1. Unit #5
Acids and Bases

2. Properties of Acids:
Give foods a tart or sour taste.
(Ex. Lemons, Limes, Vinegar)
2. When in solution, they are electrolytes. (Conduct electricity)
3. React with indicators to change color. (Litmus (in acid turns from blue to red)

3. Properties of Acids (con’t):
4. React with many metals to form Hydrogen gas and the salt of the metal.
5. React with bases to form a salt and water.
Salt -
A compound composed of the cations of an aqueous base and the anions of an aqueous acid.

4. Properties of Bases:
Give foods a bitter taste. (Ex. Bile,
Baking soda)
2. When in solution, they are electrolytes. (Conduct electricity)
3. React with indicators to change color. (Litmus (in base turns from red to blue)

5. Properties of Bases (con’t):
4. React with acids to form a salt and water.
5. Feel slippery (Like soap water)

6. Three Types of Acids / Bases
1. “Arrhenius” concept of acids / bases.
2. “Bronsted - Lowery” concept of acids / bases.
3. “Lewis” concept of acids / bases.

7. Arrhenius concept -
Presented in 1883, however it was not universally accepted until 1903.
Acid -
A substance that, when dissolved in water, increases the concentration of the Hydrogen ion.
A substance that, when dissolved in water, increases the concentration of the Hydroxide ion.
Base -

8. Notice:
The H+ ion is not a “bare proton,” but a proton chemically bonded to water
H2O + H+
H3O+

9. Limitations of Arrhenius concept:
1. All acids and bases must be in aqueous solution.
2. All bases must be hydroxides (OH-).

10. Strength of Arrhenius Acids / Bases.
Strong acid -
A substance that completely ionizes in aqueous solution to give the hydronium ion (H3O+) and an anion.

11. Examples of strong Arrhenius acids:
HNO3
HCl
HBr HI
H2SO4
HClO4 (Perchloric acid)

12. What happens when the previous acids are added to water ?
HCl + H2O
H3O+ + Cl-

13. Strength of Arrhenius Acids / Bases (con’t).
Strong base -
A substance that completely ionizes in aqueous solution to give the hydroxide ion (OH-) and a cation.

14. Examples of strong Arrhenius bases:
The hydroxides of the group IA elements.

15. What happens when the previous bases are added to water ?
H2O
NaOH
Na+ + OH-

16. Strength of Arrhenius Acids / Bases (con’t).
Weak acid and bases -
Substances that do not completely ionizes in aqueous solution and exist in reversible reactions with the corresponding ion.

17. Examples of weak acids:
H3PO4
CH3COOH (aka: HC2H3O2)
H2CO3
H2S
HCN
Ethanoic or acidic acid

18. What happens when the previous acids are added to water ?
CH3COOH + H2O
CH3COO- + H3O+

19. Examples of weak bases:
NH4OH
Ca(OH)2
Mg(OH)2
“Milk of Magnesia”

20. What happens when the previous bases are added to water ?
H2O
Ca(OH)2
Ca OH-

21. Once again:
Limitations of the Arrhenius concept:
1. All species are aqueous.
2. All bases are hydroxides.
These limitations prompted others to find a “Broader” explanations for the acid / base concept.

22. Bronsted - Lowery concept -
Presented in 1923, 39 years after the Arrhenius concept, 20 years after its (Arrhenius) formal acceptance.
A substance that donates Hydrogen ions.
Acid -
A substance that accepts Hydrogen ions.
Base -

23. Miscellaneous components of this concept:
1. All Arrhenius acids / bases are still acids / bases with this definition.
2. Acids / bases can be ions as well as molecular substances.

24. Miscellaneous components of this concept (con’t):
3. Acid / base reactions are not restricted to aqueous solutions.
HCl(g) + NH3(g)
NH4Cl(s)
4. Some species may be either an acid or a base (amphoteric) depending on what the other reactant is.

25. Makes the sol’n basic.
Example #1:
NH3 + H2O
NH OH-
acid
Which reactant has accepted the hydrogen ion ?
Which reactant has donated the hydrogen ion ?
Which reactant is the Bronsted - Lowery acid ?
Which reactant is the Bronsted - Lowery base ?

26. Makes the sol’n acid.
Example #2:
HCl + H2O
H3O+ + Cl-
acid
Which reactant has accepted the hydrogen ion ?
Which reactant has donated the hydrogen ion ?
Which reactant is the Bronsted - Lowery acid ?
Which reactant is the Bronsted - Lowery base ?

27. Amphoteric - A substance that can act as both, an acid or a base.
A.K.A. Amphiprotic species

28. CO3-2 + H2O HCO3- + OH- Example of an amphiprotic species: HCO3- Acid
Which reactant has accepted the hydrogen ion ?
Which reactant has donated the hydrogen ion ?
Which reactant is the Bronsted - Lowery acid ?
Which reactant is the Bronsted - Lowery base ?

29. H2CO3 + F- HCO3- + HF Example of an amphiprotic species: HCO3- base
Which reactant has accepted the hydrogen ion ?
Which reactant has donated the hydrogen ion ?
Which reactant is the Bronsted - Lowery acid ?
Which reactant is the Bronsted - Lowery base ?

30. Conjugate acid - H2CO3 + F- HCO3- + HF
A substance formed when a base gains a hydrogen ion.
base
H2CO3 + F-
HCO HF
Conjugate acid
What is the conjugate acid?

31. Conjugate base - H2CO3 + F- HCO3- + HF
A substance that remains when an acid has donated a hydrogen ion.
acid
H2CO3 + F-
HCO HF
Conjugate base
What is the conjugate base?

32. Conjugate base Acid Give the conjugate base for each acid below. HBr
NH4+
HCO3-
HC2H3O2
H2SO4

33. Conjugate acid Base Give the conjugate acid for each base below. CN-
H2O
HCO3-
SO4-2
H2PO4-

34. Label each as acid or base, and conjugate acid or base.
HCO HF
H2CO3 + F-

36. Label each as acid or base, and conjugate acid or base.
HCO OH-
CO H2O

37. Label each as acid or base, and conjugate acid or base.
H3O+ + CO3-2
HCO H2O

38. Label each as acid or base, and conjugate acid or base.
HNO3 + F-
NO HF

39. Label each as acid or base, and conjugate acid or base.
H2S + CN-
HS- + HCN

40. Strength of “Bronsted - Lowery” acids & bases:
Strong acid -
A substance that loses its proton (H+) more easily than the other reactant.
Strong base -
A substance that holds on to its proton (H+) more strongly than the other reactant.

41. Weaker acid or base -
Term used to compare the conjugate acid/base to the original acid/base.
A reaction will proceed in the direction of the weaker acid / base.

42. Strong acid
Weaker base
H2SO4 HI
HBr
HCl
HNO3
H3O+
H3PO4 HF
HSO4- I
Br-
Cl-
NO3-
H2O
H2PO4 F-
Weaker acid
Strong base

43. Note:
The strongest acids have the weakest conjugate bases, and the strongest bases have the weakest conjugate acids.

44. pH and Acid / Base Strength
pH Scale 14
Neutral
Strong
Acid
Strong
Base

45. pH + pOH = 14 pH = -log [H3O] Concentration in Molarity
pOH = -log [OH]
pH + pOH = 14

46. Kw = ion product constant for water.
Kw = [H+] x [OH-] = 1.0 x (mol/L)2
[H+] and [OH-] are inter-dependant.
If [H+] then [OH-]
and
If [H+] then [OH-]

47. Example #1
If [H+] = 1x10-5 M , what is p[OH-] ?
Kw = [H+] x [OH-]
(1 x M2)
= (1 x 10-5 M) x p[OH-]
= p[OH-]
(1 x 10-9 M)

48. Example #2
A sample of orange juice has a Hydrogen ion concentration of 2.9 x 10-4 M.
What is the pH ?

49. Example #3
What is the pH of a sample of gastric juice whose [H+] is M ?

50. Example #4
If the pH = 4, what is the molarity of the hydroxide ion ?

51. Example #5
If [H+] = M , what is the pOH ? What is the pH ? What is the concentration of OH- ?

52. Classify each as either Acidic, Neutral, or Basic
[H+] = 6 x M =
pH 9.22
Basic
[OH-] = 3 x 10-2 M =
pOH 1.5
pH 12.5
Basic
[H+] = 2 x 10-7 M =
pH 6.70
Acidic
[OH-] = 1 x 10-7 M =
pOH 7
pH 7
Neutral

53. Example #6
If the hydroxide ion concentration is
1.0 x 10-3 M , what is the [H+] ? Is the solution acidic, basic, or neutral ?

54. Molecular Structure and Acid Strength
Acid strength is determined by two factors:
1. Polarity of the bond “holding” the acidic Hydrogen.
2. Strength of the bond “holding” the acidic Hydrogen.

55. The periodic table can explain the previous two factors
The periodic table can explain the previous two factors. Bond Polarity first.
As you go from left to right across the periodic table, atomic radii decrease.
Why ?
As the number of protons increases, it pulls the valence electrons in closer to nucleus.

56. and
As atomic size decreases, the atom’s electronegativity increases.
and
As electronegativity increases, so does bond polarity; which make removal of the proton easier.

57. and
The easier the proton is removed, the greater the acid strength.

58. The second factor is strength of the bond holding the Hydrogen.
As you go from top to bottom down the periodic table, atomic radii increases.
Why ?
With each row you go down, you add another energy level (a new valence shell).

59. and
with each additional valence shell, the distance between the nucleus and the bonding shell increases.
and
The bond holding the proton (Hydrogen ion) gets weaker the further from it is from the nucleus.

60. and
The weaker the bond, the easier the proton is removed, and the greater the acid strength.

61. The effect of the previous two factors on Binary acids work as follows:
An acid composed of only two elements.

62. Consider the Group VII elements:HF
1.78
Strong bond
Weak acid H
HCl
0.96
Weaker bond
Stronger acid H
HBr
0.76
Weaker bond
Stronger acid H HI
0.46
Weak bond
Strong acid

63. The General Trend
As you go down a column, atomic size increases, strength of bond holding the proton decreases, and Acid strength increases.

64. Effects on Binary acids con’t:
Consider moving across the second energy level:
Oxygen
Fluorine
1.24
1.78
Electronegativity -----
H2O < HF
Acids
Less polar bond
More polar bond
Weaker acid
Stronger acid

65. The General Trend
As you go across a row, atomic size slowly decreases, but electronegativity increases greatly. Resulting in a more polar the bond and a stronger Acid.

66. The effect of the previous two factors on Oxoacids work as follows:
An acid composed of H-O-”Y” . A substance with the Oxygen atom(s) bonded to a central atom, with one or more of the H+ bonded to the Oxygen(s).

67. The acidic Hydrogen is always bonded to an oxygen atom, which is attached to the other element (“Y”). O
Ex: H2SO4 H O S O H O
Acidic hydrogens

68. Bond polarity appears to be the dominant factor in determining the relative strength of Oxoacids.
More polar bond = Stronger acid

69. Consider the three following Oxoacids:
HClO Hypochlorous acid H-O-Cl
HBrO Hypobromous acid H-O-Br
HIO Hypoiodous acid H-O-I
Which has the most polarity in the bonding?
HClO
Which has the least polarity in the bonding?
HIO

70. Rank the previous three acids from weakest to strongest.
Weaker acid Stronger acid
HIO
HBrO
HClO

71. If the Oxoacid has numerous oxygen attached to the “Y” atom; the stronger acid is the acid with the most oxygen attached to “Y”.

72. Consider these acids:
HClO Hypochlorous acid
HClO2 Chlorous acid
HClO3 Chloric acid
HClO4 Perchloric acid

73. Rank the previous acids from weak to strong.
Weaker
acid
Stronger
acid
HClO
HClO2
HClO3
HClO4
Can you explain why the additional oxygen cause this effect ?

74. The effect of the previous two factors on
Polyprotic acids work as follows:
Polyprotic acid -
An acid that contains more than one acidic Hydrogens.
Example: H3PO4

75. Rank the following from weakest to strongest acid:
H3PO4 Phosphoric acid
H2PO4- Dihydrogen phosphate
HPO4-2 Hydrogen phosphate (Biphosphate)
PO4-3 Phosphate

76. Weaker
acid
Stronger
acid
PO4-3
HPO4-2
H2PO4-
H3PO4
Can you explain why the additional Hydrogens cause this affect ?
Clue: consider the definition of a Bronsted - Lowery base and the negative charge.

77. The Lewis Acid / Base Concept