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Acid-Base Chemistry
Review Unit 6
Chapters 4, 15, 16
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Properties of Acids
• Acids– Taste sour– Turn litmus red, phth colorless– React with
• Metals to make H2 and salt (redox)
• Carbonates to make CO2 and H2O and salt
• Base to make H2O and salt (neutralization, K=1014)
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Properties of Bases
• Taste bitter
• Feel slippery
• Turn litmus blue, phth pink
• React with– Fats, oils, and waxes to make soap and water
– Acids to make salt and H2O
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Models of Acid-Base Behavior
• Arrhenius– Acids dissolve in water to form H+/H3O+ ions– Bases dissolve in water to form OH- ions– Built on the conjugate species of water
• Brönsted-Lowry– Acids are proton (H+) donors– Bases are proton acceptors
• Lewis– Acids are electron-pair acceptors– Bases are electron-pair donors– Model of choice for coordination chemists
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Strong vs. Weak Acids
• Acids ionizeHX + H2O <---> X- + H3O+
• Ka=[H3O+] [X-]/[HX]• Ka= hydronium x conjugate/original
– Strong acids ionize completely, irreversibly, no K• HCl, HBr, HI, HNO3, H2SO4, HClO4
– Weak acids ionize < 5%, reversibly, use K• Reversibility means the species produced (X-) can
reaccept a proton, making it a conjugate base
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Strong vs. Weak Bases
• Bases hydrolyze (split water)– B + H2O <---> HB+ + OH-
• Kb = [OH-] [HB+]/[B]
• Kb = hydroxide x conjugate/original
– Strong bases are the soluble hydroxides• Group I and heavy Group II hydroxides
– Weak bases hydrolyze < 5%• Reversibility means the species produced (HB+) can
redonate a proton
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pH
• Def’n: pH = -log [H3O+]
• [H3O+] = 10-pH
• [H3O+] [OH-] = 1.0 x 10-14
– H2O(l) + H2O(l) <===> H3O+(aq) + OH-
(aq)
– Kw = [H3O+] [OH-] = 1.0 x 10-14
• pH + pOH = 14
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Key Equilibria
• Autoionization of water– Occurs in any aqueous solution; Kw applies in all– 2 H2O <===> H3O+ + OH- Kw = 1.0 x 10-14
• Ionization of an acid– HX(aq) + H2O(l) <===> X-
(aq) + H3O+(aq)
– Ka = [H3O+] [X-]/[HX]
• Hydrolysis of a base– B(aq) + H2O(l) <===> HB+
(aq) + OH-(l)
– Kb = [OH-] [HB+]/[B]
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Uses for Ka and Kb (simple sol’ns)
• Measure pH ---> Find Ka or Kb
• Given Ka ---> Find pH
• Given Kb ---> Find pOH
• All can be solved with The Grid, but
• Grid always gives– Ka = x2/([original] - x) where x = [H3O+] or
– Kb = x2/([original] - x) where x = [OH-]
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Buffers
• Sol’ns that are resistant to changes in pH• Roughly equimolar mix of acid and conjugate
– Conjugates don’t neutralize each other– Conjugates can neutralize all other acids and bases
• Types of buffer problems– Design a buffer with a certain pH– Find pH of a buffer with given composition– Find pH of a buffer after something is added
• All are solved with either Ka or Kb
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Buffer Form of the Ka Equation
• Ka = [H3O+] [conj base]/[acid] applies to buffers just as for simple solutions
• Differences– Due to common ion effect, there is no dissociation– So [acid]original = [acid]equilibrium (same for base)
• We rearrange Ka/b equation in “buffer form”– [H3O+] = Ka (acid/base) [OH-] = Kb (base/acid)– Can use original [ ]’s = equilibrium [ ]’s– Can use M’s or moles in “the ratio”
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Making a Buffer
• Mix HF(aq) with NaF(s) or
• Partially neutralize HF– Neutralizing HF produces F-
– HF + OH- ---> H2O + F-
• An equimolar buffer has a “ratio” of 1, so its pH = pKa of the weak acid chosen
• Fine-tune pH by tweaking the “ratio”
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pH Curves• Measure pH as base is titrated into an acid• Equivalence point is when moles H3O+ = moles OH-
• Big change in pH at the equivalence point– Biggest for strong acid/strong base titration– Smallest for weak acid/weak base titration– Choose indicator with pKa ~ pH at equiv point
• pH at equivalence point– = 7 for strong acid/strong base (no conjugates left)– > 7 for weak acid/strong base (conj base left)– < 7 for strong acid/weak base (conj acid left)
• Use equivalence point to calculate [ ] of UK• Use half-equivalence point to find pKa
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Common Weak Species
• Acids– Organic acid (RCOOH), HCO3
-, Zn+2, NH4+
• Bases – NH3, amines (RNH2), conjugates
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Polyprotic Acids
• First ionization is more extensive than second
• Ka1 > Ka2
• pH curve has two eq. points
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Miscellaneous
• Anhydrides– Metal oxides are base anhydrides
• CaO + H2O ---> Ca+2 + 2 OH-
– Nonmetal oxides are acid anhydrides• SO2 + H2O ---> H2SO3
• Amphiprotic– HCO3
- can either gain or lose a proton
• Amphoteric– Al species can act as acids or bases (in a Lewis
way)
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Types of Problems• Simple solution
– Only one ingredient in water– Ka = x2/([original] - x) where x = [H3O+]
• Buffer– Acid and it’s conjugate– [H3O+] = Ka (acid/base) or use Kb
• Titration– Acid and a non-conjugate base– Two steps
• Neutralization goes to completion• What remains is
– Simple solution if you go to the equivalence point– Buffer is you only partially neutralize