Unit 12 Weak Acids and Weak Bases

What makes a Weak Acid? Weak acids (covalently bonded H compounds) are ionized by water less than 100% HBrO + H 2 O  H 3 O + + BrO - According to Bronsted-Lowry, hypobromous acid is the acid and the hypobromite ion is the conjugate base.

What makes a Weak Base? Weak bases are the conjugate bases of the weak acids (the anions) or they are the organic bases—the amines The amines are derivatives of ammonia NH 3 + H 2 O   NH OH - For Bronsted-Lowry, ammonia is the base (proton acceptor) and the ammonium ion is the conjugate acid (proton donor)

Weak: Don’t ionize 100% Bronsted-Lowry model for Acid-Base Reactions HA + H 2 O  H 3 O + + A - HA acid and A - conjugate base B + H 2 O  BH + + OH - B base and BH + conjugate acid

Conjugate Acid/Base Conjugate AcidConjugate Base HFF-F- HSO 4 - SO 4 2- NH 4 + NH 3

Examples Label the acid, base, conjugate acid, and conjugate base for #2 on page 374 Determine it if substance would act as an acid or base for #4 on page 374 **remember acids lose an H + and bases gain an H +

Examples On page 375 let’s look at number 26

Homework 1 Page 374 (1,3,5, 25)

Equilibrium Expression For HA + H 2 O  H 3 O + + A - K a = [H + ] [A - ] / [HA] For Base + H 2 O  BH + + OH - K b = [ BH + ] [OH - ] / [Base]

EQUILIBRIUM Weak acids and bases set up an equilibrium situation. EQUILIBRIUM: the rate of the forward rxn = rate of the reverse rxn K is the equilibrium constant K a = [H + ]*[A - ] / [HA] K b = [BH + ] * [OH - ] / [B]

Weak Solutions If you put a weak acid in water, the water pulls H + from the acid because the covalent bond is weaker in the acid. [H + ] = [A - ] If you put a weak base in water, the weak base pulls an H + from the water. [OH - ] = [BH + ]

Example: Weak Acid Calculate the pH of a 2M HBrO solution. What is the % ionization?

Example: Weak Base What is the pH of a 3M solution of ammonia, a weak base?

Example: Finding Ka Aspirin is a weak acid. A 0.1 M solution has a pH = What is the Ka?

Example: Finding Ka Saccharin is a weak acid. A 0.1 M solution ionizes 22%. Calculate the concentration of all species, the pH, and Ka.

Example: Finding Kb Cocaine is a weak base. A M solution has a pH of 9.7, what is the Kb?

Homework 2 & 3 Page odd Page , 53-58

Polyprotic Acids Water pulls off 1 H + at a time. Ionization occurs in stages Water will be less effective at pulling off an H + from the negative anion at each step. The #H + = # ionizations

Multiprotic Acids H 3 A  H + + H 2 A - K a1 H 2 A -  H + + HA 2- K a2 HA 2-  H + + A 3- K a3

Multiprotic Acid Example What is the pH and concentration of all species present in a 1.5M solution of phosphoric acid (in soft drinks)?

Homework 4 Page 375 (45-48)

How to determine if the salts are acidic, basic or neutral in water solutions? NaNO 2 NaF HCl NaCN HCO 3 -

Homework 5 Page 376 (59, 60)