1. Unit 4: Equilibrium, Acids & Bases Part 2: Acids and Bases
Bronsted Lowry Acids and Bases
Autoionization of Water pH
Strong Acids and Bases
Weak Acids and Bases
Ionization Constants
Buffers
Titrations
Lewis Acids and Bases

2. HCl (g) H+ (aq) + Cl- (aq)
Review
Arrhenius Acid: Substance that increases the concentration of H+ ions when dissolved in water
HCl (g) H+ (aq) + Cl- (aq)
Arrhenius Base: Substance that increases the concentration of OH- ions when dissolved in water.
NaOH (s) Na+ (aq) + OH- (aq)
H2O
H2O

3. Review
The Arrhenius definition of acids and bases is limited to aqueous solutions
Two other common definitions for acids and bases.
Bronsted-Lowry acids and bases
Lewis acids and bases

4. Bronsted-Lowry Acids & Bases
any substance that can transfer a proton (H+ ion) to another substance
a proton donor
HCl (g) + H2O (l) H3O+ (aq) + Cl- (aq)
A Bronsted-Lowry acid must have a hydrogen that can be lost as H+
B-L acid

5. Bronsted-Lowry Acids & Bases
Bronsted-Lowry Base
any substance that can accept a proton (H+ ion) from another substance
A proton acceptor
HCl (g) + H2O (l) H3O+ (aq) + Cl- (aq)
A B-L base must have an atom with a lone pair of electrons that can form a new bond to a hydrogen ion.
B-L acid
B-L base

6. Bronsted-Lowry Acids & Bases
The H+ ion interacts strongly with the nonbonding pairs of electrons on water molecules, forming the hydronium ion
The hydronium ion is responsible for the characteristic properties of aqueous solutions of acids.

7. Bronsted-Lowry Acids & Bases
H3O+ is a more realistic depiction of the hydrogen ion in solution but for convenience we often use H+ (aq) to depict the hydrated hydrogen ion.
HCl (g) + H20 (l)  H3O+ (aq) + Cl- (aq)
HCl (aq)  H+ (aq) + Cl- (aq)

8. Bronsted-Lowry Acids & Bases
Some substances like water are amphoteric
Capable of acting as either an acid or a base
H2O (l) + HCl (g)  H3O+ (aq) + Cl- (aq)
NH3 (g) + H2O (l)  NH4+ (aq) + OH- (aq)
base
acid

9. Bronsted-Lowry Acids & Bases
Examples of other amphoteric substances include:
NaHCO3
NaH2PO4
NaHSO4
Alcohols such as ethanol:

10. Bronsted-Lowry Acids & Bases
In any acid-base equilibrium, both forward and reverse reactions involve proton transfers.
HNO2 (aq) + H2O (l) NO2 – (aq) + H3O+ (aq)
Forward Reaction:
B-L acid =
B-L base =
Reverse Reaction:
acid =
base =

11. Bronsted-Lowry Acids & Bases
The reaction between a BL acid and base produces a new acid (the conjugate acid) and a new base (the conjugate base).
HNO2 (aq) + H2O (l) NO2 – (aq) + H3O+ (aq)
HNO2 and NO2- are called a conjugate acid-base pair.
H2O and H3O+ are also a conjugate acid-base pair.
BL acid
BL base
conjugate base
conjugate acid

12. Bronsted-Lowry Acids & Bases
Conjugate acid:
The acid formed when a base gains a proton
Conjugate acid of H2O
H3O +
Conjugate acid of SO4 2-
HSO4 -
NOTE: The conjugate acid is always shown on the product side.

13. Bronsted-Lowry Acids & Bases
Conjugate base:
The base formed by removing a proton from an acid
Conjugate base of H2O
OH -
Conjugate base of H2SO4
HSO4 -
NOTE: The conjugate base is always shown on the product side.

14. Bronsted-Lowry Acids & Bases
Conjugate acid-base pair:
An acid and a base that differ only in the presence or absence of a single proton
HNO2 and NO2-
H3O+ and H2O
HCO3 - and CO32-
NH4+ and NH3

15. Bronsted-Lowry Acids & Bases
Example: Identify the acid, base, conjugate acid and conjugate base for the following reaction.
HSO4– (aq) + CO32- (aq) SO42- (aq) + HCO3- (aq)

16. Bronsted-Lowry Acids & Bases
Example: Identify the BL acid, BL base, conjugate acid and conjugate base in the following reactions.

17. Bronsted-Lowry Acids & Bases
In any acid-base equilibrium, both forward and reverse reactions involve proton transfers.
HNO2 (aq) + H2O (l) NO2 – (aq) + H3O+ (aq)
Conjugate
base
Conjugate
acid
acid
base
How can we predict the position of the chemical equilibrium?

18. Bronsted Lowry Acids & Bases
The relative strengths of the acid and the conjugate acid can be used to predict the position of the equilibrium.
Equilibrium favors the formation of the weaker acid.
The stronger acid more effectively loses a proton than its conjugate acid

19. Bronsted Lowry Acids & Bases
Every substance can be categorized as either:
strong acid
ionizes completely
weak acid
ionizes partially
solutions contain mixture of acid molecules, hydronium ion, and conjugate base
negligible acidity
no tendency to form H+ in solution

20. Bronsted-Lowry Acids & Bases
The seven most common strong acids:
HCl hydrochloric acid
HBr hydrobromic acid
HI hydroiodic acid
HNO3 nitric acid
HClO3 chloric acid
HClO4 perchloric acid
H2SO4 sulfuric acid
You must know these acids by name and formula.

21. Bronsted-Lowry Acids & Bases
Examples of common weak acids:
acetic acid,
citric acid,
phosphoric acid
Examples of substances with negligible acidity:
CH4 H2
OH –
NH3

22. Bronsted-Lowry Acids & Bases
An inverse relationship exists between the strength of an acid and its conjugate base or between a base and its conjugate acid.
Strong acids form very weak conjugate bases (usually have negligible basicity).
Weak acids form stronger (but still fairly weak) conjugate bases
Substances with negligible acidity form very strong bases.

23. Bronsted-Lowry Acids and Bases
The stronger the acid, the weaker the conjugate base
The stronger the base, the weaker the conjugate acid

24. Bronsted-Lowry Acids & Bases
Example: Does the following acid/base equilibrium favor the reactants or products?