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

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;

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).

Complete Ionization of Hydrochloric Acid

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

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) + 2 OH-(aq); Weak bases ionize only partially in aqueous solution: NH3(aq) + H2O  NH4+(aq) + OH-(aq); PO43-(aq) + H2O  HPO42-(aq) + OH-(aq)

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.

Brønsted-Lowry’s Acid-Base Reaction

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.

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

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

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

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-

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

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-

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

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.

Relative Strength of Acids and Their Conjugate Bases

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)

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-;

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.