chapter 18 - Chapter 18 Acid-Base Equilibria I. II. III....

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Chapter 18 Acid-Base Equilibria I. Acids & Bases in Water II. Autoionization of Water and pH III. Brønsted-Lowry Acids and Bases IV. Weak Acid Equilibria V. Weak Bases VI. Trends in Acid Strength VII. Acidic & Basic Salt Solutions VIII. Generalizing the Brønsted-Lowry Concept IX. Lewis Acids & Bases I. Acids & Bases Arrhenius Acid-Base Theory : (oldest & simplest) Arrhenius acid = a substance that has H in its formula & yields H + ions when dissolved in water Arrhenius base = a substance that has OH in its formula & yields OH - ions when dissolved in water Ex : HCl( aq ) + NH 3 ( aq ) → Cl - ( aq ) + NH 4 + ( aq ) acid base (donor) (acceptor) Q : How can a proton be an H + ion? Ans : hydrogen atom (H) 1 proton + 1 electron hydrogen ion (H + ) 1 proton Brønsted-Lowry acid = proton (H + ) donor Brønsted-Lowry base = proton (H + ) acceptor
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Conjugate base = a species with one fewer H + than its conjugate acid Ex : OH - is the conjugate base of H 2 O. Conjugate acid = a species with one more H + than its conjugate base Ex : H 3 O + is the conjugate acid of H 2 O. Conjugate acid-base pair = two species whose formulas differ by a single H + Ex : HCN / CN - H 2 O/H 3 O + H 2 O/OH - Conjugate Acid-Base Pairs Exercise : What is the conjugate acid of each of the following: (a) HS - (b) CH 3 CH 2 NH 2 (c) SO 4 2- Solution : Exercise : What is the conjugate base of each of the following: (a) HS - (b) C 6 H 5 COOH (c) H 2 SO 4 Solution : Exercise : Identify the Brønsted acids and bases on the left and their conjugates on the right: ) O( H ) ( PO H ) ( HPO ) ( O H 2 - 4 2 2- 4 3 l aq aq aq +  → + + a) ) O( H (aq) CO (aq) OH (aq) HCO 2 2- 3 - - 3 l +  + b) acid base c) ) ( HCO ) ( SO ) ( CO ) ( HSO - 3 - 2 4 - 2 3 - 4 l aq aq aq +  +
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Strong acids are strong electrolytes: ) ( NO ) ( O H 100% ) ( O H ) ( HNO 3 3 2 3 aq aq l aq + + → + Weak acids are weak electrolytes: ) ( F ) ( O H ) O( H ) HF( - 3 % 1 ~ 2 aq aq l aq +  + + Relative Acid-Base Strength Some substances have negligible acidity in water: reaction no ~ ) O( H ) OH( CH 2 3 + l aq • The stronger an acid, the weaker its conjugate base. • The weaker an acid, the stronger its conjugate base. • The stronger a base, the weaker its conjugate acid. • The weaker a base, the stronger its conjugate acid. Common Strong Acids: HCl HClO 3 HNO 3 HBr HClO 4 H 2 SO 4 HI Common Strong Bases = hydroxides of Group 1A metals: LiOH, NaOH, KOH,… Heavy Group 2A metals: Ca(OH) 2 , Ba(OH) 2 , Sr(OH) 2 most common Strong Acids & Strong Bases Memorize all of these! Exercise: Identify the stronger base of each pair: a) Br - or CN - b) ClO - or Cl - c) OH - or NO 3 - d) F - or CH 3 - f) H 3 O + or H 2 O
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Figure 18.2 The extent of dissociation for weak acids. Weak acid: HA( aq ) + H 2 O( l ) H 2 O + ( aq ) + A - ( aq ) Weak acids dissociate very slightly into ions in water. Strong acids dissociate completely into ions in water. HA( g or l ) + H 2 O( l ) H 3 O + ( aq ) + A - ( aq ) HA( aq ) + H 2 O( l ) H 3 O + ( aq ) + A - ( aq ) K c >> 1 K c << 1 K c = [H 3 O + ][A - ] [H 2 O][HA] K c [H 2 O] = K a = [H 3 O + ][A - ] [HA] stronger acid higher [H 3 O + ] larger K a smaller K a lower [H 3 O + ] weaker acid Acid-Dissociation Constant, K a II. Autoionization of Water Water is a very weak electrolyte.
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This note was uploaded on 05/08/2008 for the course CHEM 4:12 taught by Professor Larsen during the Spring '08 term at University of Iowa.

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chapter 18 - Chapter 18 Acid-Base Equilibria I. II. III....

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