1. CHAPTER 9 Acids & Bases General, Organic, & Biological Chemistry Janice Gorzynski Smith

2. CHAPTER 9: Acids & Bases 2 Smith. General Organic & Biologicial Chemistry 2nd Ed. Learning Objectives:  Define Acids & Bases: Bronsted-Lowry  Understand the difference between strong & weak acids  Hydronium ion formation  Identify conjugate acids & bases  Define & calculate K a, K b, K w  Water is both an acid & a base  Titrations to calculate analyte concentration  Buffers  Weak acid + conjugate base  Weak base + conjugate acid

3. 3 Acid/Base Brønsted-Lowry Definition Smith. General Organic & Biologicial Chemistry 2nd Ed. The Brønsted–Lowry definition is more widely used: A Brønsted–Lowry acid is a proton (H + ) donor. A Brønsted–Lowry base is a proton (H + ) acceptor. H 3 O + (aq) + Cl − (aq)HCl(g) + H 2 O(l) This proton is donated. HCl is a Brønsted–Lowry acid because it donates a proton to the solvent water. H 2 O is a Brønsted–Lowry base because it accepts a proton from HCl.

4. 4 Acid/Base Brønsted-Lowry Definition Smith. General Organic & Biologicial Chemistry 2nd Ed. A monoprotic acid contains one acidic proton. HCl A diprotic acid contains two acidic protons. H 2 SO 4 A triprotic acid contains three acidic protons. H 3 PO 4 A Brønsted–Lowry acid may be neutral or it may carry a net positive or negative charge. HCl, H 3 O +, HSO 4 − A Brønsted–Lowry acid must contain a hydrogen atom.

5. 5 Acid/Base Brønsted-Lowry Definition Smith. General Organic & Biologicial Chemistry 2nd Ed. A base must contain a lone pair of electrons that can be used to form a new bond to the proton. N H H H + H 2 O(l) N H H H H + + OH − (aq) This e − pair forms a new bond to a H from H 2 O. H 2 O water NH 3 ammonia Lone pairs make these neutral compounds bases. NaOH sodium hydroxide KOH potassium hydroxide The OH − is the base in each metal salt.

6. 6 Acid/Base Brønsted-Lowry Definition Smith. General Organic & Biologicial Chemistry 2nd Ed. HA+ B A − HB+B+ + gain of H + acidbase loss of H + This e − pair stays on A. This e − pair forms a new bond to H +.

7. 7 Acid/Base Conjugate Acids & Bases Smith. General Organic & Biologicial Chemistry 2nd Ed. The product formed by loss of a proton from an acid is called its conjugate base. HA+ B A − HB+B+ + gain of H + acidbaseconjugate base conjugate acid loss of H + The product formed by gain of a proton by a base is called its conjugate acid. HBr+ + acidbaseconjugate base conjugate acid H2OH2O Br − H3O+H3O+

8. 8 Acid/Base Water: An Acid & Base Smith. General Organic & Biologicial Chemistry 2nd Ed. Amphoteric compound: A compound that contains both a hydrogen atom and a lone pair of e − ; it can be either an acid or a base. HOH H 2 O as a base add H + HOH H + conjugate acid HOH H 2 O as an acid remove H + HO − conjugate base

9. 9 Acid/Base Strong Acids Smith. General Organic & Biologicial Chemistry 2nd Ed. When a covalent acid dissolves in water, the proton transfer that forms H 3 O + is called dissociation. When a strong acid dissolves in water, 100% of the acid dissociates into ions. Common strong acids are HI, HBr, HCl, H 2 SO 4, and HNO 3. A single reaction arrow is used, because the product is greatly favored at equilibrium. H 3 O + (aq) + Cl − (aq)HCl(g) + H 2 O(l)

10. 10 Acid/Base Weak Acids Smith. General Organic & Biologicial Chemistry 2nd Ed. When a weak acid dissolves in water, only a small fraction of the acid dissociates into ions. Unequal reaction arrows are used, because the reactants are usually favored at equilibrium. H 3 O + (aq) + CH 3 COO − (aq)CH 3 COOH(l) + H 2 O(l) Common weak acids are H 3 PO 4, HF, H 2 CO 3, and HCN.

11. 11 Acid/Base Strong & Weak Bases Smith. General Organic & Biologicial Chemistry 2nd Ed. When a strong base dissolves in water, 100% of the base dissociates into ions. Na + (aq) + − OH(aq)NaOH(s) + H 2 O(l) Common strong bases are NaOH and KOH. When a weak base dissolves in water, only a small fraction of the base dissociates into ions. NH 4 + (aq) + − OH(aq)NH 3 (g) + H 2 O(l)

12. 12 Acid/Base Strong/Weak Acids/Bases Smith. General Organic & Biologicial Chemistry 2nd Ed. A strong acid readily donates a proton, forming a weak conjugate base. HCl strong acid Cl − weak conjugate base A strong base readily accepts a proton, forming a weak conjugate acid. OH − strong base H 2 O weak conjugate acid

13. 13 Acid/Base Strong/Weak Acids/Bases Smith. General Organic & Biologicial Chemistry 2nd Ed.

14. 14 Acid/Base K a & K b Smith. General Organic & Biologicial Chemistry 2nd Ed. The position of the equilibrium depends upon the strengths of the acids and bases. The stronger acid reacts with the stronger base to form the weaker acid and the weaker base. A Brønsted–Lowry acid–base reaction represents an equilibrium. HA + B A − HB+B+ + acidbaseconjugate base conjugate acid

15. 15 Acid/Base K a & K b Smith. General Organic & Biologicial Chemistry 2nd Ed. K a = [H 3 O + ][ ] A − [HA] acid dissociation constant The stronger the acid, the larger the K a value. Equilibrium favors formation of the weaker acid, the acid with the smaller K a value. H 3 O + (aq) + (aq)HA(g) + H 2 O(l)A −

16. 16 Acid/Base K a & K b Smith. General Organic & Biologicial Chemistry 2nd Ed.

17. 17 Acid/Base K a & K b Smith. General Organic & Biologicial Chemistry 2nd Ed. K b = [OH - ][BH + ] [ B] Base dissociation constant OH - (aq) + BH + (aq)B (g) + H 2 O(l) The stronger the base, the larger the K b value.

18. 18 Acid/Base KwKw Smith. General Organic & Biologicial Chemistry 2nd Ed. H OH base HOH H + conjugate acid HOH conjugate base HO − ++ loss of H + gain of H + K w = [H 3 O + ][OH − ] ion-product constant K w = (1.0 x 10 −7 ) x (1.0 x 10 −7 ) K w = 1.0 x 10 −14 K w is a constant for all aqueous solutions at 25 o C.

19. 19 Acid/Base pH The lower the pH, the higher the concentration of H 3 O + : Acidic solution: pH 1 x 10 −7 Basic solution: pH > 7  [H 3 O + ] < 1 x 10 −7 Neutral solution: pH = 7  [H 3 O + ] = 1 x 10 −7 Smith. General Organic & Biologicial Chemistry 2nd Ed.

20. 20 Acid/Base pH Smith. General Organic & Biologicial Chemistry 2nd Ed.

21. 21 Acid/Base Common Acid/Base Reactions Smith. General Organic & Biologicial Chemistry 2nd Ed. Neutralization reaction: An acid-base reaction that produces a salt and water as products. HA(aq) + MOH(aq) acid base OH(l) + MA(aq) H watersalt The acid HA donates a proton (H + ) to the OH − base to form H 2 O. The anion A − from the acid combines with the cation M + from the base to form the salt MA.

22. 22 Acid/Base Common Acid/Base Reactions Smith. General Organic & Biologicial Chemistry 2nd Ed. A net ionic equation contains only the species involved in a reaction. HCl(aq) + NaOH(aq) H—OH(l) + NaCl(aq) Written as individual ions: H + (aq) + Cl − (aq) + Na + (aq) + OH − (aq) H—OH(l) + Na + (aq) + Cl − (aq) Omit the spectator ions, Na + and Cl –. H + (aq) + OH − (aq)H—OH(l) What remains is the net ionic equation:

23. 23 Acid/Base Common Acid/Base Reactions Smith. General Organic & Biologicial Chemistry 2nd Ed. A bicarbonate base, HCO 3 −, reacts with one H + to form carbonic acid, H 2 CO 3. H + (aq) + HCO 3 − (aq) Carbonic acid then decomposes into H 2 O and CO 2. H 2 O(l) + CO 2 (g) H 2 CO 3 (aq) HCl(aq) + NaHCO 3 (aq) H 2 O(l) + CO 2 (g) NaCl(aq) + H 2 CO 3 (aq) For example:

24. 24 Acid/Base Common Acid/Base Reactions Smith. General Organic & Biologicial Chemistry 2nd Ed. A carbonate base, CO 3 2–, reacts with two H + to form carbonic acid, H 2 CO 3. 2 H + (aq) + CO 3 2– (aq) H 2 O(l) + CO 2 (g) H 2 CO 3 (aq) 2 HCl(aq) + Na 2 CO 3 (aq) H 2 O(l) + CO 2 (g) 2 NaCl(aq) + H 2 CO 3 (aq) For example:

25. 25 Acid/Base Titration Smith. General Organic & Biologicial Chemistry 2nd Ed. Calculations: Know: the concentration of Base & the Volume of Base added. 1.V Base (L) x M Base (moles/L) = moles Base added to the acid 2.Mole : Mole ratio of Acid : Base is 1:1 (if monoprotic), therefore moles Base added = moles of Acid reacted 3.Determine the Molarity of the Acid solution initially by dividing the # moles that reacted with the initial volume of the solution (L). AH + B  A- + BH + Acid + Base  Conjugate Base + Conjugate Acid HCl + NaOH  NaCl + H 2 O H + (acid) + OH - (base)  H 2 O (neutral water) InitialMidway Final

26. 26 Acid/Base Titration Caclulations Smith. General Organic & Biologicial Chemistry 2nd Ed. Moles of base Moles of base Volume of acid Volume of acid mole–mole conversion factor mole–mole conversion factor M (mol/L) conversion factor M (mol/L) conversion factor Moles of acid Moles of acid Volume of base M (mol/L) conversion factor [1] [2] [3]

27. 27 Acid/Base Buffers Smith. General Organic & Biologicial Chemistry 2nd Ed. A buffer is a solution whose pH changes very little when acid or base is added. Most buffers are solutions composed of roughly equal amounts of: A weak acid The salt of its conjugate base The buffer resists change in pH because Added base, OH −, reacts with the weak acid Added acid, H 3 O +, reacts with the conjugate base

28. 28 Acid/Base Buffers Smith. General Organic & Biologicial Chemistry 2nd Ed. If an acid is added to the following buffer equilibrium, then the excess acid reacts with the conjugate base, so the overall pH does not change much.

29. 29 Acid/Base Buffers Smith. General Organic & Biologicial Chemistry 2nd Ed. If a base is added to the following buffer equilibrium, then the excess base reacts with the conjugate acid, so the overall pH does not change much.

30. 30 Acid/Base Buffers Smith. General Organic & Biologicial Chemistry 2nd Ed.

31. 31 Acid/Base Buffers Smith. General Organic & Biologicial Chemistry 2nd Ed. The effective pH range of a buffer depends on its K a. [H 3 O + ][ ] A − [HA] K a = Rearranging this expression to solve for [H 3 O + ]: [ ] A − [HA] =[H 3 O + ]xKaKa H 3 O + (aq) + (aq)HA(aq) + H 2 O(l)A − determines the buffer pH

32. 32 Acid/Base Buffers Smith. General Organic & Biologicial Chemistry 2nd Ed. Normal blood pH is between 7.35 and 7.45. The principle buffer in the blood is carbonic acid/ bicarbonate (H 2 CO 3 /HCO 3 − ). CO 2 (g) + H 2 O(l) H 2 CO 3 (aq) H2OH2O H 3 O + (aq) + HCO 3 − (aq) CO 2 is constantly produced by metabolic processes in the body. The amount of CO 2 is related to the pH of the blood.