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 H 2 and salt (redox) Carbonates to make CO 2 and H 2 O and salt Base to make H 2 O and salt (neutralization, K=10 14 )

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 H 2 O

Models of Acid-Base Behavior Arrhenius –Acids dissolve in water to form H + /H 3 O + 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 ionize HX + H 2 O X - + H 3 O + K a =[H 3 O + ] [X - ]/[HX] K a = hydronium x conjugate/original –Strong acids ionize completely, irreversibly, no K HCl, HBr, HI, HNO 3, H 2 SO 4, HClO 4 –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 + H 2 O HB + + OH - K b = [OH-] [HB + ]/[B] K b = 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 [H 3 O + ] [H 3 O + ] = 10 -pH [H 3 O + ] [OH - ] = 1.0 x –H 2 O (l) + H 2 O (l) H 3 O + (aq) + OH - (aq) –K w = [H 3 O + ] [OH - ] = 1.0 x pH + pOH = 14

Key Equilibria Autoionization of water –Occurs in any aqueous solution; K w applies in all –2 H 2 O H 3 O + + OH - K w = 1.0 x Ionization of an acid –HX (aq) + H 2 O (l) X - (aq) + H 3 O + (aq) –K a = [H 3 O + ] [X - ]/[HX] Hydrolysis of a base –B (aq) + H 2 O (l) HB + (aq) + OH - (l) –K b = [OH - ] [HB + ]/[B]

Uses for K a and K b (simple sol’ns) Measure pH ---> Find K a or K b Given K a ---> Find pH Given K b ---> Find pOH All can be solved with The Grid, but Grid always gives –K a = x 2 /([original] - x) where x = [H 3 O + ] or –K b = x 2 /([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 K a or K b

Buffer Form of the K a Equation K a = [H 3 O + ] [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 K a/b equation in “buffer form” –[H 3 O + ] = K a (acid/base) [OH - ] = K b (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 - ---> H 2 O + F - An equimolar buffer has a “ratio” of 1, so its pH = pK a 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 H 3 O + = 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 pK a ~ 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 pK a

Common Weak Species Acids –Organic acid (RCOOH), HCO 3 -, Zn +2, NH 4 + Bases –NH 3, amines (RNH 2 ), conjugates

Polyprotic Acids First ionization is more extensive than second K a1 > K a2 pH curve has two eq. points

Miscellaneous Anhydrides –Metal oxides are base anhydrides CaO + H 2 O ---> Ca OH - –Nonmetal oxides are acid anhydrides SO 2 + H 2 O ---> H 2 SO 3 Amphiprotic –HCO 3 - 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 –K a = x 2 /([original] - x) where x = [H 3 O + ] Buffer –Acid and it’s conjugate –[H 3 O + ] = K a (acid/base) or use K b 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