Updates Assignment 04 is is due today (in class) Midterms marked (in the box); solutions are posted Assignment 03 is in the box

Acids and Bases Chapter 16

HA (aq) + H 2 O (l) H 3 O + (aq) + A - (aq) Weak Acids (HA) and Acid Ionization Constants HA (aq) H + (aq) + A - (aq) K a = [H + ][A - ] [HA] K a is the acid ionization constant KaKa weak acid strength 16.5

NH 3 (aq) + H 2 O (l) NH 4 + (aq) + OH - (aq) Weak Bases and Base Ionization Constants K b = [NH 4 + ][OH - ] [NH 3 ] K b is the base ionization constant KbKb weak base strength 16.6 Solve weak base problems like weak acids except solve for [OH - ] instead of [H + ].

16.6

16.7 Ionization Constants of Conjugate Acid-Base Pairs HA (aq) H + (aq) + A - (aq) A - (aq) + H 2 O (l) OH - (aq) + HA (aq) KaKa KbKb H 2 O (l) H + (aq) + OH - (aq) KwKw K a K b = K w For a Weak Acid and Its Conjugate Base, K a and K b are related by K w, Ka =Ka = KwKw KbKb Kb =Kb = KwKw KaKa

Polyprotic Acids Have more than one acidic proton. If the difference between the K a for the first dissociation and subsequent K a values is 10 3 or more, the pH generally depends only on the first dissociation.

16.8 Note that when K a decreases for each subsequent proton dissociation, K b increases Remember K a K b is a constant!

Acid Strength Depends on parameters such as solvent, temperature, concentration, and the molecular structure of the acid By considering the properties of two acids in the same solvent at the same temperature and concentration, the variation of acid strength with structure can be studied. Bond strength and bond polarity play prominent roles in determining acid strength –As H-X bond strength increases, acid strength decreases –As H-X bond polarity increases, acid strength increases

Molecular Structure and Acid Strength H X H + + X - The stronger the bond The weaker the acid HF << HCl < HBr < HI 16.9 Hydrohalic acids

Strength of Hydrohalic Acids H-I bond is weakest, HI should be strongest acid H-F bond is most polar, HF should be strongest acid Empirically, HI is a much stronger acid so bond strength is the determining factor for rationalizing the strengths of the hydrohalic acids HF << HCl < HBr < HI

Molecular Structure and Acid Strength Z O HZ O-O- + H + -- ++ The O-H bond will be more polar and easier to break if: Z is very electronegative or Z is in a high oxidation state 16.9 Oxoacids

Molecular Structure and Acid Strength 1. Oxoacids having different central atoms (Z) that are from the same group and that have the same oxidation number. Acid strength increases with increasing electronegativity of Z H O Cl O O H O Br O O Cl is more electronegative than Br HClO 3 > HBrO You may find p. 107 helpful in reviewing how to assign oxidation numbers. Oxoacids

Molecular Structure and Acid Strength 2. Oxoacids having the same central atom (Z) but different numbers of attached groups. Acid strength increases as the oxidation number of Z increases. HClO 4 > HClO 3 > HClO 2 > HClO 16.9

Acid-Base Properties of Salts Neutral Solutions: Basic Solutions: Acid Solutions: Salt - ionic compound formed by the reaction between an acid and a base Salts are strong electrolytes that completely dissociate in water and, in some cases, react with water Salt hydrolysis – reaction of a cation or anion of a salt with water Salt hydrolysis usually affects the pH of a solution

Acid-Base Properties of Salts Neutral Solutions: Salts containing an alkali metal or alkaline earth metal ion (except Be 2+ ) and the conjugate base of a strong acid (e.g. Cl -, Br -, and NO 3 - ). NaCl (s) Na + (aq) + Cl - (aq) H2OH2O Basic Solutions: Salts derived from a strong base and a weak acid. CH 3 COONa (s) Na + (aq) + CH 3 COO - (aq) H2OH2O CH 3 COO - (aq) + H 2 O (l) CH 3 COOH (aq) + OH - (aq) 16.10

Acid-Base Properties of Salts Acid Solutions: Salts derived from a strong acid and a weak base. NH 4 Cl (s) NH 4 + (aq) + Cl - (aq) H2OH2O NH 4 + (aq) NH 3 (aq) + H + (aq) Salts with small, highly charged metal cations (e.g. Al 3+, Cr 3+, and Be 2+ ) and the conjugate base of a strong acid. Al(H 2 O) 6 (aq) Al(OH)(H 2 O) 5 (aq) + H + (aq)

Acid Hydrolysis of Al

Acid-Base Properties of Salts Solutions in which both the cation and the anion hydrolyze: K b for the anion > K a for the cation, solution will be basic K b for the anion < K a for the cation, solution will be acidic K b for the anion  K a for the cation, solution will be neutral 16.10