L18 - Lecture 18 Basically Weak: Weak Acids and Bases Key...

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Unformatted text preview: Lecture 18 Basically Weak: Weak Acids and Bases Key Idea: The pH depends on the acid or base concentration and the degree of dissociation. Brnsted-Lowry Theory Acids are H+ donors HCOOH (aq) + H2O (l) ! H3O+ (aq) + Cl- (aq) Bases are H+ acceptors NH3 (aq) + H2O (l) ! NH4+ (aq) + OH- (aq) Observe what water does in these equilibria! Water is an amphiprotic solvent: it can both accept and donate a proton NH3 OH- Autoionization of water H 2O(l ) + H 2O(l ) ! H 3O + (aq ) + OH ! (aq ) K w = [ H 3O + ][OH ! ] K w = 1.0 "10!14 at 25 0 C NH4+ At 25 oC: What is the pH of pure water? What is the pOH of pure water? Relationship between [H3O+] and [OH-] H3O+ concentration more acidic 1.0 x 10 0 1.0 x 10 -1 1.0 x 10 -2 1.0 x 10 -3 1.0 x 10 -4 1.0 x 10 -5 1.0 x 10 -6 1.0 x 10 -7 more basic 1.0 x 10 -8 1.0 x 10 -14 [H3O+] [OH-] = 10 -14 OH- concentration 1.0 x 10 -14 1.0 x 10 -13 1.0 x 10 -12 1.0 x 10 -11 1.0 x 10 -10 1.0 x 10 -9 1.0 x 10 -8 1.0 x 10 -7 1.0 x 10 -6 1.0 x 10 0 pH 0 1 2 3 4 5 6 7 8 14 pH = pOH = 7.0 However, at 50 oC: Kw = 5.48 x 10-14 pH = pOH = 6.6!! At all temperatures, pure water has "neutral" pH!! pH + pOH = 14 Diluting Acids 10 mL 0.010 M HCl pH = 2 H+ Can you turn an acid into a base by dilution? Cl1 liter 0.00010 M HCl pH = 4 Calculating pH How can you calculate the pH of acid or base solutions? Strong acids and bases: rely on stoichiometry Weak acids and bases: ICE equilibrium calculations 100 mL 0.0010 M HCl pH = 3 Mix 10 mL 0.010 M HCl with 90 mL water Mix 100 mL 0.0010 M HCl with 900 mL water Conjugate Acid-Base Pairs acid acid remove H+ Conjugate Acid-Base Pairs conjugate base conjugate base HA + H 2O ! H 3O + + A! [ H 3O + ][ A! ] [ HA] A! + H 2O ! HA + OH ! [ HA][OH ! ] [ H 3O + ] K w " = [ A! ] [ H 3O + ] K a HA (aq) + H2O (l) ! H3O+ (aq) + A- (aq) ! add H+ Ka = Kb = base conjugate acid K a ! Kb = K w A strong acid has a weak conjugate base and vice versa. Relative strengths of the conjugate acid-base pairs strong acids HCl H2SO4 HNO3 H3O+ HSO4! HF weak acids CH3COOH H2CO3 H2S NH4+ HCN HCO3! H2O H2 CH4 Cl! HSO4! NO3! H 2O SO42! F! CH3CO2! HCO3! HS! NH3 CN! CO32! OH! H! CH3! weak bases extremely weak bases Factors that influence the degree of dissociation HA (aq) + H2O (l) ! H3O+ (aq) + A- (aq) ! Breaking bonds of reactants: For organic acids = always OH Base strength increases Stability of the ions formed: One ion is always hydronium. The anion varies. Thus, acid strength depends on the stability of the anion. Acid strength increases extremely weak acids strong bases Correlation of acid strength and structure Which is the strongest acid? A. A. B. C. B. C. D. Reactions between strong acids and bases Predict the products. HCl (aq) + NaOH (aq) ! HNO3 (aq) + NaOH (aq) ! Ca(OH)2 (s) + 2 HCl (aq) ! HNO3 (aq) + HCl (aq) ! Ca(OH)2 (s) + NaOH (aq) ! Net ionic equation: For a stoichiometric reaction, what is the pH of the resulting solution? Reactions between weak and strong Predict the products. CH3COOH (aq) + OH- (aq) ! NH4+ (aq) + OH- (aq) ! F- (aq) + H3O+ (aq) ! HCO3- (s) + H3O+ (aq) ! Wrap-up points Water is amphiprotic. It can accept H+ and donate H+. At 25C, Kw = 10-14. pH = pOH = 7. Water is neutral because pH = pOH. A strong acid has a weak conjugate base and vice versa. The degree of dissociation for organic acids depends on the stability of the conjugate base. To calculate pH for strong acids and bases, rely on stoichiometry. To calculate pH for weak acids and bases, do ICE calculations. ...
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This note was uploaded on 04/03/2008 for the course CHEM 1A taught by Professor Nitsche during the Fall '08 term at University of California, Berkeley.

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