Acid-Base Equilibria

1
Acid-Base Equilibria
2
Arrhenius

• acids increase H when dissolved in water
• acids can be classified as monoprotic, diprotic
  or triprotic
• bases increase OH- when dissolved in water
• bases can be classified as monobasic, dibasic, or
  tribasic

3
A-B Strength

• strong acids bases ionize completely and are
  strong electrolytes
• 7 acids 8 bases (memorize them!!!)
• diprotic acids dibasic bases do NOT ionize
  completely, only their first H or OH- ionizes
  completely.
• Strong acid strong base neutral salt
• strong A-B are not equilibrium expressions, but
  all other A-B are reversible

4
Bronsted-Lowry A-B

• restricts definition to H
• acids donate H
• bases accept H
• allows the classification of less traditional A-B
• conjugate A-B pairs 2 formulas in an equation
  whose formulas differ by a H

5
What is the acid, base, and the conjugates?

• HClO H2O H3O ClO-
• CO32- H2O OH- HCO3-

6
Wait, water can go both ways?

• amphoteric substances can behave as either an
  acid or base depending on what they react with.
• water and anions with protons (H) attached are
  most common amphoterics

7
Lewis A-B Theory

• very general definition of A-B using electron
  pairs
• acid electron pair acceptor
• base electron pair donor
• In the following equation, which species acts as
  a Lewis acid and which acts as a base?
• BF3 NH3 BF3NH3

8
Weak A-B

• only partially ionize in water and are weak
  electrolytes
• can be written as equilibrium expressions with a
  Ka or Kb value
• K value indicates how much the acid or base will
  ionize (high K higher ionization)
• larger K values indicate a stronger acid or base
• For di- and tri- protic/basic, there will be 2 K
  values (one for the first ionization and one for
  the second)

9
Autoionization of Water

• H2O H2O OH- H3O
• reversible equilibrium where water can donate a
  proton to itself
• Kw 1.0 x 10-14 at room temp.
• For any conjugate A-B pair, Kw Ka x Kb
• What is the Ka value for NH4?

10
Example

• Is an aqueous solution of Na2HPO4 acidic or basic?

11
pH scale

• pH -logH
• works for pH ranges from 2-12 and approximates pH
  outside that window
• The exponent on the H is an indicator of
  approximate pH.

12
(No Transcript)
13
Strong A-B

• no equilibrium b/c all acid/base ionizes
• use original acid concentration to calculate pH
• Calculate the H and pH in a solution of 0.37M
  hydrochloric acid.
• Calculate the OH- and pH in a 0.58M solution of
  NaOH

14
Weak Acids

• use RICE to find equilibrium concentrations
• R HA H2O H3O A-
• I I 0 0
• C -x x x
• E I-x x x
• b/c Ka for most weak acids is less than 10-3, I-x
  is about equal to I, so Ka x2/I

15
Example

• What is the pH of a 0.20M solution of acetic acid
  (CH3COOH)?

16
Weak Bases

• similar calculations as acids (replace H3O with
  OH-)
• R B H2O OH- HB
• I I 0 0
• C -y y y
• E I-y y y
• b/c Kb for most weak bases is less than 10-3, I-y
  is about equal to I, so Kb y2/I

17
Example

• What is the pH of a 0.68M solution of aqueous
  ammonia?

18
Classify the following as weak/strong acids/bases

• chloric acid
• ammonium chloride
• calcium hydroxide
• ethyl amine
• sodium cyanide

19
Example

• Measurements show that the pH of a 0.10M solution
  of acetic acid is 2.87. What is the Kb of
  potassium acetate?

20
A-B properties of salt solutions

• for the most part anions are slightly basic
  (because they attract protons) and cations are
  slightly acidic (because they can donate protons)
• ions from strong A-B are the only neutral ions
• To determine if a salt is acidic or basic, look
  at the ions it forms
• ignore any neutral ions
• if anion is left, salt is basic
• if cation is left, salt is acidic
• if both cation anion are neutral, salt is
  neutral
• if both cation anion are not neutral, the
  A-B-ness cant be determined from the formula

21
Classify the following salts as acidic, basic, or
neutral

• NaNO2
• CH3NH3Cl
• NaCl
• MgSO4
• Al2(SO3)3

22
A-B-ness chemical structure

• 3 factors affect attraction of electrons (acidity
  increases with stronger attraction of electrons)
• ionic charge when comparing similar atoms, more
  positive ions are stronger acids
• oxidation on central atom when comparing
  similar formulas with the same central atom, the
  higher the ox, the stronger the acid
• electronegativity when comparing similar
  formulas with different central atoms, the higher
  the EN, the stronger the acid

23
Common Base Reactions

• Strong bases also include hydrides (H-), nitrides
  (N3-), and carbides (C22-)
• NaH H2O ? H2 Na OH-
• Mg3N2 6H2O ? 2NH3 3Mg2 6OH-
• Ca2C2 2H2O ? C2H2 Ca2 2OH-
• strong bases also include oxides of groups 12
  metals
• Li2O H2O ? 2Li 2OH-
• CaO H2O ? Ca2 2OH-

24
Common Acid Reactions

• nonmetal oxides (aka. acid anhydrides) turn into
  acids when placed in water
• SO2 H2O H2SO3
• CO2 H2O H2CO3
• Cl2O7 H2O ? 2H 2ClO4-