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Chapter 2: Water. WHY DO WE NEED TO DO THIS AGAIN!!!. Properties of water. Very polar Oxygen is highly electronegative H-bond donor and acceptor High b.p., m.p., heat of vaporization, surface tension. Water dissolves polar compounds. solvation shell or hydration shell.

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Properties of water l.jpg
Properties of water

  • Very polar

  • Oxygen is highly electronegative

  • H-bond donor and acceptor

  • High b.p., m.p., heat of vaporization, surface tension


Water dissolves polar compounds l.jpg
Water dissolves polar compounds

solvation shell

or

hydration shell


Non polar substances are insoluble in water l.jpg
Non-polar substances are insoluble in water

Many lipids are amphipathic



Hydrogen bonding of water l.jpg
Hydrogen Bonding of Water

One H2O molecule can

associate with 4

other H20 molecules

  • Ice: 4 H-bonds per water molecule

  • Water: 2.3 H-bonds per water molecule

Crystal lattice of ice




Slide12 l.jpg

Relative Bond Strengths

Bond typekJ/mole

H3C-CH3 88

H-H 104

Ionic 40 to 200

H-bond 2 - 20

Hydrophobic interaction 3 -10

van der Waals 0.4 - 4



Ionization of water14 l.jpg

H20 + H20 H3O+ + OH-

Ionization of Water

H20 H+ + OH-

Keq=1.8 X 10-16M

Keq= [H+] [OH-]

[H2O]

[H2O] = 55.5 M

[H2O] Keq = [H+] [OH-]

(1.8 X 10-16M)(55.5 M ) = [H+] [OH-]

1.0 X 10-14 M2 = [H+] [OH-] = Kw

If [H+]=[OH-] then [H+] = 1.0 X 10-7


Ph scale l.jpg
pH Scale

  • Devised by Sorenson (1902)

  • [H+] can range from 1M and 1 X 10-14M

  • using a log scale simplifies notation

  • pH = -log [H+]

  • Neutral pH = 7.0


Weak acids and bases equilibria l.jpg
Weak Acids and Bases Equilibria

  • Strong acids / bases – disassociate completely

  • Weak acids / bases – disassociate only partially

  • Enzyme activity sensitive to pH

  • weak acid/bases play important role in

  • protein structure/function


Acid conjugate base pairs l.jpg
Acid/conjugate base pairs

HA + H2O A- + H3O+

HA A- + H+

HA = acid ( donates H+)(Bronstad Acid)

A- = Conjugate base (accepts H+)(Bronstad Base)

Ka & pKa value describe tendency to loose H+

large Ka = stronger acid

small Ka = weaker acid

Ka = [H+][A-]

[HA]

pKa = - log Ka




Buffers l.jpg
Buffers

  • Buffers are aqueous systems that resist changes in pH when small amounts of a strong acid or base are added.

  • A buffered system consist of a weak acid and its conjugate base.

  • The most effective buffering occurs at the region of minimum slope on a titration curve

    (i.e. around the pKa).

  • Buffers are effective at pHs that are within +/-1 pH unit of the pKa


Henderson hasselbach equation l.jpg
Henderson-Hasselbach Equation

HA = weak acid

A- = Conjugate base

1) Ka = [H+][A-]

[HA]

2) [H+] = Ka [HA]

[A-]

3) -log[H+] = -log Ka -log [HA]

[A-]

* H-H equation describes

the relationship between

pH, pKa and buffer concentration

4) -log[H+] = -log Ka +log [A-]

[HA]

5) pH = pKa +log [A-]

[HA]


Case where 10 acetate ion 90 acetic acid l.jpg
Case where 10% acetate ion 90% acetic acid

  • pH = pKa + log10[0.1 ]

  • ¯¯¯¯¯¯¯¯¯¯

  • [0.9]

    • pH = 4.76 + (-0.95)

    • pH = 3.81


  • Case where 50 acetate ion 50 acetic acid l.jpg
    Case where 50% acetate ion 50% acetic acid

    • pH = pKa + log10[0.5 ]

  • ¯¯¯¯¯¯¯¯¯¯

  • [0.5]

    • pH = 4.76 + 0

    • pH = 4.76 = pKa


  • Case where 90 acetate ion 10 acetic acid l.jpg
    Case where 90% acetate ion 10% acetic acid

    • pH = pKa + log10[0.9 ]

  • ¯¯¯¯¯¯¯¯¯¯

  • [0.1]

    • pH = 4.76 + 0.95

    • pH = 5.71


  • Cases when buffering fails l.jpg
    Cases when buffering fails

    • pH = pKa + log10[0.99 ]

  • ¯¯¯¯¯¯¯¯¯¯

  • [0.01]

    • pH = 4.76 + 2.00

    • pH = 6.76

    • pH = pKa + log10[0.01 ]

  • ¯¯¯¯¯¯¯¯¯

  • [0.99]

    • pH = 4.76 - 2.00

    • pH = 2.76


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