Acids and Bases

1. Acids and Bases The First of Three Lectures

2. Today’s Outline • Theories of Acids and Bases • Strong vs. Weak Acids and Bases • Water: An Acid and a Base?

3. Acid-Base Theory • How would you describe an acid or a base? • Similar questions about acids and bases have been asked ever since their discovery in the 1200’s • Because of these curiosities, several theories were generated in attempts to clarify what makes an acid an acid (or a base a base)

4. Acid-Base Theory - Arrhenius • Arrhenius defined the terms acid and base as follows: • An acid is a substance that produces H+ ions when dissolved in water • A base is a substance that produces OH- ions when dissolved in water

5. Acid-Base Theory - Arrhenius • We can demonstrate these definitions as follows: H2O HNO3(l) H+(aq) + NO3-(aq) H2O Ca(OH)2(s) Ca2+(aq) + 2OH-(aq)

6. Acid-Base Theory - Arrhenius • What type of reaction deals with the reaction of an acid and a base? • Neutralization Reaction • The products of a Neutralization Reaction are water and a salt (an ionic compound containing the cation from a base and the anion from the acid)

7. Acid-Base Theory - Arrhenius • Write the total balanced equation for the reaction of sodium hydroxide and hydrochloric acid • We call this the molecular equation • Can you write the total and net ionic equations? NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l) Base Acid Salt Water

8. Acid-Base Theory - Arrhenius • The total ionic equation shows all of the ions in the solution • The net ionic equation shows only those ions involved in the reaction Na+(aq) + OH-(aq) + H+(aq) + Cl-(aq) Na+(aq) + Cl-(aq) + H2O(l) H+(aq) + OH-(aq) H2O(l)

9. Acid-Base Theory - Arrhenius • Are there any errors, or oversights, in the Arrhenius Theory? • Consider ammonia, NH3, which is a well known base: • What happens when ammonia is placed in water? H2O NH3(g) ????

10. Acid-Base Theory - Arrhenius • Why do we study a theory with so many gaps? • Provides ability to track molecular and net ionic equations • Continues to build on our knowledge about the history of chemistry

11. Brnsted-Lowry Theory • Bronsted and Lowry came up with an alternative definition for acids and bases

12. Brnsted-Lowry Theory • In the Bronsted-Lowry definition:An acid is a substance from which a proton can be removed (proton donor)A base is a substance that can remove a proton from an acid (proton acceptor)

13. Brnsted-Lowry Theory • Let’s look at an example: • What is H3O+? HF(aq) + H2O(l) H3O+(aq) + F-(aq)

14. Brnsted-Lowry Theory • Let’s look at an example: • What is H3O+? • Hydronium Ion - indicates the low probability of finding a base proton in solution • Covalently binds a proton to the O atom HF(aq) + H2O(l) H3O+(aq) + F-(aq)

15. Brnsted-Lowry Theory • In this reaction, identify the acid and base • According to the B-L theory, any acid-base reaction involves a proton transfer between an acid and base (forward and reverse) HF(aq) + H2O(l) H3O+(aq) + F-(aq) Acid Base Acid Base

16. Brnsted-Lowry Theory • There are two pairs of substances that differ by a proton • We classify them, generally, as conjugate acid-base pairs (two substances, one acid and one base, in an acid-base reaction that differ by the gain or loss of a proton) HF(aq) + H2O(l) H3O+(aq) + F-(aq)

17. Brnsted-Lowry Theory • By convention, we refer to the substance on the left side as the acid (or base) and the substance on the right as its conjugate base (or conjugate acid) HF(aq) + H2O(l) H3O+(aq) + F-(aq) Conjugate Acid Conjugate Base Acid Base

18. Brnsted-Lowry Theory • In the following equations, identify the acid (base) and its corresponding conjugate base (conjugate acid) HCO3-(aq) + OH-(aq) CO32-(aq) + H2O(l) H2CO3(aq) + CN-(aq) HCN(aq) + HCO3-(l) HCO3-(aq) + HNO3(aq) H2CO3(aq) + NO3-(aq)

19. Acid - Base Strength • How can we define a strong acid or base? • Hint: it has nothing to do with how much damage it can cause to your skin

20. Acid - Base Strength • Let’s think about it from an equilibrium point of view • Can we assume that there are certain acids that will favor the product side of this reaction (which implies that the base can easily extract the proton)? HA(aq) + H2O(l) H3O+(aq) + A-(aq)

21. Acid - Base Strength • By learning a small number of the strong acids and bases, we can determine the relative strength of any acid or base we encounter

22. Acid - Base Strength • Strong Acids: • HCl, HBr, HI, HNO3, HClO4, H2SO4 • Strong Bases: • NaOH, KOH, Ca(OH)2, Sr(OH)2, Ba(OH)2 • All other acids or bases are considered weak, and do not completely dissociate in aqueous solution

23. Acid - Base Strength • Let’s consider H2SO4 for a moment • What kind of acid is this? • Strong, diprotic acid • Since there are two protons available to be removed, which one will be easier? • The first proton will be easiest to remove • Therefore, H2SO4 is a strong acid, while HSO4- is a weak acid

24. Acid - Base Strength • Is water an acid or base?

25. Acid - Base Strength • Does water act like an acid or a base in the following two reactions? • So which is it, an acid or a base? HF(aq) + H2O(l)  H3O+(aq) + F-(aq) NH3(aq) + H2O(l)  NH4+(aq) + OH-(aq) Base Acid

26. Acid - Base Strength • Water is an amphoteric substance: it can behave as both an acid and as a base • It can undergo a process called self-ionization in which two identical molecules react to give ions H2O(l) + H2O(l)  H3O+(aq) + OH-(aq) Kw = [H3O+][OH-]

27. Acid - Base Strength • Kw is the ion-product constant for water • At 25oC, Kw = 1.0 x 10-14 • Does this mean that water readily ionizes into its composite ions? • NO! • At 25oC, [OH-] = [H3O+] = 1x10-7M  0 M

28. Announcements • Extra Credits: Chapter 15 • 30, 38, 44, 50, 56, 64.