Acid-Base Chemistry

Review Unit 6

Chapters 4, 15, 16

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)

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

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

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

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

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

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]

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-]

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

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”

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”

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

Common Weak Species

• Acids– Organic acid (RCOOH), HCO3

-, Zn+2, NH4+

• Bases – NH3, amines (RNH2), conjugates

Polyprotic Acids

• First ionization is more extensive than second

• Ka1 > Ka2

• pH curve has two eq. points

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)

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