1. Unit 5: Acids, Bases and Titrations

2. Acid/Base Definitions
Arrhenius Model
Acids produce hydrogen ions in aqueous solutions
Bases produce hydroxide ions in aqueous solutions
Bronsted-Lowry Model
Acids are proton donors
Bases are proton acceptors
Lewis Acid Model
Acids are electron pair acceptors
Bases are electron pair donors

3. Acid Dissociation Acid Proton Conjugate base
HA  H A-
Acid Proton Conjugate
base
Alternately, H+ may be written in its hydrated form, H3O+ (hydronium ion)

4. Dissociation of Strong Acids
Strong Acids are assumed to dissociate completely in solution.
Does this suggest a large or small Ka?
Does this suggest a reactant or product favored equilibrium?

5. Dissociation Constants: Strong Acids

6. Strong Acids to know
AP College Board would like you to know the following strong acids:
HCl, HBr, HI, HClO4, H2SO4, HNO3

7. Dissociation of Weak Acids
Weak acids are assumed to dissociate only slightly (less than 5%) in solution.
Does this suggest a large or small Ka?
Does this suggest a reactant or product favored equilibrium?

8. Dissociation Constant: Weak Acids

9. Auto-Ionization of Water
H2O + H2O  H3O+ + OH-
At 25, [H3O+] = [OH-] = 1 x 10-7M
Kw is a constant at 25 C:
Kw = [H3O+][OH-]
Kw = (1 x 10-7)(1 x 10-7) = 1 x 10-14

10. Homework Pg
#16, 21a, 21b, 28-32, 35, 37, 38

11. The pH Scale
The pH scale is a log based scale used to measure the acidity of a solution.
Smaller pH values are more acidic.
Bigger pH values are more basic.
pH levels at or very near 7 are considered to be neutral.

12. pH and pOH pH = - log 10 ( 𝐻 3 𝑂 + ) pOH = - log 10 ( 𝑂𝐻 − )
Relationship between pH and pOH
pH + pOH = 14

13. Weak vs Strong and pH
Since strong acids are assumed to completely dissociate, more hydronium ions will be created.
lower concentration  higher pH
Since weak acids are assumed to dissociate slightly, less hydronium ions are created.
Higher concentration  lower pH
Since pH depends on both concentration and strength, it takes a much larger concentration for weak acids to have the pH of a stronger acid at a smaller concentration.

14. Finding Concentration from pH or pOH
𝐻 3 𝑂 + = 10 −𝑝𝐻
𝑂𝐻 − = 10 −𝑝𝑂𝐻

15. pH and pOH Calculations

16. Homework
Pg. 674
#41-48

17. Weak Acid Equilibrium Problem
What is the pH of a 0.50M solution of acetic acid, HC2H3O2, Ka = 1.8 x 10-5 ?
Step #1: Write the dissociation equation
HC2H3O2  C2H3O2- + H+

18. Weak Acid Equilibrium Problem cont.
What is the pH of a 0.50M solution of acetic acid, HC2H3O2, Ka = 1.8 x 10-5 ?
Step #2: ICE it!
HC2H3O2  C2H3O2- + H+ I
0.50 C -X +X E X X

19. Weak Acid Equilibrium Problem cont.
What is the pH of a 0.50M solution of acetic acid, HC2H3O2, Ka = 1.8 x 10-5 ?
Step #3: Set up the law of mass action
HC2H3O2  C2H3O2- + H+

20. Weak Acid Equilibrium Problem cont.
What is the pH of a 0.50M solution of acetic acid, HC2H3O2, Ka = 1.8 x 10-5 ?
Step #4: Solve for x, which is also [H+]
HC2H3O2  C2H3O2- + H+
[H+] = 3.0 x 10-3 M

21. Weak Acid Equilibrium Problem cont.
What is the pH of a 0.50M solution of acetic acid, HC2H3O2, Ka = 1.8 x 10-5 ?
Step #5: Convert [H+] to pH
HC2H3O2  C2H3O2- + H+

22. Strong Bases Strong bases are metallic hydroxides
Group I hydroxides (NaOH, KOH) are very soluble
Group II hydroxides (Ca, Ba, Mg, Sr) are less soluble
pH of strong bases is calculated directly from the concentration of the base in solution

23. Weak Bases

24. Dissociation of Bases
B + H2O  BH+ + OH-

25. Relationship Between Ka and Kb
Kw = [H3O+][OH-]
Ka x Kb = Kw

26. Homework P
#49, 50, 53-59, 61, 63, 65, 66, 71-79, 82, 83, 86

27. Polyprotic Acids H3PO4 + H2O  H3O+ + H2PO4-
An acid that can donate more than one proton.
H3PO4 + H2O  H3O+ + H2PO4-
H2PO4- + H2O  H3O+ + HPO42-
HPO42- +H2O  H3O+ + PO43-
With weak acids, all species are minor and so the Hydronium from each separate reaction affects the pH, but H3PO4 has the dominant equilibrium.

28. Strong Polyprotic Acids
The initial acid is a strong acid, but the acidic ions produced are not.
H2SO4 + H2O  H3O+ + HSO4-
HSO4 + H2O  H3O+ + SO42-
The hydronium produced by the second acid dissociation may not be large enough to affect the pH.
To find out we can create an ICE box problem and assume x is small. If the assumption holds true, then the second dissociation doesn’t produce enough hydronium.
If the assumption proves false, then the second dissociation does affect the pH.

29. Salts that produce Neutral Soln.
If the salt is made with the cation of a strong base and the anion of a strong acid, the resulting solution will have no effect on the [H+] when dissolved in water.
Examples:
KCl
NaNO3

30. Salts that produce Basic Soln.
If the salt has a cation with a neutral properties (Na+ or K+) and an anion that is the conjugate base of a weak acid, the resulting solution is basic.
Example:
NaC2H3O2

31. Salts that produce Acidic Soln.
Salts that have an anion that is not a base and a cation that is the conjugate acid of a weak base produce acidic solutions.
Example:
NH4Cl
Salts with a highly charged metal ion also produce an acidic solution.
AlCl3

32. Check it out!
Pg. 660
Table 14.6
The above table gives a good break down of what types of salts produce which type of solution.

33. Structural Impact
Weaker bonds and increased electronegativity/polarity increases the strength of the acid.
For acids containing oxygen, the more oxygen atoms attached to the central atom in the acid, the stronger the acid is.

34. Oxyacids
H-O-X
Acidic Solution: If X has a high electronegativity, the O-X bond will be strong and covalent. This will result in a relatively weaker O-H bond to break releasing a proton.
Basic Solution: If X has a low electronegativity, the O-X bond will be weak and ionic. This will result in a relatively stronger O-H bond to stay intact as the O-X bond will break in polar water, releasing the metal cation and the hydroxide ion

35. Homework
Pg. 676
#93-99, 101, 102, 105, 106,

36. Bell Work Recall: Which acids and bases are strong?
Check with a partner.

37. Lewis Acids and Bases Lewis Acid: Electron pair acceptor
Lewis Base: Electron pair donor

38. Lewis Acids cont.
A stronger Lewis acid is one that is smaller and more positively charged than another acid.
B3+ is a strong Lewis acid as it has a large positive charge, making it more attracted to an electron pair.

39. Practice Ni2+(aq) + 6NH3(aq) ↔ Ni(NH3)62+(aq)
Identify the Lewis acid and base for each reaction:
Ni2+(aq) + 6NH3(aq) ↔ Ni(NH3)62+(aq)
H+(aq) + H2O(l) ↔ H3O+(aq)
HCl(aq) + H2O(l)  H3O+(aq) +Cl-(aq)

40. Amphoterism
The ability to act like both an acid and a base.

41. Solving Acid Base Problems
Take a look at each species and identify if it is an acid or a base.
Identify the strength
If a strong acid or base is present, assume it goes to completion.
Calculate the concentration of the product
If a weak acid or base is present, draw an ICE box and fill out any given information.
Assume the acid or base dissociation is small
Define the equilibrium concentrations in terms of “x”
Solve for “x”
Check the assumption
If there the species is polyprotic, choose the reaction that will most affect the pH.

42. Homework
Pg. 677 #

43. (a) Calculate the [H+] of a 0.14–molar solution of HOCl.
HOCl ↔ OCl- + H+
Hypochlorous acid, HOCl, is a weak acid commonly used as a bleaching agent. The acid–dissociation constant, Ka, for the reaction represented above is 3.2×10–8.
(a) Calculate the [H+] of a 0.14–molar solution of HOCl.
(b) Write the correctly balanced net ionic equation for the reaction that occurs when NaOCl is dissolved in water and calculate the numerical value of the equilibrium constant for the reaction.
(e) Household bleach is made by dissolving chlorine gas in water, as represented below.
Cl2(g) + H2O → H+ + Cl– + HOCl(aq)
Calculate the pH of such a solution if the concentration of HOCl in the solution is molar.

45. FRQ Practice
A comparison of the theories Arrhenius, Brønsted and Lewis shows a progressive generalization of the acid base concept. Outline the essential ideas in each of these theories and select three reactions, one that can be interpreted by all three theories, one that can be interpreted by two of them, and one that can be interpreted by only one of the theories. Provide these six interpretations.

46. Answer Examples: Interpreted by all three HCl + H2O → H+(aq) + Cl–(aq)
Arrhenius
acid = produce H+ ions in aqueous solution
base = produce OH– ions in aqueous solution
Brønsted–Lowry
acid = proton(H+) donor; base = proton acceptor
Lewis
acid = e– pair acceptor; base = e– pair donor
Examples:
Interpreted by all three
HCl + H2O → H+(aq) + Cl–(aq)
NaOH + H2O → Na+(aq) + OH–(aq)
Interpreted by two
NH3 + HCl ↔ NH4+ + Cl–
Interpreted by only one
BF3 + NH3 → F3B:NH3

47. Mo’ Practice… Mo’ Perfect
Predict whether solutions of each of the following salts are acidic, basic, or neutral. Explain your prediction in each case
(a) Al(NO3) (b) K2CO (c) NaBr

48. Answers (a) acidic; Al3+ + H2O ↔ AlOH2+ + H+; hydrolysis of Al3+;
Al(OH2)n3+ as Brønsted acid, etc.
(b) basic; CO32– + H2O ↔ HCO3– + OH– ; or
hydrolysis of CO32– as conjugate to a weak acid, etc.
(c) neutral; Na+ from strong base; Br– from strong acid

49. Because, why not.
Methylamine CH3NH2, is a weak base that ionizes in solution as shown by the following equation.
CH3NH2 + H2O ↔ CH3NH3+ + OH–
(a) At 25ºC the percentage ionization in a molar solution of CH3NH2 is 4.7%. Calculate [OH–], [CH3NH3+], [CH3NH2], [H3O+], and the pH of a molar solution of CH3NH2 at 25ºC
(b) Calculate the value for Kb, the ionization constant for CH3NH2, at 25ºC.

50. Answers

51. And just like the energizer bunny…
Sodium benzoate, C6H5COONa, is the salt of a weak acid, benzoic acid, C6H5COOH. A 0.10 molar solution of sodium benzoate has a pH of 8.60 at room temperature.
(a) Calculate the [OH–] in the sodium benzoate solution described above.
(b) Calculate the value for the equilibrium constant for the reaction:
C6H5COO– + H2O ↔ C6H5COOH + OH–
(c) Calculate the value of Ka, the acid dissociation constant for benzoic acid.

52. Answer

57. Give a brief explanation for each of the following.
(a) For the diprotic acid H2S, the first dissociation constant is larger than the second dissociation constant by about 105 (K1 ~ 105 K2).
(b) In water, NaOH is a base but HOCl is an acid.

58. Answer:
(a) After the first H+ is lost from H2S, the remaining species, HS–, has a negative charge. This increases the attraction of the S atom for the bonding electrons in HS–. Therefore, the bond is stronger, H+ is harder to remove, and K2 is lower.
(b) Polar H2O can separate ionic NaOH into Na+(aq) and OH–(aq), giving a basic solution. In HOCl, chlorine has a high attraction for electrons due to its greater charge density. This draws electrons in the H–O bond towards it and weakens the bond. H+ can be removed, making an acidic solution.