1. Chapter 17: Acids & Bases

2. Acids
Arrhenius acid – A substance that produces H+ (also H3O+) ions in aqueous solution
Brønsted-Lowry acid - a substance that donates H+ ions

5. Structure of an acid
Chapter 15, Unnumbered Figure, Page 660

6. Chapter 15, Unnumbered Figure 1, Page 661
Hydrogens connected to more electronegative atoms are more likely to act as an acidic hydrogen
High EN atoms: F, Cl, Br, I, O

8. Practice
Decide if the following molecules are acids or not, based on their structures.

9. Strong acids
Strong acid - an acid that completely ionizes or dissociates in water

10. Weak acids
Weak acid - an acid that partially ionizes or dissociates in water

11. Chapter 15, Unnumbered Figure 2, Page 661

12. Chapter 15, Unnumbered Figure 4, Page 661

13. Bases
Arrhenius base – A substance that produces OH- ions in aqueous solution
Brønsted-Lowry base - a substance that accepts H+ ions
Arrow pushing mechanism

14. Strong bases
Strong base - a base that completely ionizes or dissociates in water

15. Weak bases NH3(aq) + H2O(l) NH4+(aq) + OH-(aq)
Weak base - a base that partially ionizes or dissociates in water

17. Conjugate acid-base pairs
Each acid has a conjugate base and every base has a conjugate acid

19. Chapter 15, Figure 15.12
Strength of Conjugate Acid–Base Pairs

20. Problems Identify the conjugate bases for the following acids HBr H2S
H2CO3
Identify the conjugate acids for the following bases
NO2-
NH3
OH-
Problems

21. Polyprotic acids
Polyprotic acids - acids that can donate more than one H+

23. Acid ionization constant (Ka)
Acid ionization constant (Ka) - the equilibrium constant for the ionization reaction of an acid with water

26. Questions
Write the Ka expression for the dissociation of HCN (hydrocyanic acid) in water.
Write the Ka expression for the dissociation of HClO2 (chlorous acid) in water.
If the Ka for HCN is 4.9 x and the Ka for HF is 3.5 x 10-4, which acid is stronger?

27. Base ionization constant (Kb)
Base ionization constant (Kb) - the equilibrium constant for the ionization reaction of a base with water

29. Autoionization of water
Also called the self ionization of water
1 out of 10 million water molecules form ions through self ionization
All aqueous solutions contain both H3O+ and OH–

30. Ion Product Constant for Water (Kw)
The numerical value obtained by multiplying the molar concentrations for hydronium and hydroxide ions present in pure water
Kw = [H3O+][OH-] = 1.00 x 25 oC
the concentration of H3O+ and OH– are equal in pure water
[H3O+] = [OH–] = 25°C
Ion Product Constant for Water (Kw)

31. H+ OH- [H+] vs. [OH-] Acid Base

32. Calculate the [OH] at 25°C when the [H3O+] = 1
Calculate the [OH] at 25°C when the [H3O+] = 1.5 x 10-9 M, and determine if the solution is acidic, basic, or neutral
Calculate the [H3O+] at 25°C in a solution that has a [OH] 1.3 x M and determine if the solution is acidic, basic, or neutral
Calculate the [H3O+] at 25°C in a solution that has a [OH] 1.0 x 10-7 M and determine if the solution is acidic, basic, or neutral
Questions

33. pH and pOH
pH is a measure of the concentration of H+ or H3O+ in solution
pOH is a measure of the concentration of OH- in solution

34. pH and pOH Acidic solutions Basic solutions Neutral solutions

35. Determining [H3O+] and pH of an acidic solution : Strong acids
The two sources of [H3O+] in a solution are from the autoionization of water and the weak/strong acid
Strong acids completely ionize in water, so the initial acid concentration is the [H3O+]
Determining [H3O+] and pH of an acidic solution : Strong acids

36. Determining [H3O+] and pH of an acidic solution : Weak acids
For weak acids we must use the Ka value to determine [H3O+] from initial [HA]
Determining [H3O+] and pH of an acidic solution : Weak acids

37. x is small approximation
[HA]
[H3O+]
[A-]
Initial
Change -x +x
Equilibrium
[HA] - x x
If Ka is very small, the x value is very small since small amount of products made
This allows simplification of denominator expression

38. Exceptions to x is small approximation
Can only apply this rule if the [HA] is much larger than Ka
If they are close in within a factor of 1 to 100, then the % error is large, so you can’t use this approximation
Ka = 1x10-10 [HA] = 1 x Can use approximation
Ka = 1x10-4 [HA] = 1 x Can’t use approximation
If you can’t use approx. then you have to use quadratic formula like from before

39. Practice HCN(aq) + H2O(l) CN-(aq) + H3O+(aq)
Determine the [H3O+] of a M HCN which has a Ka value of 4.9 x 10-10?
HCN(aq) + H2O(l) CN-(aq) + H3O+(aq)
[HCN]
[H3O+]
[CN-]
Initial
Change -x +x
Equilibrium
[HCN] - x x
approx

40. Percent ionization of weak acids
A way to quantify the ionization of a weak acid
Example
[HA] initial = M
[H3O+] equilibrium = M
Percent ionization of weak acids

41. Finding pH for a mixture of acids
If you have a mixture of a strong and a weak acid with relative concentrations, the weak acid becomes essentially negligible
Calculate as if only the strong acid is present
If you have a mixture of two weak acids with relative concentrations, consider the weaker of the two acids to be negligible, if differing by a magnitude of 103
Calculate as if only the stronger acid is present
Finding pH for a mixture of acids

42. Finding pH for mixture of acids
If both weak acids have relative concentrations and Ka values that are close…
Calculate the [H+] of the stronger weak acid using ice table
Then take the calculated [H+] from the ice table and use it as the initial concentration in a ice table with the weaker weak acid
Finding pH for mixture of acids

43. Practice
What is the pH of a mixture of M Benzoic acid (HC7H5O2 , Ka1 = 6.5 x 10-5) and M Acetic acid (HC2H3O2, Ka2 = 1.8 x 10-5)?
HC7H5O2 (aq) + H2O (l)  H3O+ (aq) + C7H5O2-(aq)
HC2H3O2 (aq) + H2O (l)  H3O+ (aq) + C2H3O2-(aq)
Chapter 15, Unnumbered Table, Page 698

44. Conjugate acids and bases
Anions can be conjugate bases of acids
The Kb of a conjugate base is related to the Ka of the acid it came from
Ka Kb = Kw = 1014
In general
Stronger acid = weaker conjugate base
Weaker acid = stronger conjugate base
Conjugate acids and bases

45. Acid strength trends
If a Hydrogen has more of a “+” charge, or is more easily donated, it is a stronger acid
Weaker bond to H atom, stronger acid
Higher electronegativity of atom connected to H, stronger acid
The more oxygen atoms in close proximity, stronger acid

46. Acid strength: Bond energy
Chapter 15, Unnumbered Table, Page 698

47. Acid strength: Electronegativity
Chapter 15, Unnumbered Table, Page 698

48. Acid strength: Oxygens in close proximity
Chapter 15, Unnumbered Table 2, Page 699

49. Acid rain
Combustion of fossil fuels release SO2 and NO2 into the atmosphere
2SO2+O2+2H2O  2H2SO4 sulfuric acid
4NO2+O2+2H2O  4HNO3 nitric acid
These acids + rain = acid rain
Can cause building degradation, hazardous to aquatic life, hindrance to tree survivability, overall bad for the environment

50. Chapter 15, Figure 15.16
Acid Rain

51. Chapter 17 Review Acids and bases Strong vs weak acids/bases
Conjugate acid/base pairs
Poly protic acids
Acid/base ionization constant Ka/Kb
Kw ion product of water
pH and pOH
ICE tables for acids, x is small approx
% ionization
Acid strength trends and Acid rain
Chapter 17 Review