Updates Assignment 04 is is due today (in class) Midterms marked (in the box); solutions are posted...
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Transcript of Updates Assignment 04 is is due today (in class) Midterms marked (in the box); solutions are posted...
Updates
• Assignment 04 is is due today (in class)
• Midterms marked (in the box); solutions are posted
• Assignment 03 is in the box
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+].
16.7
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