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Understanding Water Properties and pH: A Comprehensive Overview

Dive into the properties of water, its hydrogen bonding, ionization, and the role of pH in chemical equilibria, with examples and explanations of weak acids, bases, buffers, and the Henderson-Hasselbach equation.

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Understanding Water Properties and pH: A Comprehensive Overview

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  1. Water

  2. Properties of water • Very polar • Oxygen is highly electronegative • H-bond donor and acceptor • High b.p., m.p., heat of vaporization, surface tension

  3. Water dissolves polar compounds solvation shell

  4. Non-polar substances are insoluble in water Many lipids are amphipathic

  5. 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

  6. Relative Bond Strengths Bond typeKJ/mole H3C-CH3 88 H-H 104 Ionic 10-20 H-bond 3-6 Hydrophobic interaction 1-3 van der Waals 1

  7. Biological Hydrogen Bonds d+ d- H-Bond distance = ~0.2 nm d+ d- DNA strands held together by H-bonds

  8. 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

  9. 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

  10. 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

  11. Conjugate acid conjugate base pairs HA + H2O A- + H3O+ HA A- + H+ HA = Conjugate 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

  12. pKa values determined by titration

  13. Phosphate has three ionizable H+ and three pKas

  14. 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

  15. Henderson-Hasselbach Equation HA = Conjugate 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]

  16. Case where 10% acetate ion 90% acetic acid • pH = pKa + log10[0.1 ] • ¯¯¯¯¯¯¯¯¯¯ • [0.9] • pH = 4.76 + (-0.95) • pH = 3.81

  17. Case where 50% acetate ion 50% acetic acid • pH = pKa + log10[0.5 ] • ¯¯¯¯¯¯¯¯¯¯ • [0.5] • pH = 4.76 + 0 • pH = 4.76 = pKa

  18. Case where 90% acetate ion 10% acetic acid • pH = pKa + log10[0.9 ] • ¯¯¯¯¯¯¯¯¯¯ • [0.1] • pH = 4.76 + 0.95 • pH = 5.71

  19. 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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