1. Aqueous Equilibria: Acids & Bases
McMurry and Fay ch. 14

2. Acid-base definition (Arrhenius)
Acid: increases H+ ions in solution
Base: increases OH- ions in solution

3. Arrhenius Acids & Bases
Acids: HCl  H+ + Cl- H2SO4  2 H+ + SO42- Bases: KOH  K+ + OH- NaOH  Na+ + OH-

4. Ammonia as a base
Ammonia (NH3) neutralizes acids. How can it make OH- in solution?

5. An important equilibrium constant
H2O H+ + OH-
(self-ionization of water)
[H+][OH-] = Kw

6. Ammonia as a base
NH3 + H2O  NH3 + H+ + OH-  NH4+ + OH-

7. Acid-base definition (Brønsted-Lowry)
Acid: proton donor
Base: proton acceptor
(the word proton refers to H+)

8. Brønsted-Lowry acid-base reactions are reversible
NH3 + H2O NH OH-
accepts
proton
BASE
donates
proton
ACID
donates
proton
ACID
accepts
proton
BASE

9. Brønsted-Lowry acid-base reactions are reversible
NH3 + H2O NH OH-
CONJUGATE
ACID
CONJUGATE
BASE
accepts
proton
BASE
donates
proton
ACID
donates
proton
ACID
accepts
proton
BASE

10. Conjugate acid-base pairs
Two substances that differ by only one proton
The acid has one more proton than the base
The base has one less proton than the acid

11. In the following reaction, which is the conjugate base of CH3COOH
In the following reaction, which is the conjugate base of CH3COOH? CH3COOH + H2O CH3COO- + H3O+
CH3COOH
H2O
CH3COO-
H3O+

12. Hydronium ion H+ is not very stable by itself
Charge on proton in solution is stabilized by IMFs
H+ ion is frequently written as H3O+ H+ O H
In this class we will use H+ and H3O+ interchangeably

13. Acid strength: old version
The stronger the acid,
the more H+ it generates
Acid strength: old version
HCl (aq) H+ (aq) + Cl- (aq)
(strong acid because no HCl remains; all turns into H+)
CH3COOH (aq) CH3COO-(aq) + H+ (aq)
(weak acid because little H+ is made; most remains as CH3COOH)

14. Acid strength: Equilibrium Version
Strength depends on equilibrium positions
HCl + H2O H3O+ + Cl-
CH3COOH + H2O CH3COO- + H3O+
In strong acids, the conjugate base form is favored at equilibrium
In weak acids, the acid form is favored at equilibrium

15. Characteristics of a Strong Acid
Weak bond (easy to break) between H & anion is weak
Anion is stable in solution

16. Acid strength: Binary acids in the same group of the periodic table
As valence shell increases, bond strength decreases
Easier to break bonds between H & larger atoms H F Cl H
Acid strength increases Br H I H

17. Acid strength: Binary acids in the same row of the periodic table- H H H H N N -
As you move to the right, electronegativity increases
More electronegative atoms are more happy as ions
More electronegative elements are stronger acids H H H H H H H H O - O - F H F

18. Acid strength: Oxoacids
Oxoacids are acids composed of hydrogen, oxygen, and some other element (examples: H2SO4, H3PO4, HNO3)
General rule: as central atom electronegativity increases,
strength of the O-H bond decreases
 Higher electronegativity = stronger acid strength

19. Acid strength: Oxoacids
For a given element, there can be oxoacids with different numbers of oxygen atoms bound to the central atom H2SO4 H2SO3 O O O S O O S O O

20. Acid strength: Oxoacids
More resonance structures = more stable conjugate base
General rule: The larger the number of lone oxygen atoms,
the stronger the acid

21. Lewis Acids and Bases
Explain acid-base chemistry that occurs outside of aqueous solutions Lewis acid: electron pair acceptor Lewis base: electron pair donor

22. Lewis Base examples:
H N H H O H H .. .. ..

23. Lewis acid-base example
NH3 + BF3