Chapter 15 Acids and Bases Week 1-1 (09)

Chapter 15 Acids and Bases Week 1-1 (09) - CHAPTER 15 ACID-...

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CHAPTER 15 ACID- BASE EQUILIBRIA Introduction to Acids, Bases and the Equilibrium Concept Water (H 2 O) – the most important molecule on earth. Even in pure water, there are small amounts of ions from the equilibrium below (“self- ionization of water” or “auto-ionization of water”). H 2 O (l) H + (aq) + OH - (aq) More accurately: H 2 O (l) + H 2 O (l) H 3 O + (aq) + OH - (aq) H 3 O + (aq) = hydronium ion; often abbreviated as H + (aq) OH - (aq) = hydroxide ion [H 3 O + ] = [OH - ] in pure water Definitions of Acids and Bases 1) ACIDS: give [H 3 O + ] > [OH - ] in solution (vinegar, lemon juice) 2) BASES: give [H 3 O + ] < [OH - ] in solution (bicarb) Understanding “acidity” and “basicity” involves knowing the [H + ] and [OH - ] concentrations in solution. Historically, the first definitions of acids and bases were the “ Arrhenius Definitions ”. 1) ACID = a substance with H in its formula, and which dissociates in water to give H 3 O + (aq) (= H + (aq)) Generic acid = HA (e.g., HCl, HNO 3 , H 2 SO 4 , etc.) 15-1
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2) BASE = a substance with OH in its formula, and dissociates to yield OH - Generic base = MOH (e.g., NaOH, Ca(OH) 2 , etc.) Neutralization : the reaction between an acid and a base H + (aq) + OH - (aq) H 2 O (l); Strengths of Acids and Bases : (i.e., the amount of H + or OH - produced per mole of substance dissolved). S TRONG ACIDS AND BASES DISSOCIATE COMPLETELY (~100%) IN SOLUTION MUST KNOW THEM ! Strong Acids : HCl, HBr, HI, HClO 4 , HNO 3 , H 2 SO 4 - plus few rare ones Strong Bases : MOH and M(OH) 2 , where M = Li + , Na + , K + , Rb + , Cs + , Ca 2+ , Sr 2+ , Ba 2+ ** A LL OTHER ACIDS AND BASES ARE WEAK ** Because strong acids and bases dissociate “completely” (i.e. ~100%), we do not consider them equilibria, (i.e., K c >>> 1) and we write them as a one-directional reaction. Strong Acid : HA (g or l) + H 2 O (l) H 3 O + (aq) + A - (aq) Strong Base : MOH (s) + H 2 O (l) M + (aq) + OH - (aq) Weak Acids : dissociate only partially in solution; it is an equilibrium. HA (aq) + H 2 O (l) H 3 O + (aq) + A - (aq) 15-2 kJ 55.9 - rxn H Δ =
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**Amount of dissociation varies, depending on K of the acid ** Section 15.1 The Brønsted-Lowry Definition The Arrhenius definition of acids and bases doesn’t cover all possibilities, e.g., some bases do not contain OH in their formula. The Brønsted-Lowry definition is much better. Acid : an H + donor . Must contain H + in its formula (all Brønsted- Lowry acids are also Arrhenius acids). Base : an H + acceptor . Must contain a lone-pair capable of binding an H + (e.g., NH 3 , F - , OH - , etc.). Brønsted-Lowry bases are not necessarily Arrhenius bases, but Arrhenius bases contain the Brønsted-Lowry base, OH - . This gives acid/base reactions
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This note was uploaded on 05/18/2011 for the course CHM 2046 taught by Professor Veige/martin during the Spring '07 term at University of Florida.

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Chapter 15 Acids and Bases Week 1-1 (09) - CHAPTER 15 ACID-...

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