module 8 | other aspects of aqueous equilibria

common ion effect

• shift in position of equilibrium upon adding a substance that provides more of an ion that’s already there

• le chatelier’s principle; if there’s extra stuff on the right, it’ll shift left, and vice versa

buffer

solution that resists change in pH w/addition of either an acid or base

must contain significant amounts of weak acid/base with conjugate base/acid

• acid neutralizes any OH- ions, base neutralizes any H3O+ ions

three ways to make a buffer

1. mixing weak acid/base w/conjugate salt (ex. CH3COOH&NaCH3COO, NH3&NH4Cl)

2. mixing two salts that make a conjugate acid-base pair (NaH₂PO₄ and NA₂HPO₄)

3. reacting SOME of a weak base w/a strong acid or SOME of a weak acid w/a strong base - calculations

effectiveness of buffer depends on

• pka/pkb of buffering system

• pH of environment 

henderson-hasselbalch equation

• used when rule of 100 applies - initial acid/salt concentrations at least 100x greater than ka

• pH = pK + log ([conjugate b/a]/[b/a'])

calculating pH changes in a buffer system

1. stoichiometry calculation (how addition changes relative amounts—moles—of acid and conj. base) - added acid reacts w/A- to make more HA, added base reacts w/HA to make more A-

2. equilibrium calculation (calculate pH based on new amounts of acid/conj base)

buffer capacity

• the max amount of acid/base that a buffer can neutralize before the pH changes a lot

• limited capacity of H3O/OH that can be soaked up

  • eventually, all HA reacts w/added OH- and all A- reacts w/added H3O+

max buffer capacity

when [HA] and [A-] are large and equal to each other

buffer range

most effective buffer range → pka ± 1

• choose an acid w/pka close to desired pH

equivalence point

• moles of acid = moles of base

  • moles of H3O+ = moles of OH-

• stoichiometrically equivalent

• inflection point

• depends on pH of salt solution

strong acid and strong base titration

• equivalence point = 7

• ex. HCl + NaOH → NaCl + H2O

weak acid and strong base titration

• equivalence point > 7

• ex. CH3COOH + NaOH → CH3COONa + H2O

half-equivalence point

• during titration of weak acid/base w/strong acid/base, HALF of WEAK substance has been converted to conjugate

• pH = pKa for weak acid titration

• volume at half equivalence point * 2 = volume at equivalence point

strong acid and weak base titration

• pH @ equivalence point < 7

• ex. HCl + NH3 → NH4 + Cl-

weak acid and weak base titration

• pH of equivalence point depends on Ka of weak acid and Kb of weak base in products

• ex. if Ka = Kb, neutralization products have neutral pH

ksp

solubility product constant

• larger Ksp = more soluble