Updates
• Assignment 05 is is due Monday, Mar. 05
(in class)
• Midterm 2 is Thurs., March 15
– Huggins 10, 7-8pm
– For conflicts: ELL 221, 6-7pm (must arrange at
least one week in advance)
Acids and Bases
Chapter 16
Binary acids (HX, H2X, H3X, H4X)
•
Bond strength determines acidity within the same group (column), size
•
Bond polarity determines acidity within the same period (row),
electronegativity
Rationalizing acidity
•
Two considerations
–
–
•
Stable
–
–
–
–
–
–
•
“Unhappy”
High energy
High number of charges
High charge density on atom(s)
Weak bonds
A highly favored reaction: unstable reactants forming stable products
–
–
•
“Happy”
Low energy
Low number of charges
Low charge density on atom(s)
Charge distributed over multiple atoms
Strong bonds
Unstable
–
–
–
–
–
•
Stability of reactants
Stability of products
Strong acid: unstable acid leads to stable conjugate base
Similarly, strong base: unstable base leads to stable conjugate acid
Weak acids and bases: intermediate in stability
Rationalizing acidity
•
Two considerations
–
–
•
Stable
–
–
–
–
–
–
•
“Unhappy”
High energy
High number of charges
High charge density on atom(s)
Weak bonds
A highly favored reaction: unstable reactants forming stable products
–
–
•
“Happy”
Low energy
Low number of charges
Low charge density on atom(s)
Charge distributed over multiple atoms
Strong bonds
Unstable
–
–
–
–
–
•
Stability of reactants
Stability of products
Strong acid: unstable acid leads to stable conjugate base
Similarly, strong base: unstable base leads to stable conjugate acid
Weak acids and bases: intermediate in stability
Oxyacids
Central atoms derived from same group (same oxidation state)
•
More electronegative central atom polarizes the OH bond more,
facilitating ionization
•
More electronegative central atom better able to stablize resulting
negative charge following ionization, making a happier (more stable)
conjugate base
Oxyacids
Identical central atoms with different oxidation states
•
Acidity increases as oxidation number of central atom increases
•
Said another way: for oxyacids with the same central atom, acidity increases as
the number of oxygens attached increases
•
Additional electronegative oxygen atoms pull electron density from the O-H
bond, further increasing its polarity, facilitating ionization
•
Increasing the number of oxygens also helps stabilize the conjugate base by
increasing its ability to “spread out” its negative charge (a happier conjugate
base)
Carboxylic acids
• Portion in blue known as carboxyl group and is often written as
–COOH
• Acids that contain a carboxyl group are called carboxylic acids,
and they form the largest category of organic acids (organic
referring to carbon-containing compounds, based on C-C
bonds)
Carboxylic acids
•
Acetic acid is a weak acid (Ka = 1.8 x 10-5),
whereas methanol is not an acid in water
•
Additional oxygen increases polarity of O-H bond
and helps to stabilize the conjugate base
•
The conjugate base (called a carboxylate anion)
can exhibit resonance (p. 284), which contributes
further to the stability of the anion by spreading the
negative charge over several atoms
•
When the three hydrogens are replaced with
fluorines, the acidity increases (Ka = 5.0 x 10-1)
Acid-base properties of salt solutions
•
Salt solutions can be neutral, acidic, or basic, owing to the reaction of a
cation or anion (or both) with water
•
These three solutions contain the acid-base indicator bromthymol blue.
(a) The NaCl solution is neutral (pH = 7.0)
(b) The NH4Cl solution is acidic (pH = 3.5)
(c) The NaClO solution is basic (pH = 9.5)
Acid-base properties of salt solutions
•
Anions
–
–
•
Conjugate base of strong acid DOES NOT react with water (Cl-)
Conjugate base of weak acid reacts with water (CH3COO-)
Cations
–
–
–
Conjugate acid of weak base reacts with water (NH4+)
Most metals can react with water (Al3+, Cr3+, Fe3+, Bi3+, Be2+)
Ions of alkali metals and heavier alkaline earth metals DO NOT react with water (Na+)
Acid-base properties of salt solutions
(a) The NaCl solution is neutral (pH = 7.0)
(b) The NH4Cl solution is acidic (pH = 3.5)
(c) The NaClO solution is basic (pH = 9.5)
•
Anions
–
–
•
Conjugate base of strong acid DOES NOT react with water (Cl-)
Conjugate base of weak acid reacts with water (acetate)
Cations
–
–
–
Conjugate acid of weak base reacts with water (NH4+)
Most metals can react with water (Al3+, Cr3+, Fe3+, Bi3+, Be2+)
Ions of alkali metals and heavier alkaline earth metals DO NOT react with water (Na+)
Acid-base properties of salt solutions
(a) The NaCl solution is neutral (pH = 7.0)
(b) The NH4Cl solution is acidic (pH = 3.5)
(c) The NaClO solution is basic (pH = 9.5)
• Examples where either the cation or the
anion of the salt reacts -but not both!
Combined effect of cation and anion in solution
•
If salt solution contains an anion that reacts with water, we expect
solution to be basic
•
If salt solution contains a cation that reacts with water, we expect
solution to be acidic
•
What happens when salt is made from a cation and anion that both
react with water?
– Whether solution is basic, neutral, or acidic depends on the relative abilities
of the ions to react with water
•
When a solution contains both the conjugate base of a weak acid and
the conjugate acid of a weak base, the ion with the larger equilibrium
constant, Ka or Kb, will have the greater influence on the pH
•
Stated another way: When a solution contains a reactive anion and a
reactive cation (toward water), the ion with the larger equilibrium
constant, Ka or Kb, will have the greater influence on the pH
Combined effect of cation and anion in solution
• When a solution contains both the conjugate
base of a weak acid and the conjugate acid of
a weak base, the ion with the larger
equilibrium constant, Ka or Kb, will have the
greater influence on the pH
• Take NH4F as an example
–
–
–
–
Both NH4+ and F- will react with water
Ka for NH4+ is 5.6 x 10-10
Kb for F- is 1.4 x 10-11
Therefore, Ka is larger, and solution will be acidic
Anions that act as both an acid and a base
HCO3-(aq) + H2O
H3O+ (aq) + CO32- (aq)
Ka = 4.8 x 10-11
HCO3-(aq)
H2CO3 (aq) + OH- (aq)
Kb = 2.4 x 10-8
+ H2O
• Bicarbonate ion can ionize (rxn 1) or undergo
hydrolysis (rxn 2)
• Ionization involves bicarbonate acting as an acid;
hydrolysis involves bicarbonate acting as a base
• Because Kb is larger, we predict that hydrolysis will
predominate, yielding a basic solution
Acid-Base Properties of Salts
16.10
Acidic, Basic and Amphoteric Oxides, p. 264
(shown in highest oxidation states)
CO2 (g) + H2O (l)
H2CO3 (aq)
N2O5 (g) + H2O (l)
2HNO3 (aq)
16.11
Lewis Acids
• Lewis acids are defined as electron-pair
acceptors.
• Atoms with an empty valence orbital can be Lewis
acids.
Lewis Bases
• Lewis bases are defined as electron-pair donors.
• Anything that could be a Brønsted–Lowry base is
a Lewis base.
• Lewis bases can interact with things other than
protons, however (broadest definition).
Chemistry In Action: Antacids and the Stomach pH Balance
NaHCO3 (aq) + HCl (aq)
NaCl (aq) + H2O (l) + CO2 (g)
Mg(OH)2 (s) + 2HCl (aq)
MgCl2 (aq) + 2H2O (l)