This document discusses the basics of acids and bases according to three definitions: 1) Arrhenius definition - acids produce H3O+ in water and bases produce OH- in water. 2) Brønsted-Lowry definition - acids donate protons and bases accept protons. 3) Lewis definition - acids accept electron pairs and bases donate electron pairs. It also covers the pH scale, differences between strong and weak acids/bases, and conjugate acid-base pairs. Neutralization reactions between acids and bases are described.

1. Acid-Base Equilibria (Pt. 1)
The Basics of Acids and
Bases
By Shawn P. Shields, Ph.D.
This work is licensed by Dr. Shawn P. Shields-Maxwell under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0
International License.

2. Overview of Acid/Base Definitions
There are three main classifications of
acids and bases
1) The Arrhenius definition is the most
restrictive.
2) The Brønsted-Lowry description is
more broad.
3) The Lewis classification of acids and
bases is the most general of all.

3. Arrhenius Acids and Bases
Arrhenius Acids are compounds that
produce hydronium H3O+ (also called
“protons” H+) in aqueous soln (water).
𝐇𝐂𝐥 𝐚𝐪 + 𝐇 𝟐 𝐎 𝐥 → 𝐇 𝟑 𝐎+
𝐚𝐪 + 𝐂𝐥−
(𝐚𝐪)
𝐇𝐂𝐥 𝐚𝐪 → 𝐇+
𝐚𝐪 + 𝐂𝐥−
(𝐚𝐪)
hydronium
“proton”

4. Arrhenius Acids and Bases
Arrhenius Bases are compounds that
produce hydroxide OH in aqueous
solution (water).
𝐍𝐚𝐎𝐇 𝐚𝐪 → 𝐎𝐇−
𝐚𝐪 + 𝐍𝐚+
(𝐚𝐪)

5. Brønsted-Lowry Acids and Bases
Brønsted-Lowry acids donate (or “give
away”) protons H+.
𝐇𝐂𝐥 𝐚𝐪 + 𝐇 𝟐 𝐎 𝐥 → 𝐇 𝟑 𝐎+
𝐚𝐪 + 𝐂𝐥−
(𝐚𝐪)
H+ on HCl is
donated to water
to produce
hydronium
hydronium

6. Brønsted-Lowry Acids and Bases
Brønsted-Lowry bases accept
protons H+.
𝐇𝐂𝐥 𝐚𝐪 + 𝐇 𝟐 𝐎 𝐥 → 𝐇 𝟑 𝐎+
𝐚𝐪 + 𝐂𝐥−
(𝐚𝐪)
𝐍𝐇 𝟑 𝐚𝐪 + 𝐇 𝟐 𝐎 𝐥 ⇌ 𝐍𝐇 𝟒
+
𝐚𝐪 + 𝐎𝐇−
(𝐚𝐪)
Water (H2O) accepts H+ from HCl
(Water acts as a base)
NH3 accepts H+ from water (H2O)
(NH3 acts as a base)

7. Lewis Acids and Bases
A Lewis acid is an electron pair acceptor.
A Lewis base is an electron pair donor.
𝐁𝐅𝟑 𝐚𝐪 + 𝐍𝐇 𝟑 𝐥 → 𝐅𝟑 𝐁 − 𝐍𝐇 𝟑 (𝐚𝐪)
Lewis acid Lewis base

8. The pH Scale
The acidity of a substance can be
determined by measuring the
concentration of hydronium H3O+ (or
H+) in solution.
pH is related to the concentration of
hydronium H3O+.
pH =  log [H3O+]

9. The pH Scale
pH =  log [H3O+]
The lower the pH, the more acidic the
solution.
The higher the pH, the more basic.
acid
14neutral0
7 base

10. Strong versus Weak Acids
Analyze the two solutions shown below.
What is different about them? What is similar?
H3O+
Cl
H3O+
F
HF

11. Strong versus Weak Acids
The solution on the left (HCl) shows the acid dissolved
completely into ions.
The beaker on the right (HF) shows a solution with very
few HF molecules dissolved in solution
H3O+
Cl
H3O+
F
HF

12. Strong versus Weak Acids
HCl is a strong acid
𝐇𝐂𝐥 𝐚𝐪 + 𝐇 𝟐 𝐎 𝐥 → 𝐇 𝟑 𝐎+
𝐚𝐪 + 𝐂𝐥−
(𝐚𝐪)
HF is a weak acid
𝐇𝐅 𝐚𝐪 + 𝐇 𝟐 𝐎 𝐥 ⇌ 𝐇 𝟑 𝐎+
𝐚𝐪 + 𝐅−
(𝐚𝐪)

13. Strong and Weak Bases
Bases can be categorized in exactly
the same way.
Strong bases dissolve completely
into ions in solution
Example:
𝐍𝐚𝐎𝐇 𝐚𝐪 → 𝐍𝐚+
𝐚𝐪 + 𝐎𝐇−
𝐚𝐪

14. Strong and Weak Bases
Weak bases only partially dissolve
into ions in solution.
Examples:
𝐌𝐠 𝐎𝐇 𝟐 𝐚𝐪 ⇌ 𝐌𝐠 𝟐+
𝐚𝐪 + 𝟐 𝐎𝐇−
𝐚𝐪
𝐍𝐇 𝟑 𝐚𝐪 + 𝐇 𝟐 𝐎 𝐥 ⇌ 𝐍𝐇 𝟒
+
𝐚𝐪 + 𝐎𝐇−
(𝐚𝐪)

15. Conjugate Acid-Base Pairs
When an acid donates a proton, a
conjugate base is formed as well.
𝐇𝐂𝐥 𝐚𝐪 + 𝐇 𝟐 𝐎 𝐥 → 𝐇 𝟑 𝐎+
𝐚𝐪 + 𝐂𝐥−
(𝐚𝐪)
𝐇𝐅 𝐚𝐪 + 𝐇 𝟐 𝐎 𝐥 ⇌ 𝐇 𝟑 𝐎+
𝐚𝐪 + 𝐅−
(𝐚𝐪)
acid conjugate
base for HCl
acid conjugate
base for HF

16. Conjugate Acid-Base Pairs
When a base accepts a proton, a
conjugate acid is formed as well.
𝐍𝐇 𝟑 𝐚𝐪 + 𝐇 𝟐 𝐎 𝐥 ⇌ 𝐍𝐇 𝟒
+
𝐚𝐪 + 𝐎𝐇−
(𝐚𝐪)
base conjugate
acid for NH3

17. Conjugate Acid-Base Pairs
Conjugate acid-base pairs are two
related species differing only by a
proton (H+)
𝐇𝐅 𝐚𝐪 + 𝐇 𝟐 𝐎 𝐥 ⇌ 𝐇 𝟑 𝐎+
𝐚𝐪 + 𝐅−
(𝐚𝐪)
𝐍𝐇 𝟑 𝐚𝐪 + 𝐇 𝟐 𝐎 𝐥 ⇌ 𝐍𝐇 𝟒
+
𝐚𝐪 + 𝐎𝐇−
(𝐚𝐪)
base conjugate acid for NH3
acid conjugate
base for HF

18. Neutralization Reactions
An acid and a base react to form
water and a “salt”.
𝐇𝐂𝐥 𝐚𝐪 + 𝐍𝐚𝐎𝐇 𝐚𝐪 → 𝐇 𝟐 𝐎 𝐥 + 𝐍𝐚𝐂𝐥(𝐚𝐪)
𝐇𝐅 𝐚𝐪 + 𝐍𝐇 𝟑 𝐚𝐪 ⇌ 𝐇 𝟐 𝐎 𝐥 + 𝐍𝐇 𝟒 𝐅(𝐚𝐪)
acid “salt”
acid “salt”
base
base

19. What you Should Be Able to Do
Recognize acids and bases.
Write the chemical reaction for
the dissolution of an acid in water.
Write the chemical reaction for
the dissolution of a base in water.

20. What you Should Be Able to Do
Identify strong and weak acids and
bases. (See the list of strong acids
and bases to memorize.)
Identify conjugate acid-base pairs.
Write the chemical reaction for an
acid reacting with a base
(neutralization reaction).

21. Next up,
The Autoionization of Water
(Pt 2)