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Acids and BasesChapter 16

HA (aq) + H2O (l) H3O+ (aq) + A- (aq)

Weak Acids (HA) and Acid Ionization Constants

HA (aq) H+ (aq) + A- (aq)

Ka =[H+][A-][HA]

Ka is the acid ionization constant

Ka

weak acidstrength

16.5

NH3 (aq) + H2O (l) NH4+ (aq) + OH- (aq)

Weak Bases and Base Ionization Constants

Kb =[NH4

+][OH-][NH3]

Kb is the base ionization constant

Kb

weak basestrength

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) + H2O (l) OH- (aq) + HA (aq)

Ka

Kb

H2O (l) H+ (aq) + OH- (aq) Kw

KaKb = Kw

For a Weak Acid and Its Conjugate Base, Ka and Kb are related by Kw,

Ka = Kw

Kb

Kb = Kw

Ka

Polyprotic Acids

• Have more than one acidic proton.

• If the difference between the Ka for the first dissociation and subsequent Ka values is 103 or more, the pH generally depends only on the first dissociation.

16.8

• Note that when Ka decreases for each subsequent proton dissociation, Kb increases

• Remember KaKb 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 H Z 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

HClO3 > HBrO3

16.9

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.

HClO4 > HClO3 > HClO2 > HClO

16.9

Acid-Base Properties of Salts

16.10

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 SaltsNeutral Solutions:

Salts containing an alkali metal or alkaline earth metal ion (except Be2+) and the conjugate base of a strong acid (e.g. Cl-, Br-, and NO3

-).

NaCl (s) Na+ (aq) + Cl- (aq)H2O

Basic Solutions:

Salts derived from a strong base and a weak acid.

CH3COONa (s) Na+ (aq) + CH3COO- (aq)H2O

CH3COO- (aq) + H2O (l) CH3COOH (aq) + OH- (aq)

16.10

Acid-Base Properties of Salts

Acid Solutions:

Salts derived from a strong acid and a weak base.

NH4Cl (s) NH4+ (aq) + Cl- (aq)

H2O

NH4+ (aq) NH3 (aq) + H+ (aq)

Salts with small, highly charged metal cations (e.g. Al3+, Cr3+, and Be2+) and the conjugate base of a strong acid.

Al(H2O)6 (aq) Al(OH)(H2O)5 (aq) + H+ (aq)3+ 2+

16.10

Acid Hydrolysis of Al3+

16.10

Acid-Base Properties of Salts

Solutions in which both the cation and the anion hydrolyze:

• Kb for the anion > Ka for the cation, solution will be basic

• Kb for the anion < Ka for the cation, solution will be acidic

• Kb for the anion Ka for the cation, solution will be neutral

16.10