
        
        because deltaH is positive, the increase in deltaS drives these changes
    - - hydrogen bonds readily form between an electronegative atom (the hydrogen acceptor) and a hydrogen atom covalently bonded to another electronegative atom (the hydrogen donor)
    - - hydrogen atoms covalently bonded to carbon atoms do not hydrogen bond
      - alcohols, aldehydes, ketones, and compounds containing N-H bonds all form hydrogen bonds with water
    - - H-bonds are strongest when the acceptor atom is in line with the covalent bond between the donor atom and H, as this maximizes electrostatic interaction
  - - - for water at 25C, ε=78.5
      - for nonpolar benzene at 25C, ε=4.6
  - - - the magnitude of the charges (Q)
      - the distance between the charged groups (r)
      - the dielectric constant of the solvent (ε)
  - - - their movement into aqueous solution decreases entropy by constraining their motion
      - biologically important gases CO2, O2, N2 are nonpolar
    - - nonpolar compounds interfere with the hydrogen bonding among H2O molecules, which increases enthalpy (deltaH) and decreases entropy (deltaS)
      - the free-energy chagne (deltaG = deltaH - TdeltaS) for dissolving a nonpolar solute in water is unfavorable:
        
        deltaH has a positive value
        
        deltaS has a negative value
        
        deltaG has a positive value
    - - H2O molecules form a highly ordered cagelike shell around each solute molecule
        
        this maximizes solvent-solvent hydrogen bonding
        
        H2O molecules are not as highly oriented as those in clathrates
  - - - vapor pressure
      - boiling point
      - melting point (freezing point)
      - osmotic pressure
    - - ...is the force necessary to resist water movement
      - ...is approximated by the van't Hoff equation
        
        osmotic pressure = icRT, where R is the gas constant and T is the absolute temperature
        
        depends on...
        
        the van't Hoff factor, a measure of the extent to which the solution dissociates into 2+ ionic species (i)
        
        for nonionizing solutes, i=1
        
        for solutes that dissociate into two ions, i=2
        
        the solute's molar concentration (c)
      - Calculating osmotic pressure for solutions of several solutions
        
        for solutions of several (n) solutes, osmotic pressure is the sum of the contributions of each species
      - effects of extracellular osmolarity on water movement
        
        an isotonic solution has an osmolarity equal to that of a cell's cytosol
        
        a hypertonic solution has a higher osmolarity than that of the cytosol
        
        a hypotonic solution has a lower osmolarity than that of the cytosol