1. Chapter 19 Notes: Part II
Acid/Base Theories

2. There are three ways to define acids and bases.
This reflects the fact that science is always revising itself.

3. 1) Arrhenius Acids/Bases
An acid is a chemical that gives off hydrogen ions in solution.
A base is a chemical that gives off hydroxide ions in solution.
Example: HCl + NaOH  NaCl + H2O
**BASICALLY…acids start with H+ and bases end with OH-!!!

4. Arrhenius Acids/Bases
There are three ways to describe an acid:
Monoprotic-gives off one H+ in sol’n
Diprotic-gives off two H+
Triprotic-gives off three H+

5. Bronsted-Lowry Acids/Bases
The Arrhenius definition is a good one, but does not encompass everything that shows acidic/basic qualities.
To account for this, Johannes Bronsted and Thomas Lowry proposed a new idea.

6. 2) Bronsted-Lowry Acids/Bases
An acid is a hydrogen-ion (proton) donor.
A base is a hydrogen-ion (proton) acceptor.

7. Conjugate Acids/Bases
A conjugate acid is the particle that is formed when a base gains a hydrogen ion.
A conjugate base is the particle that is formed when an acid donates a hydrogen ion.

8. Conjugate Acids/Bases
A conjugate acid/base pair consist of two substances related by the loss of a single hydrogen ion.

9. Conjugate Pairs

10. NH3 + H2O  NH4+ + OH- Which is the initial acid?
Which is the initial base?
What is the conjugate acid?
What is the conjugate base?
proton donator
H2O
NH3
NH4+
OH-
proton acceptor
formed after base gains H+
formed after acid donates H+

11. HCl + H2O  H3O+ + Cl- Which is the initial acid?
Which is the initial base?
What is the conjugate acid?
What is the conjugate base?
HCl
H2O
H3O+
Cl-

12. Amphoteric Substances
Note that in one of the previous examples H2O was acidic and basic in the other.
A substance that can act as an acid or a base is called amphoteric.

13. 3) Lewis Acids/Bases
A Lewis acid is a substance that can accept a pair of electrons to form a covalent bond.
A Lewis base is a substance that can donate a pair of electrons to a covalent bond.

14. **MUST draw Lewis dot structures to determine whether a compound will gain or lose e-!!!3 7 5 1
Ex… BF3 + NH3
F3BNH3 B F x N H F B x N H x
ACID
(not happy)
accept e- pair
BASE
(happy)
donate e- pair

15. pH scale 7 = neutral Over 7 = base
The pH scale is a way of expressing the strength of acids and bases.
Instead of using very small numbers based on the Molarity of the H+ (or OH-) ion ,
We use the NEGATIVE power of 10 on the Molarity of the H+ (or OH-) ion. pH = - log [H+] Under 7 = acid
7 = neutral Over 7 = base

16. The Inverse Relationship between the pH and pOH Scales
For any neutral solution,
pH + pOH = (at 25°C)

18. pH of Common Substances

19. pH
[H+]
[OH-]
pOH