1. Living By Chemistry SECOND EDITION
Unit 4: TOXINS
Stoichiometry, Solution Chemistry, and Acids and Bases

2. Lesson 85: Pass the Proton
Acid-Base Theories

3. ChemCatalyst
Which of these four solutions conduct electricity: 0.10 M HCl (hydrochloric acid), 0.10 M CH3COOH (acetic acid), 0.10 M NaCl (sodium chloride), 0.10 M C12H22O11 (sugar)? Explain.

4. Key Question
How are acids and bases defined?

5. You will be able to:
define Arrhenius and Brønsted-Lowry acids and bases
explain the behavior of acids and bases on a particulate level
explain the difference between strong and weak acids and bases

6. Prepare for the Activity
Work in groups of four.

7. Discussion Notes Acids are substances that add H+ to solution.
Bases are substances that add OH- to solution.
Neutral substances do not add H+ or OH- to solution.
The definitions of acids and bases have changed over time.

8. Discussion Notes (cont.)
Arrhenius Definition of Acids and Bases:
An acid is any substance that adds hydrogen ion (H+) to solution.
A base is any substance that adds hydroxide ion (OH-) to solution.

9. Discussion Notes (cont.)
Brønsted-Lowry Definition of Acids and Bases:
An acid is a proton donor.
A base is a proton acceptor.
Acids and bases that break apart (dissociate) completely in solution are called strong acids and strong bases.
Acids and bases that do not dissociate completely in solution are called weak acids and weak bases.

10. Wrap Up How are acids and bases defined?
According to the Arrhenius definition, an acid is a substance that adds hydrogen ions, H+, to an aqueous solution.
According to the Arrhenius definition, a base is a substance that adds hydroxide ions, OH–, to an aqueous solution.

11. Wrap Up (cont.)
Some substances add OH– by removing H+ from water. Brønsted and Lowry define an acid as a proton (H+) donor and a base as a proton (H+) acceptor.
Strong acids and strong bases dissociate completely into ions, while weak acids and weak bases do not.

12. Check-In
Which substances do you expect will conduct electricity: hydrocyanic acid, HCN; magnesium hydroxide, Mg(OH)2; methanol, CH3OH? Explain your thinking.