1. Acids and Bases Topics to be covered: Definitions of acids and bases;
Bronsted’s conjugate acid-base pairs concept;
Definition of strong vs. weak acid/base

2. Acids and Bases Arrhenius definition: Brnsted-Lowry definition:
Acid: substance that increases [H3O+] in solution;
Base: substance that increases [OH-] in solution;
Brnsted-Lowry definition:
Acid: reactant that donates proton in a reaction;
Base: reactant that accepts proton in a reaction;

3. Strong and Weak Acids
Strong acids ionize completely in aqueous solution:
HCl(aq) + H2O  H3O+(aq) + Cl-(aq);
H2SO4(aq) + H2O  H3O+(aq) + HSO4-(aq);
Weak acids ionize only partially in aqueous solution:
HF(aq) + H2O ⇌ H3O+(aq) + F-(aq);
HOCl(aq) + H2O ⇌ H3O+(aq) + ClO-(aq).

4. Complete Ionization of Hydrochloric Acid

5. Dissociations of Strong and Weak Acids
a) Strong acid
b) Weak acid

6. Strong and Weak Bases NaOH(aq)  Na+(aq) + OH-(aq);
Strong bases ionize completely in aqueous solution:
NaOH(aq)  Na+(aq) + OH-(aq);
Ba(OH)2(aq)  Ba2+(aq) OH-(aq);
Weak bases ionize only partially in aqueous solution:
NH3(aq) + H2O  NH4+(aq) + OH-(aq);
PO43-(aq) + H2O  HPO42-(aq) + OH-(aq)

7. Brønsted-Lowry: Conjugate Acids & Bases
Let “HA” represent hydrogen bonded to a random negative ion, A
Consider the following equilibrium:
HA B ⇌ BH A-
Acid Base Conjugate Conjugate
acid base
A- is the conjugate base to acid HA;
HA & A- are conjugate acid-base pair;
BH+ is the conjugate acid to base B;
BH+ & B are also conjugate acid-base pair.

8. Brønsted-Lowry’s Acid-Base Reaction

9. Brønsted’s Conjugate Acid-Base Pairs
Notice: The base becomes the conjugate acid after the proton transfer and the acid becomes the conjugate base after the proton transfer.

10. Examples of Conjugate Acid-Base Pairs
HNO3 – NO3- H2SO4 – HSO4- H3O+ – H2O HF – F- H3PO4 – H2PO4- CH3COOH – CH3COO- H2PO4- – HPO42- NH4+ – NH3 H2O – OH-

11. Exercise #1: Conjugate Acids & Bases
Write the formulas of the conjugate bases for the following acids: (a) HF (b) HSO4- (c) H2CO3 (d) HPO42-

12. Exercise #1: Conjugate Acids & Bases
Answers: (a) F- (b) SO42- (c) HCO3 - (d) PO43-

13. Exercise #2: Conjugate Acids and Bases
Write the formulas of the conjugate acids for the following bases:
(a) NH3
(b) CO32-
(c) C5H5N
(d) HPO32-

14. Exercise #2: Conjugate Acids and Bases
Answers
(a) NH4+
(b) HCO3-
(c) HC5H5N+
(d) H2PO3-

15. Acid Strength and Ionization Constants
Identify the acid, base, conjugate acid, and conjugate base in each of the following reactions and identify the conjugate acid base pairs. (a) HSO4- + NH3 ↔ NH4+ + SO42- (b) SCN- + HF ↔ HSCN + F-

16. Acid Strength and Ionization Constants
Answers (a) HSO4- + NH3 ↔ NH4+ + SO42- A B CA CB (b) SCN- + HF ↔ HSCN + F- B A CA CB

17. Relative Strength of Acids and Their Conjugate Bases
Acids Conjugate Bases
Very Strong Very Weak
Strong Weak
Weak Strong
Very Weak Very Strong
______________________________
Strong acids lose protons very readily  weak conjugate bases;
Weak acids do not lose protons very readily  strong conjugate bases.

18. Relative Strength of Acids and Their Conjugate Bases

19. Acid-Base Properties of Water
Water auto-ionizes (ionizes with itself) to produce both H3O+ and OH, thus it has both acid and base properties.
Auto-ionization of water:
2H2O  H3O+(aq) + OH-(aq)

20. Polyprotic Acids Their hydrogen atoms ionize in stages. For example,
Acids with more than one ionizable hydrogen ion are called polyprotic acids. Examples: H2SO3, H2SO4, H3PO4, H3C6H5O7, etc.
Their hydrogen atoms ionize in stages. For example,
H3PO4(aq) + H2O ⇄ H3O+(aq) + H2PO4-(aq);
H2PO4-(aq) + H2O ⇄ H3O+(aq) + HPO42-(aq);
HPO42-(aq) + H2O ⇄ H3O+(aq) + PO43-(aq);
Acid strength decreases in the order:
H3PO4 >> H2PO4- >> HPO42-;

21. Work time!
Finish part 2, part 6, and part 7 of the “Acid-Base Homework Packet” given to you last Tuesday. The whole packet will be due next Monday. Don’t forget that your Titration lab is due tomorrow.