1. Acids and Bases

2. Acids, Bases and Equilibrium
When an acid is dissolved in water, the H+ ion (proton) produced by the acid combines with water to produce the hydronium ion, H3O+
HCl and other strong electrolytes ionize completely in water
Weak acids like acetic acid ionize only to a very small extent in water

3. Acids, Bases and Equilibrium
The equilibrium concept is used to describe to what extent an acid or base ionizes in water
Ionization constants: K>1 indicates a strong acid or base, K<1 refers to a weak acid or base

4. Strong Electrolytes
Strong Acids – HCl, HBr, HI, HNO3, HClO4, H2SO4 (for first H+ only)
Strong Bases – LiOH, NaOH, KOH, RbOH, CsOH, Sr(OH)2, Ba(OH)2
All others are probably weak

5. Arrhenius Acids and Bases
An acid is a substance that, when dissolved in water, increases the concentration of hydrogen ions, H+, in the water
A base is a substance that, when dissolved in water increases the concentration of hydroxide ion, OH-, in the water

6. Bronsted-Lowry Acids and Bases
An acid is any substance that can donate a proton to any other substance
Examples of Bronsted acids – molecular compounds (HNO3), cations (NH4+), hydrated metals (Fe(H2O)63+), or anions (H2PO4-)
Theory is not restricted to compounds in water

7. Bronsted-Lowry Acids and Bases
A Bronsted base is a substance that can accept a proton from any other substance
Bronsted bases can be molecular compound (NH3), anions (CO32-), or cations (Al(H2O)5(OH)2+)

8. Polyprotic Acids + Bases
Acid Form
Amphiprotic Form
Base Form
H2S
HS-
S2-
H3PO4
H2PO4-
HPO42-
PO43-
H2CO3
HCO3-
CO32-
H2C2O4
HC2O4-
C2O42-

9. Conjugate Acid-Base Pairs
A pair of compounds or ions that differ by the presence of one H+ ion is called a conjugate acid-base pair
Every reaction between a Bronsted acid and Bronsted base involves H+ transfer and has two conjugate acid-base pairs

19. Conjugate Acid-Base Pairs
HCl
+ H2O 
Cl-
+ H3O+
HCO3- 
CO32-
CH3CO2H
CH3CO2-
H2O
+ NH3
OH-
+ NH4+
+ CO32-
+ HCO3-

20. Water and the pH scale
Water autoionizes to a small extent, producing low concentrations of H3O+ and OH- ions (water conducts electricity)
The equilibrium for autoionization of water lies far to the left
At 25oC, Kw=1.0 x (the ionization constant for water)

21. Water and the pH scale
Kw increases with temperature because the autoionization of water is endothermic
Kw is valid in pure water and any aqueous solution
In pure water and dilute aqueous solutions, the concentration of water is considered to be constant at 55.5 M

22. Water and the pH scale
[H3O+] = [OH-] = 1.0 x 10-7 M in pure water, a neutral solution
In acidic solution, [H3O+]>[OH-]
In basic solution, [H3O+]<[OH-]

23. Water and the pH scale pH = -log[H3O+] pOH = -log[OH-]
pKw = pH + pOH = 14.00

24. Relationship between hydronium and hydroxide ion concentrations, pH and pOH

25. Equilibrium Constants for Acid and Bases
The strength of acids and bases of the same concentration can be compared by measuring pH
The relative strength of an acid can be expressed with an equilibrium constant
Ka = [H3O+][A-]/[HA]

26. Equilibrium Constants for Acid and Bases
Kb = [BH+][OH-]/[B]
Weak acid, Ka<1, pH>2, small [H3O+]
Weak base, Kb<1, pH<12, small [OH-]
A large value of K indicates ionization products are strongly favored; small K value indicates reactants are favored

27. Equilibrium Constants for Acid and Bases
Weak acids have strong conjugate bases; small Ka corresponds with large Kb
Consider the acids and conjugate bases in table 15.2 on page 668
Notice trends: as acid strength declines in a series, the relative conjugate base strength increases

28. Equilibrium Constants for Acid and Bases
Use Tables 15.3 on page 671, 15.4 on page 679, and 15.5 on page 683
Which is the stronger acid, H2SO4 or H2SO3?
Is benzoic acid stronger or weaker than acetic acid?
Which has the stronger conjugate base, acetic acid or formic acid?
Which is the stronger base, ammonia or methylamine?
Which has the stronger conjugate acid, ammonia or methylamine?

29. Calculations with Equilibrium Constants
The principles of the equilibria can be applied to aqueous solutions of weak acids and bases. The equilbrium constants Ka and Kb can be determined if the concentrations of the various species present in the solution are known. These are often determined by measuring pH.

30. Calculations with Equilibrium Constants
If the acid or base is weak, and the initial concentration of acid (or base) is at least 100x Ka (or Kb), then the approximation that
[acid]initial = [acid]equilibrium is valid. Otherwise the quadratic equation must be solved.

31. Calculations with Equilibrium Constants
(ex) For the weak acid:
HA (aq) + H2O (l)  H3O+ (aq) + A- (aq) Ka = [H3O+ ][A-]/[HA] and because [H3O+ ] = [A-], Ka = [H3O+ ]2/[HA]
The assumption that [HA]equil = [HA]initial is valid if [HA]initial > 100 x Ka

32. Calculations with Equilibrium Constants
Calculating a Ka value from a measured pH
A solution prepared from mol butanoic acid dissolved in sufficient water to give 1.0 L of solution has a pH of Determine Ka for butanoic acid. The acid ionizes according to the balanced equation:
CH3CH2CH2CO2H + H2O  H3O+ + H3CH2CH2CO2-

33. CH3CH2CH2CO2H + H2O  H3O+ + H3CH2CH2CO2-
Calculate initial molarity of butanoic acid
Calculate equilibrium molarity of hydronium ion from pH
Construct ice table
Write equilibrium constant expression and substitute in values from ice table
Calculate Ka

34. Calculating Equilibrium Concentrations and pH from Ka
What are the equilibrium concentrations of acetic acid, acetate ion, and hydronium ion for a 0.10 M solution of acetic acid (Ka = 1.8 x 10-5)? What is the pH of the solution?
Write chemical equation for ionization of acetic acid in water
ICE it up
Write equilibrium constant expression, substitute values from ice table, solve for x
Use x to find equilibrium concentrations
Use concentration of hydronium ion to solve for pH

35. Calculating the pH of a salt solution
What is the pH of a M solution of sodium acetate?
Write equation for ionization reaction of acetate ion in water (it is a base!)
Put it on ICE
Write equilibrium constant expression and substitute ICE values
Solve for x, solve for concentration of hydroxide ion
Solve for concentration of hydronium ion, solve for pH

36. Calculating the pH after the reaction of an acid with a base
Calculate the pH after mixing 15 mL of 0.12 M acetic acid with 15 mL of 0.12 M NaOH. What are the major species in solution at equilibrium (besides water) and what are their concentrations?
Write balanced equations
Stoichiometry problem to solve for “initial” concentration of acetate anion
ICE, equilibrium constant express Kb, pOH and such

37. Polyprotic acids and bases
The pH of many inorganic polyprotic acids depends primarily on the hydronium ion generated in the first ionization step
Each successive loss of a proton is about more difficult than the previous step
The hydronium ion produced in the second step can be neglected; calculate using the k of the first ionization

38. Molecular Structure, Bonding, and Acid Strength
In a series of acids, as bond strength decreases, acid strength increases
Adjacent electronegative atoms that pull electrons from a hydrogen increase the strength of an acid (inductive effect)
Acids that have resonance structures are stronger because they are more stable after they lose a proton than acids without resonance

39. Lewis Acids and Bases
A Lewis acid is a substance that can accept a pair of electrons from another atom to form a new bond
A Lewis base is a substance that can donate a pair of electrons to another atom to form a new bond
This is also know as coordinate covalent chemistry

40. Lewis Acids and Bases A + B:  B:A acid + base  acid-base adduct
The acid must have an empty orbital available or be able to make one available
A base must have a pair of nonbonding electrons
The formation of hydronium ion is an example of this type of reaction

41. Lewis Acids and Bases
Good Lewis bases include hydroxide ion and ammonia and water
Metal cations are good Lewis acids; transition metals form complexes known as coordination complexes
Nonmetal oxides such as carbon dioxide are Lewis acids
Some metal hydroxides are amphoteric – acting like an acid in the presence of a base and a base in the presence of an acid (ex. Al(OH)3)

42. Acid and Base Properties of Some Ions in Aqueous Sol’n
Neutral
Basic
Acidic
Anions
Cl- NO3-
Br- ClO4- I-
CH3CO2- CN- OCl-
SO HPO42-
HCO2- PO43- NO2-
CO32- HCO3- F-
S2- HS- SO32-
HSO4-
H2PO4-
HSO3-
Cations
Li+
Na+ Ca2+
K+ Ba2+
Al(H2O)5(OH)2+ and analogous ions
Al(H2O)63+ and hydrated transition metal cations
NH4+

43. Acid and Base Properties of Some Ions in Aqueous Sol’n
Anions that are conjugate bases of strong acids are such weak bases that they have no effect on pH
There are numerous basic anions that are conjugate bases of weak acids
Acidic anions arise from polyprotic acids

44. Acid and Base Properties of Some Ions in Aqueous Sol’n
Alkali metal and alkaline earth metal cations have no effect on pH (conj. acid of strong base)
All metal cations are hydrated in water, M(H2O)6n+, but when M is +2 or +3 and particularly a transition metal, the ion acts as an acid

45. Aqueous Solutions of Salts
Cation
Anion
pH of Solution
From strong base (Na+)
From strong acid (Cl-)
=7 neutral
From strong base (K+)
From weak acid (CH3CO2-)
>7 basic
From weak base (NH4+)
<7 acidic
From any weak base (BH+)
From any weak acid (A-)
Depends on relative strengths of acid and base

46. For each of the following salts in water, predict whether the pH will be greater than, less than, or equal to 7
KBr
NH4NO3
AlCl3
Na2HPO4

47. pKa = -log Ka
A logarithmic scale is used to report and compare acid strengths
The pKa value becomes smaller as the acid strength increases
KaKb = Kw for an acid and its conjugate base
Also, pKw = pKa + pKb

48. Equilibrium Constants and Acid-Base Reactions
All proton transfer reactions proceed from the stronger acid and base to the weaker acid and base.
Write the net ionic equation for the possible reaction between acetic acid and sodium hydrogen sulfate, NaHSO4. Does the equilibrium lie to the left or right?

49. Types of Acid-Base Reactions
Reaction of a Strong acid with a Strong Base
Net ionic equation:
H3O+ + OH-  2H2O
K = 1/Kw = 1.0 x 1014
Mixing equal molar quantities of a strong base with a strong acid produces a neutral solution (pH=7, at 25o )

50. Types of Acid-Base Reactions
Unless both the acid and base involved in the neutralization reaction are strong, the pH of the solution that results will not be neutral.

51. Types of Acid-Base Reactions
Reaction of a Weak Acid with a Strong Base
(ex.) net ionic equation:
CH3CO2H + OH-  H2O + CH3CO2-
Adding these two reactions give the net ionic equation above;
CH3CO2H + H2O  H3O+ + CH3CO2- Ka=1.8x10-5
H3O+ + OH-  2 H2O Ka=1/Kw=1.0x1014
The K for the net reaction is the product of the two equilibrium constants Kneut=Ka x 1/Kw=1.8x109

52. Types of Acid-Base Reactions
Mixing equal molar quantities of a strong base with a weak acid produces a salt whose anion is the conjugate base of the weak acid. The solution is basic, with the pH depending on Kb for the anion.
In the example above, the salt that results from the reaction is acetate, the conjugate base of a weak acid. Therefore the solution will be basic at the equivalence point.

53. Types of Acid-Base Reactions
Reaction of a Strong Acid with a Weak Base
(ex.) net ionic equation:
H3O+ + NH3  NH4+ + H2O
Adding these two reactions give the net ionic equation above:
NH3 + H2O  NH4 + OH- Kb = 1.8x10-5
H3O+ + OH-  2 H2O K = 1/Kw = 1.0x1014
The K for the net reaction is the product of the two equilibrium constants K = Kb x 1/Kw = 1.8x109

54. Types of Acid-Base Reactions
Mixing equal molar quantities of a strong acid and a weak base produces a salt whose cation is the conjugate acid of the weak base. The solution is acidic, with the pH depending on the Ka for the cation.
In the above example, the solution at the equivalence point contains the ammonium ion, the conjugate acid of a weak base, and the solution is acidic.

55. Types of Acid-Base Reactions
Reaction of a Weak Acid with a Weak Base
(ex.) net ionic equation:
CH3CO2H + NH3  NH4+ + CH3CO2-
The reaction is product-favored because acetic acid is stronger than ammonium ion and ammonia is a stronger base than acetate ion (use table)
K = Ka x Kb / Kw (what is the K for this ex?)

56. Types of Acid-Base Reactions
When a weak acid reacts with a weak base, the pH of the solution at the equivalence point depends upon which is the stronger, the acid or the base
If equal molar solutions are mixed the resulting solution contains ammonium acetate; is it acidic or basic? (use table)

57. Types of Acid-Base Reactions
Mixing equal molar quantities of a weak acid and a weak base produces a salt whose cation is the conjugate acid of the weak base and whose anion is the conjugate base of the weak acid. The solution pH depends on the relative Ka and Kb values.