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Acid-Base Equilibrium 1 AP Chemistry Acid-Base Theory and Equilibrium Constants.

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Presentation on theme: "Acid-Base Equilibrium 1 AP Chemistry Acid-Base Theory and Equilibrium Constants."— Presentation transcript:

1 Acid-Base Equilibrium 1 AP Chemistry Acid-Base Theory and Equilibrium Constants

2 2 Models of Acids and Bases Arrhenius Concept: Acids produce H + (or H 3 O +, hydronium ion) in solution; bases produce OH ion. Arrhenius Concept: Acids produce H + (or H 3 O +, hydronium ion) in solution; bases produce OH ion. HCl + H 2 O Cl + H 3 O + HCl + H 2 O Cl + H 3 O + acid base acid base Brønsted-Lowry: Acids are proton (H + ) donors bases are proton acceptors. Brønsted-Lowry: Acids are proton (H + ) donors bases are proton acceptors.

3 3 HA(aq) + H 2 O(l) H 3 O + (aq) + A (aq) HA(aq) + H 2 O(l) H 3 O + (aq) + A (aq) Conjugate Acid-Base Pairs conjugate base: everything that remains of the acid molecule after a proton is lost. conjugate base: everything that remains of the acid molecule after a proton is lost. conjugate acid: formed when the proton is transferred to the base. conjugate acid: formed when the proton is transferred to the base. conj conj conj conj conj conj conj conj acid 1 base 2 acid 2 base 1

4 4 HA(aq) + H 2 O(l) H 3 O + (aq) + A (aq) HA(aq) + H 2 O(l) H 3 O + (aq) + A (aq) Acid Dissociation Constant, K a K a = [H 3 O + ][A - ] [HA] = [H + ][A - ] [HA]

5 5 Its equilibrium position lies far to the right. (HNO 3 ) Its equilibrium position lies far to the right. (HNO 3 ) Large K a value Large K a value Yields a weak conjugate base. (NO 3 ) Yields a weak conjugate base. (NO 3 ) HNO 3 + H 2 O H 3 O + + NO 3 HNO 3 + H 2 O H 3 O + + NO 3 Strong Acid: Acid Strength

6 6 Its equilibrium lies far to the left. (CH 3 COOH) Its equilibrium lies far to the left. (CH 3 COOH) Small K a value Small K a value Yields a much stronger (it is relatively strong) conjugate base than water. (CH 3 COO Yields a much stronger (it is relatively strong) conjugate base than water. (CH 3 COO CH 3 COOH + H 2 O H 3 O + + CH 3 COO CH 3 COOH + H 2 O H 3 O + + CH 3 COO Acid Strength Weak Acid:

7 7 B(aq) + H 2 O(l) OH - (aq) + HB + (aq) B(aq) + H 2 O(l) OH - (aq) + HB + (aq) Base Dissociation Constant, K b K b = + [OH - ][HB + ] [B]

8 8 Strong and weak are used in the same sense for bases as for acids. Strong and weak are used in the same sense for bases as for acids. strong = complete dissociation (hydroxide ion supplied to solution) strong = complete dissociation (hydroxide ion supplied to solution) NaOH(s) Na + (aq) + OH (aq) NaOH(s) Na + (aq) + OH (aq) Bases

9 9 weak = very little dissociation (or reaction with water) weak = very little dissociation (or reaction with water) H 3 CNH 2 (aq) + H 2 O(l) H 3 CNH 3 + (aq) + OH (aq) H 3 CNH 2 (aq) + H 2 O(l) H 3 CNH 3 + (aq) + OH (aq) Bases (cont)

10 10 Water can behave either as an acid or a base. Water can behave either as an acid or a base. H 2 O + H 2 O H 3 O + + OH H 2 O + H 2 O H 3 O + + OH conj conj acid 1 base 2 acid 2 base 1 Water is amphoteric. K w = [H + ] [OH ] K w = at 25°C

11 Acid-Base Equilibrium 2 AP Chemistry The pH Scale

12 12 pH log[H + ] and pOH = log[OH - ] pH log[H + ] and pOH = log[OH - ] pH in water ranges from 0 to 14. pH in water ranges from 0 to 14. K w = = [H + ] [OH ] K w = = [H + ] [OH ] pK w = = pH + pOH pK w = = pH + pOH As pH rises, pOH falls (sum = 14.00). As pH rises, pOH falls (sum = 14.00). The pH Scale

13 Example Problem What is the pH, pOH, and [OH-] of a M solution of nitric acid? HNO 3 + H 2 O H 3 O + + NO 3 HNO 3 + H 2 O H 3 O + + NO 3 pH log[H + ] pH = log(0.0020) pH 2.70 pH + pOH = pOH = pOH = [H + ] [OH ] = (0.0020) [OH ] = [OH ] = M

14 Acid-Base Equilibrium 3 AP Chemistry Solving Problems

15 15 List major species in solution. List major species in solution. Choose species that can produce H + and write reactions. Choose species that can produce H + and write reactions. Based on K values, decide on dominant equilibrium. Based on K values, decide on dominant equilibrium. Write equilibrium expression for dominant equilibrium. Write equilibrium expression for dominant equilibrium. List initial concentrations in dominant equilibrium. List initial concentrations in dominant equilibrium. Solving Weak Acid Equilibrium Problems

16 16 Define change at equilibrium (as x). Define change at equilibrium (as x). Write equilibrium concentrations in terms of x. Write equilibrium concentrations in terms of x. Substitute equilibrium concentrations into equilibrium expression. Substitute equilibrium concentrations into equilibrium expression. Solve for x the easy way. Solve for x the easy way. Calculate [H + ] and pH. Calculate [H + ] and pH. (continued)

17 17 % Dissociation or % Ionization % dissociation = Amount dissociated (M) x 100% Initial concentration (M)

18 Acid-Base Equilibrium 4 AP Chemistry K a and K b Relationships & Polyprotic Acids

19 K a and K b Relationship HA(aq) + H 2 O(l) H 3 O + (aq) + A (aq) K a HA(aq) + H 2 O(l) H 3 O + (aq) + A (aq) K a A - (aq) + H 2 O(l) OH - (aq) + HA(aq) K b H 2 O(l) + H 2 O(l) H 3 O + (aq) + OH (aq) H 2 O(l) + H 2 O(l) H 3 O + (aq) + OH (aq) [H 3 O + ][A - ] [HA] [OH - ][HA] [A] =[H 3 O + ][OH - ] KaKa KbKb KwKw x=

20 20... can furnish more than one proton (H + ) to the solution.... can furnish more than one proton (H + ) to the solution. Polyprotic Acids HCO 3 - H + + CO 3 2- K a2 H 2 CO 3 H + + HCO 3 - K a1 H 2 CO 3 2H + + CO 3 2- K a K a1 x K a2 = K a(overall)

21 K a and K b Relationship for Polyprotic Acids For diprotic acids K a1 x K b2 = K w K a2 x K b1 = K w For triprotic acids K a1 x K b3 = K w K a2 x K b2 = K w K a3 x K b1 = K w

22 Acid-Base Equilibrium 5 AP Chemistry Miscellaneous Acid-Base Concepts

23 23 Acidic and Basic Salts

24 24 Two factors affecting acidity in binary compounds Two factors affecting acidity in binary compounds Bond Polarity (higher is more acidic) Bond Polarity (higher is more acidic) Bond Strength (lower is more acidic) Bond Strength (lower is more acidic) Molecular Structure and Acidic Properties

25 25 1. Acidic Oxides (Acid Anhydrides): 1. Acidic Oxides (Acid Anhydrides): Nonmetal Oxides (Examples: SO 2, CO 2, CrO 3 ) Nonmetal Oxides (Examples: SO 2, CO 2, CrO 3 ) OX bond is strong and covalent. OX bond is strong and covalent. 2. Basic Oxides (Basic Anhydrides): 2. Basic Oxides (Basic Anhydrides): Metal Oxides (Examples: K 2 O, CaO) Metal Oxides (Examples: K 2 O, CaO) OX bond is ionic. OX bond is ionic. Oxides of Metal and Nonmetals

26 26 1. Acidic Oxides (Acid Anhydrides): 1. Acidic Oxides (Acid Anhydrides): H 2 O + CO 2 H 2 CO 3 H 2 O + CO 2 H 2 CO 3 N 2 O 5 + H 2 O 2HNO3 N 2 O 5 + H 2 O 2HNO3 2. Basic Oxides (Basic Anhydrides): 2. Basic Oxides (Basic Anhydrides): K 2 O + H 2 O 2KOH K 2 O + H 2 O 2KOH CaO + H 2 O Ca(OH) 2 CaO + H 2 O Ca(OH) 2 Examples of Anhydrides

27 27 Lewis Acid: electron pair acceptor Lewis Acid: electron pair acceptor Lewis Base: electron pair donor Lewis Base: electron pair donor Lewis Acids and Bases B + N F B N H F F F F F H H H H H :: Lewis Lewis Acid Base Coordinate Covalent Bond


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