2Reaction of Weak Bases with Water The base reacts with water, producing its conjugate acid and hydroxide ion:CH3NH2 + H2O CH3NH3+ + OH- Kb = 4.38 x 10-4
3Kb for Some Common Weak Bases Many students struggle with identifying weak bases and their conjugate acids.What patterns do you see that may help you?BaseFormulaConjugate AcidKbAmmonia NH3 NH4+ 1.8 x 10-5 Methylamine CH3NH2 CH3NH3+ 4.38 x 10-4 Ethylamine C2H5NH2 C2H5NH3+ 5.6 x 10-4 Diethylamine (C2H5)2NH (C2H5)2NH2+ 1.3 x 10-3 Triethylamine (C2H5)3N (C2H5)3NH+ 4.0 x 10-4 Hydroxylamine HONH2 HONH3+ 1.1 x 10-8 HydrazineH2NNH2 H2NNH3+ 3.0 x 10-6 Aniline C6H5NH2 C6H5NH3+ 3.8 x 10-10 Pyridine C5H5N C5H5NH+ 1.7 x 10-9
4Reaction of Weak Bases with Water The generic reaction for a base reacting with water, producing its conjugate acid and hydroxide ion:B + H2O BH+ + OH-(Yes, all weak bases do this – DO NOTendeavor to make this complicated!)
5Buffered SolutionsA solution that resists a change in pH when either hydroxide ions or protons are added.Buffered solutions contain either:A weak acid and its saltA weak base and its salt
6Acid/Salt Buffering Pairs The salt will contain the anion of the acid, and the cation of a strong base (NaOH, KOH)Weak AcidFormulaof the acidExample of a salt of the weak acid Hydrofluoric HF KF – Potassium fluoride Formic HCOOH KHCOO – Potassium formate Benzoic C6H5COOH NaC6H5COO – Sodium benzoate Acetic CH3COOH NaH3COO – Sodium acetate Carbonic H2CO3 NaHCO3 - Sodium bicarbonate Propanoic HC3H5O2 NaC3H5O2 - Sodium propanoate Hydrocyanic HCN KCN - potassium cyanide
7Base/Salt Buffering Pairs The salt will contain the cation of the base, and the anion of a strong acid (HCl, HNO3)BaseFormula of the baseExample of a salt of the weak acidAmmonia NH3 NH4Cl - ammonium chloride Methylamine CH3NH2 CH3NH2Cl – methylammonium chloride Ethylamine C2H5NH2 C2H5NH3NO3 - ethylammonium nitrate Aniline C6H5NH2 C6H5NH3Cl – aniline hydrochloride Pyridine C5H5N C5H5NHCl – pyridine hydrochloride
8Titration of an Unbuffered Solution A solution that is0.10 M CH3COOHis titrated with0.10 M NaOH
9Titration of a Buffered Solution A solution that is0.10 M CH3COOH and0.10 M NaCH3COO is titrated with0.10 M NaOH
11Comparing Results Unbuffered Buffered In what ways are the graphs different?In what ways are the graphs similar?
12Henderson-Hasselbalch Equation This is an exceptionally powerful tool, and it’s use will be emphasized in our problem solving.
13Weak Acid/Strong Base Titration Endpoint is above pH 7A solution that is0.10 M CH3COOHis titrated with0.10 M NaOH
14Strong Acid/Strong Base Titration Endpoint is atpH 7A solution that is0.10 M HCl is titrated with0.10 M NaOH
15Strong Acid/Strong Base Titration A solution that is0.10 M NaOH is titrated with0.10 M HClEndpoint is atpH 7It is important to recognize that titration curves are not always increasing from left to right.
16Strong Acid/Weak Base Titration A solution that is0.10 M HCl is titrated with0.10 M NH3Endpoint is belowpH 7