1. Unit 4: Acid-Base Theory
Lesson 3: Brønsted-Lowry Theory

2. Brønsted-Lowry Theory
This theory expands on Arrhenius theory and gives new definitions for acids and bases:
An acid is a substance that loses a proton to another substance.
A base is a substance that gains a proton from another substance.

3. Examples
Which of the reactants is a Bronsted-Lowry acid? Which is a Bronsted-Lowry base?
NH3 + H2O NH4+ + OH-
CH3COOH + H2O CH3COO- + H3O+
5 min

4. Brain Break: Spot 5 Differences

5. Hold up, wait a minute...
In our earlier examples, H2O was an acid in one reaction, and a base in the other!
Substances that can act as a Bronsted-Lowry acid OR base are said to be amphiprotic.

6. Amphiprotic Substances
Aside from H2O, a substance will be amphiprotic if:
it possesses a negative charge, AND
it has an easily removable hydrogen.
Examples include H2PO4- and HS-

7. Demo: The Many Faces of Aluminum
Predict: What will happen as NaOH is added to the solution of [Al(H2O)6]3+?
Observe: What changes occur?
Explain: As OH- is added, the [Al(H2O)6]3+ acts as an acid and loses a proton, then another, and another, and so on! It has seven possible forms, all of which are aqueous EXCEPT the neutral [Al(H2O)3(OH)3], which appears as a white precipitate.
10 min

8. Acid-Base Equilibrium
Bronsted-Lowry acid-base reactions are at equilibrium, so they proceed in both the forward and reverse directions.
Thus, there is an acid and a base on both sides of the equation!

9. Examples
Identify all of the Bronsted-Lowry acids and bases in the following reactions.
NH3 + H2O NH4+ + OH-
CH3COOH + H2O CH3COO- + H3O+
5 min

10. Practice:
Pg. 117 #11
Pg. 119 #13 and 14
5 min