05 Acid Base Salt

05 Acid Base Salt - BronstedLowry Acids & Bases: Acid:...

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Bronsted–Lowry Acids & Bases: • Acid: proton donor • Base: proton acceptor • The general reaction: HA(aq) + H 2 O(l) H 3 O + (aq) + A (aq) Acid Base Acid Base • HA(aq) = acid (undissociated) *A (aq) = conjugate base, after proton leaves *H 3 O + (aq) = “hydronium ion;” H + in water HA A HCl Cl HNO 3 NO 3 H 2 SO 4 HSO 4 K a = acid dissoc. Const. K C = H 3 O + [] A HA = K a Competition for H + • Consider: HCl(aq) + H 2 O(l) H 3 O + (aq) + Cl (aq) • It's a competition for H + : H 2 O vs. Cl *K a tells who wins K a (HCl) >> 1 »all completely dissociated • Strong acids: HI, HBr, HCl, HClO 4 , HNO 3 , H 2 SO 4 1
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Water defines the acid/base scale! •C l is a weaker proton acceptor than H 2 O, thus HCl is a strong acid. •F is a stronger proton acceptor than H 2 O, thus HF is a weak acid. HF(aq) + H 2 O(l) H 3 O + (aq) + F (aq) K a Water Auto–ionization: H 2 O(l) + H 2 O(l) H 3 O + (aq) + OH (aq) K c = [H 3 O + ] [OH For pure water, then Rxn: 2H 2 O(l) H 3 O + (aq) + OH (aq) Initial pure 0 0 Change –2x +x +x Final (pure) x x Solving for [H 3 O + ] and [OH ] K W = [H 3 O + ][OH ] = 1 x 10 –14 x · x = 1 • 10 –14 x = 1 • 10 –7 [H 3 O + ] = 1 • 10 –7 M [OH Pure water does not conduct electricity! 2
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Addition of Acids or Bases to Water K W = [H 3 O + ][OH ] = 1 * 10 –14 • If we increase [H 3 O + ], [OH ] must decrease by the same amount and vice versa. * Very acidic solutions: [H 3 O + ] > 1M; therefore [OH ] < 1 * 10 –14 * Very basic: [OH ] > 1M therefore [H 3 O + ] < 1 * 10 –14 Defining pH Which is more acidic, a solution with [H 3 O+] = A: 3.1 * 10 –3 B: 6.1 * 10 –4 Define pH = – log 10 [H 3 O + ] A: pH = – log 10 (3.1 * 10 –3 B: pH = – log 10 (6.1 * 10 –4 • Lower pH is more acidic * (higher is more basic) Pure water defines acidic or basic solution • pH = – log (1 * 10 –7 ) = 7 • Water defines the scale! * pH = 7 is neutral * pH > 7 is basic * pH < 7 is acidic 3
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