1. Acid-Base Theories The “Boyz”

2. Acid and Base Theories2 Arrhenius Theory of Acids Acid: molecular substances that breaks-ups in aqueous solution into H+ and anions H + (“hydrogen ions” or “protons”) H + gives acids its protons Example: HNO 3(aq)  H + (aq) + NO 3 – (aq)

3. Acid and Base Theories3 Common Acids

4. Acid and Base Theories4 Arrhenius Theory of Bases Base: releases OH – ions in aqueous solution OH – (hydroxide) OH – gives bases their properties Example: NaOH (aq)  Na + (aq) + OH – (aq)

5. Acid and Base Theories5 Common Bases

6. Acid and Base Theories6 Limitations of Arrhenius Theory H + does not exist in solution More likely to find H + attached to H 2 O (hydrated) H 3 O + Some bases, like ammonia, do not fit this definition, the solution is basic, but the compound does not dissociate, forming hydroxide ion NH 3 (g) + H 2 O (l)  NH 4 + (aq) + OH - (aq) Limited to the solvent water, but acid-base reactions can occur in other solvents

7. Bronsted and Lowry Johannes Bronsted Copenhagen (Denmark) Acid and Base Theories7 Thomas Lowry London (England)

8. Acid and Base Theories8 Brønsted–Lowry Theory of Acids and Bases Acid: proton (H+) donor HCl + NH 3  NH 4 + + Cl – HCl donates a H + to NH 3 –H+ does not exist by itself

9. Acid and Base Theories9 Bases: accept a proton H 2 O + NH 3  NH 4 + + OH – NH 3 accepts a H + from H 2 O A Brønsted -Lowry acid must have a H in its formula (like an Arrhenius acid) Any negative ion can be a Brønsted -Lowry base

10. Acid and Base Theories10 For an acid-base reaction: There must be a transfer of a proton A substance can behave as an acid, if another substance behaves as a base i.e. there is a proton donor (acid) and a proton acceptor (base) HCl (aq) + H 2 O (l)  H 3 O + (aq) + Cl - (aq) acid base conjugate conjugate acid base

11. Acid and Base Theories11 Conjugate acid-base pair The two molecules or ions related by transfer of a proton from one to the other Example. HCl (aq) and H 2 O (l) HCl donates the proton and H 2 O receives the proton

12. Acid and Base Theories12 Conjugate base of an acid The particle remaining when the proton is removed from the acid HCl (aq) – acid Remove proton  Cl - (aq) (conjugate base) Conjugate acid of a base The particle produced when a base receives a proton H 2 O (l) – base Add proton  H 3 O + (aq) (conjugate acid)

13. Conjugate Acid-Base Pair Practce H 2 SO 4 + H 2 O  HSO 4 - + H 3 O + H 2 SO 4 & HSO 4 - H 3 O + & H 2 O HCl + NH 3  Cl - + NH 4 + HCl & Cl - NH 4 + & NH 3 NH 3 + H 2 O  NH 4 + + OH - H 2 O + OH - NH 4 + & NH 3 Acid and Base Theories13

14. Acid and Base Theories14 Table 1: Comparing the Arrhenius and Brønsted-Lowry Theory TheoryArrheniusBrønsted-Lowry Acidany substance that dissociates to form H+ in aqueous solution any substance that provides a proton to another substance (or any substance from which a proton may be removed) Baseany substance that dissociates to form OH- in aqueous solution any substance that receives a proton from an acid (or any substance that removes a proton from an acid) Example HCl (aq)  H + (aq) +Cl - (aq) HCl (aq) +H 2 O (l)  H 3 O + (aq) +Cl - (aq)

15. The Acid Ionization Constant, K a The reaction of a weak acid, HA, with water forms a dynamic equilibrium involving H 3 O + and a conjugate base, A - HA (aq) + H 2 O (l) ⇋ H 3 O + (aq) + A - (aq) Acid and Base Theories15

16. The equilibrium constant, K a, is called the acid ionization constant K a is used for reactions in which an acid, HA(aq), reacts with water to form a conjugate base, A - (aq) Acid and Base Theories16

17. The equilibrium law for HA (aq) + H 2 O (l) ⇋ H 3 O + (aq) + A - (aq) Acid and Base Theories17

18. We know H + (aq) does not exist as individual ions in aqueous solution; rather, every hydrogen ion is bound to water molecule as a hydronium ion, H 3 O + (aq) For convenience, we can simply write HA(aq) separating into H + (aq) and A - (aq) Acid and Base Theories18

19. Second, when the concentration of a substance remains constant during a reaction, that substance is omitted from the equilibrium law equation In a dilution solution, we assume the concentration of water remains constant, therefore we can omit H 2 O(l) Acid and Base Theories19

20. Simplified… where [H + (aq)] is equivalent to [H 3 O + (aq)] Acid and Base Theories20

21. A Competition for Protons HA (aq) + H 2 O (l) ⇋ H 3 O + (aq) + A - (aq) acid base conjugate conjugate acid base If H 2 O has a much greater affinity for H + than does A - (if it is a stronger base), the equilibrium position will be far to the right Most dissolved acid will be in the ionized form at equilibrium Acid and Base Theories21

22. A Competition for Protons HA (aq) + H 2 O (l) ⇋ H 3 O + (aq) + A - (aq) acid base conjugate conjugate acid base If A - has a much greater affinity for H + than does H 2 O, the equilibrium position will be far to the left Most dissolved acid will be present at equilibrium as HA(aq) molecules Acid and Base Theories22

23. p. 532 Practice UC # 1, 2, 3 Learning Checkpoint 23