1. Prentice Hall ©2004 Chapter 14 Aqueous Equilibria: Acids & Bases by Dr Ayesha Mohy-ud-din

2. Prentice Hall ©2004 Acid–Base Concepts

3. Prentice Hall ©2004 pH – A Measure of Acidity Problem 14.8 Calculate the pH of each of the following Problem 14.9 Calculate the concentrations of H 3 O + and OH – in each of the following solutions: (a) Human blood (pH 7.40) (b) A cola beverage (pH 2.8) Problem 14.10 Calculate the pH of (a) 0.050 M HClO 4 (b) 6.0 M HCl (c) 4.0 M KOH (d) 0.010 M Ba(OH) 2 Problem 14.11 Calculate the pH of a solution prepared by dissolving 0.25 g of BaO in enough water to make 0.500 L of solution

4. Prentice Hall ©2004 Acid Ionization Constants Acid Ionization Constant: the equilibrium constant for the ionization of an acid. HA(aq) + H 2 O(l) æ H 3 O + (aq) + A – (aq)

5. Prentice Hall ©2004 Acid Ionization Constants 7.1 x 10 –4 4.5 x 10 –4 3.0 x 10 –4 1.7 x 10 –4 8.0 x 10 –5 6.5 x 10 –5 1.8 x 10 –5 4.9 x 10 –10 1.3 x 10 –10 HF HNO 2 C 9 H 8 O 4 (aspirin) HCO 2 H (formic) C 6 H 8 O 6 (ascorbic) C 6 H 5 CO 2 H (benzoic) CH 3 CO 2 H (acetic) HCN C 6 H 5 OH (phenol) F – NO 2 – C 9 H 7 O 4 – HCO 2 – C 6 H 7 O 6 – C 6 H 5 CO 2 – CH 3 CO 2 – CN – C 6 H 5 O – ACID K a CONJ. BASE K b 1.4 x 10 –11 2.2 x 10 –11 3.3 x 10 –11 5.9 x 10 –11 1.3 x 10 –10 1.5 x 10 –10 5.6 x 10 –10 2.0 x 10 –5 7.7 x 10 –5

6. Prentice Hall ©2004 Calculating Equilibrium Concentration in Solutions of Weak Acids

7. Prentice Hall ©2004 HA æ H + +A (M):0.500.00 (M):–x+x+x Equilib (M): 0.50–xxx Acid Ionization Constants I nitial C hange E quilibrium T able : Determine the pH of 0.50 M HA solution at 25°C. K a = 7.1 x 10 –4. Initial Change (aq) -

8. Prentice Hall ©2004 Acid Ionization Constants pH of a Weak Acid (Cont’d): 1. Substitute new values into equilibrium expression. 2. If K a is significantly (>1000 x) smaller than [HA] the expression (0.50 – x) approximates to (0.50). 3. The equation can now be solved for x and pH. 4. If K a is not significantly smaller than [HA] the quadratic equation must be used to solve for x and pH.

9. Prentice Hall ©2004 Acid Ionization Constants The Quadratic Equation: The expression must first be rearranged to: The values are substituted into the quadratic and solved for a positive solution to x and pH.

10. Prentice Hall ©2004 Acid Ionization Constants Percent Dissociation: A measure of the strength of an acid. Stronger acids have higher percent dissociation. Percent dissociation of a weak acid decreases as its concentration increases.

11. Prentice Hall ©2004 Base Ionization Constants Base Ionization Constant: The equilibrium constant for the ionization of a base. The ionization of weak bases is treated in the same way as the ionization of weak acids. B(aq) + H 2 O(l) æ BH + (aq) + OH – (aq) Calculations follow the same procedure as used for a weak acid but [OH – ] is calculated, not [H + ].

12. Prentice Hall ©2004 Base Ionization Constants 5.6 x 10 –4 4.4 x 10 –4 4.1 x 10 –4 1.8 x 10 –5 1.7 x 10 –9 3.8 x 10 –10 1.5 x 10 –14 C 2 H 5 NH 2 (ethylamine) CH 3 NH 2 (methylamine) C 8 H 10 N 4 O 2 (caffeine) NH 3 (ammonia) C 5 H 5 N (pyridine) C 6 H 5 NH 2 (aniline) NH 2 CONH 2 (urea) C 2 H 5 NH 3 + CH 3 NH 3 + C 8 H 11 N 4 O 2 + NH 4 + C 5 H 6 N + C 6 H 5 NH 3 + NH 2 CONH 3 + BASE K b CONJ. ACID K a 1.8 x 10 –11 2.3 x 10 –11 2.4 x 10 –11 5.6 x 10 –10 5.9 x 10 –6 2.6 x 10 –5 0.67 Note that the positive charge sits on the nitrogen.

13. Prentice Hall ©2004 Diprotic & Polyprotic Acids Diprotic and polyprotic acids yield more than one hydrogen ion per molecule. One proton is lost at a time. Conjugate base of first step is acid of second step. Ionization constants decrease as protons are removed.

14. Prentice Hall ©2004 Diprotic & Polyprotic Acids Very Large 1.3 x 10 –2 6.5 x 10 –2 6.1 x 10 –5 1.3 x 10 –2 6.3 x 10 –8 4.2 x 10 –7 4.8 x 10 –11 9.5 x 10 –8 1 x 10 –19 7.5 x 10 –3 6.2 x 10 –8 4.8 x 10 –13 H 2 SO 4 HSO 4 – C 2 H 2 O 4 C 2 HO 4 – H 2 SO 3 HSO 3 – H 2 CO 3 HCO 3 – H 2 S HS – H 3 PO 4 H 2 PO 4 – HPO 4 2– ACID K a CONJ. BASE K b HSO 4 – SO 4 2– C 2 HO 4 – C 2 O 4 2– HSO 3 – SO 3 2– HCO 3 – CO 3 2– HS – S 2– H 2 PO 4 – HPO 4 2– PO 4 3– Very Small 7.7 x 10 –13 1.5 x 10 –13 1.6 x 10 –10 7.7 x 10 –13 1.6 x 10 –7 2.4 x 10 –8 2.1 x 10 –4 1.1 x 10 –7 1 x 10 –5 1.3 x 10 –12 1.6 x 10 –7 2.1 x 10 –2

15. Prentice Hall ©2004 Molecular Structure and Acid Strength The strength of an acid depends on its tendency to ionize. For general acids of the type H–X: 1. The stronger the bond, the weaker the acid. 2. The more polar the bond, the stronger the acid. For the hydrohalic acids, bond strength plays the key role giving: HF < HCl < HBr < HI

16. Prentice Hall ©2004 Molecular Structure and Acid Strength The electrostatic potential maps show all the hydrohalic acids are polar. The variation in polarity is less significant than the bond strength which decreases from 567 kJ/mol for HF to 299 kJ/mol for HI.

17. Prentice Hall ©2004 Molecular Structure and Acid Strength For binary acids in the same group, H–A bond strength decreases with increasing size of A, so acidity increases. For binary acids in the same row, H–A polarity increases with increasing electronegativity of A, so acidity increases.

18. Prentice Hall ©2004 Molecular Structure and Acid Strength For oxoacids bond polarity is more important. If we consider the main element (Y): Y–O–H If Y is an electronegative element, or in a high oxidation state, the Y–O bond will be more covalent and the O–H bond more polar and the acid stronger.

19. Prentice Hall ©2004 Molecular Structure and Acid Strength For oxoacids with different central atoms that are from the same group of the periodic table and that have the same oxidation number, acid strength increases with increasing electronegativity.

20. Prentice Hall ©2004 Molecular Structure and Acid Strength For oxoacids having the same central atom but different numbers of attached groups, acid strength increases with increasing central atom oxidation number. As shown on the next slide, the number of oxygen atoms increases the positive charge on the chlorine which weakens the O–H bond and increases its polarity.

21. Prentice Hall ©2004 Molecular Structure and Acid Strength Oxoacids of Chlorine: