Arrhenius Acid is a substance that, when dissolved in water, increases the concentration of hydrogen ion, H + or ( hydronium ion H
3
O + ).
Arrhenius Base is a substance that, when dissolved in water, increases the concentration of hydroxide ions, OH .

A. Definitions
1. Bronsted Acid: H + (proton) donor
Ionizable hydrogen in the acid structure is usually bonded to an electronegative atom.
2. Bronsted Base: H + (proton) acceptor
Base structure must have an unshared pair of electrons.

B. Broader Definition For Acid-Base Reactions
Consider: HCl + NH
4
OH

NH
4
Cl + H
2
O
Consider: HF + HCO
3

H
2
CO
3
+ F -

C. Amphiprotic Compounds
Some compounds can act as a Bronsted acid in one case and a Bronsted base in another case. It depends on the molecules the amphiprotic compound is reacting with.
Example: Water
HCl + H
2
O

H
3
O + + Cl -
NH
3
+ H
2
O

NH
4
+ + OH -

D. Conjugate Acid-Base Pairs
A conjugate acid-base pair consists of two species in an acid-base reaction, one acid and one base, that differ by the loss or gain of a proton.
Conjugate acidspecies formed when a base has accepted a proton.
Conjugate basespecies formed when an acid has donated or removed a proton.
Example: Acetic Acid
CH
3
COOH
⇌
CH
3
COO + H +
CH
3
COOH + H
2
O
⇌
CH
3
COO + H
3
O +

Example Problems
1. What is the conjugate base of HNO
3
?
2. What is the conjugate acid of NH
2
?
3. Label all species in the following reaction.
H
2
PO
4
+ HCO
3
-
⇌
H
2
CO
3
+ HPO
4
2-

E. Strong vs. Weak Acids and Bases
1. Strong Acids and Bases
Essentially go to 100% ionization
Rxn normally shown with single headed arrow.
Strong Acids
HCl
HBr
HI
HNO
3
H
2
SO
4
HClO
4
HNO
3
→ H + + NO
3
-
Strong Bases
LiOH
NaOH
KOH
Ca(OH)
2
Ba(OH)
2
Sr(OH)
2

2. Weak Acids and Bases
Partially dissociate or ionize, reaching some dynamic equilibrium state.
Rxn normally shown with double headed arrow.
HCN ⇌ H + + CN -
Weak acids and bases significant in biological systems.

F. Examples of Organic Weak Acids and Bases
Carboxylic Acids: (weak acids)
RCO
2
H or RCOOH
Anion formed by ionization is resonance stabilized
Examples: pyruvic acid, lactic acid, acetic acid
Amines: (weak bases)
Based on ammonia, with differing # of “R” groups
Primary, secondary, tertiary

G. Polyprotic Acids and Bases
Polyprotic acids can donate more than one proton sequentially.
H
2
SO
4
H
3
PO
4
H
2
S
Polyprotic bases can accept more than one proton sequentially .
SO
4
2-
PO
4
3-
S 2-

Pure water slightly ionizes on its own (autoionizes) with equilibrium reactant favored.
H
2
O(l) + H
2
O(l)
⇌
H
3
O + (aq) + OH (aq)
Determine the K c value.

K c
w
K w
= ionization constant for water or
= ion product constant for water
K w
= [H + ] [OH ] = 1 x 10 -14 at 25 o C
What are [H + ] and [OH ] for pure water at 25 o C?

True for Water and Dilute Aqueous Solutions
(Including Acid and Base Solutions of Interest)
[H + ] [OH ] = 1 x 10 -14
[H
3
O + ] [OH ] = 1 x 10 -14
Neutral Solution [H + ] = [OH ]
Acidic Solution [H + ] > [OH ]
Basic Solution [H + ] < [OH ] or [H
3
O + ]

In most aqueous weak acid and base solutions, the H + concentrations are very small. Easier to express concentrations using a logarithmic relationship. p = - log
+
-

[H + ] [OH ] = 1 x 10 -14
(-log [H + ]) + (-log [OH ]) = -log (1 x 10 -14 ) pH + pOH = 14 pH + pOH = 14

Since pH is a logarithmic scale, cola drinks (pH about 2.5) are about ____ times as acidic as tomatoes (pH about 4.5)

**** [H + ] is the same as [H
3
O + ]
1) pH = -log [H + ]
2) [H + ] = 10 -pH or antilog (-pH) = [H + ]
3) pOH = -log [OH ]
4) [OH ] = 10 -pOH or antilog (-pOH ) = [OH ]
5) [H + ] [OH ] = 1 x 10 -14
6) pH + pOH = 14

Problem Solving:
1) A blood sample (considered a dilute aqueous solution) contains 7.2 x 10 -8 mol H + per liter.
a. What is the pH of the blood sample?
b. What is the [OH ] of the blood sample?
c. Is the blood sample acidic, basic, or neutral?
2) The pH of an aqueous solution is 8.7
a. What is the [H + ]?
b. What is the pOH?

Strong acids and bases undergo essentially complete dissociation (ionization).
Equilibrium expressions not needed for problem solving.
Example: What is the pH and [H + ] for a
0.05 M HCl solution?

Problem Solving for Weak Acids and Bases
1. They only dissociate or ionize partially in solution.
2. This means we have an equilibrium situation, and can solve problems using the techniques we learned earlier for dealing with equilibrium problems. (ICE tables…)
3. Equilibrium Constants (K c
)
K a for weak acids
K b for weak bases

1. Definition
Give the acid ionization constant expression for the acid: HA(aq) + H
2
O(l)
⇌
H
3
O + (aq) + A (aq) or HA(aq)
⇌
H + (aq) + A (aq)
K a
= ?

2. What does the K a tell you?
Larger K a
 stronger acid
(more of the acid ionizes in water to form H +)
See Table 15.5, for examples
Which is the strongest acid?
Acetic acid K a
= 1.8 x 10 -5
Boric Acid K a
= 7.3 x 10 -10

a
A 0.10 M solution of phenol (weak acid) has a pH of 5.43. Calculate the K a for phenol.

a
The weak monoprotic acid, parahydroxybenzoic acid, has a K a of 2.6 x 10
Answer the following questions for a 0.200 M
-5 . solution of parahydroxybenzoic acid.
1. What is the pH of the solution?
2. What percent of the parahydroxybenzoic acid has ionized in this solution?

Example: H
3
PO
4
K a
’s = 7.5 x 10 -3 , 6.2 x 10 -8, 3.6 x 10 -13
1 . Show the three acid ionization equations with their accompanying K a values.
2. Which is the strongest acid?
3. How would you go about calculating the pH of a given H
3
PO
4 solution?

(base will accept proton from H
2
O, forming OH )
1. Definition
Give the base ionization constant expression for the base: B(aq) + H
2
O(l)
⇌
BH + (aq) + OH (aq)
K b
= ?

2. Relative Strength of Weak Bases a. The K b for ammonia is 1.8 x 10 -5 .
The K b for phosphate ion is 2.8 x 10 -2 .
Which is the strongest base?
b. Which will have a higher pH: a 1.0 M ammonia solution, or a 1.0 M phosphate ion solution

b
The pain killer, morphine, is a weak base.
A 0.01 M morphine solution has a pH of
10.1. Calculate the K b for morphine.

b
The weak base methylamine (CH
3
NH
2
) has a K b of 5.0 x 10 -4 . Answer the following questions for a 0.080 M aqueous solution of methylamine.
a. Write the chemical equation.
b. Write the base ionization equilibrium expression.
c. Calculate the [OH ], pOH, and pH for solution.

a
b
A. Conjugate Acid and Base Reaction May Be
Written in Both Directions. For Example:
HA + H
2
O
⇌
H
3
O + + A K a
=
A + H
2
O
⇌
HA + OH K b
=
** Reaction will go in direction of stronger acid and base to the weaker acid and base.
** Compare K a and K b to decide direction.

B. Mathematical Relationship of K a and K b
(for previous equations)
K a

K b

H O

  
 

 
A

 
 
H O
3
  
OH
 
K a x K b
= [H + ] [OH ] = K w
K a x K b
= 1 x 10 -14
pK a
+ pK b
= 14

Consider the dihydrogen phosphate ion,
H
2
PO
4
1. K a
= 6.2 x 10 -8 .
1. What is its conjugate base?
2. Show the chemical reaction to which K a applies for H
2
PO
4
1-
4. Determine the K b value for HPO
4
2.
5. Which direction is the reaction favored?

A. Salts
1. Ionic compounds (salts) formed from acid-base reaction.
HA + MOH

M + A + H
2
O acid base salt
2. The salt (MA) could leave the resulting aqueous solution to be neutral, acidic, or basic .

B. Hydrolysis of Salts
Salts may react with water ( hydrolysis reaction involving the splitting of a water molecule ) to produce acidic or basic solutions.
For hypothetical ionic compound MA, consisting of
M x+ cation and A xanion, consider possible hydrolysis reactions and possible consequences:
M x+ + H
2
O  M(OH) (x-1)+ + H + acidic solution
A x+ H
2
O

HA (x+1)+ OH basic solution
If both cation and anion hydrolyze, compare K a and K b

C. Predicting If Hydrolysis Occurs
1. Write the hypothetical hydrolysis rxns for salt.
(one for cation and one for anion)
2. Look at the hypothetical hydrolysis products.
3. If a strong acid or strong base was hypothetically produced, these hydrolysis reactions DO NOT occur. No acid or base properties upon hydrolysis. Recall Table of Strong Acids and Bases
4. If a weak acid or base was produced, these hydrolysis reactions DO occur and produce acid or base properties. For hydrolysis products:
H + : acidic OH : Basic

D. Example Problems
Will an aqueous solution of the salt be acidic, basic, or neutral?
1) NaCl
2) NH
4
Cl
3) NH
4
CN

The cyanate ion, OCN , is a weak base. The K a for HOCN (cyanic acid) is 3.4 x 10 -8 . Calculate the pH of a 0.200 M solution of NaOCN.

A. Some Acid-Base Reactions Do Not Fit
Either Arrehenius or Bronsted –Lowry
Definitions.
Consider:
HCl + NaOH

NaCl + H
2
O
Al 3+ + 6 H
2
O

Al (H
2
O)
6
3+

B. Lewis Acids and Bases
1. Lewis Acid: electron pair acceptor
(Accepts a pair of electrons to form a new bond)
** Usually has an incomplete octet of electrons
2. Lewis Base: electron pair donor
(Donates a pair of electrons to form a new bond)
** Must have at least one lone pair of electrons
C. Identify as Lewis acid or Lewis base?
H
2
O Cr 3+ BF
3

What is acid rain and what are its environmental effects?