Post on 12-Jan-2017
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.
Overview of Acid/Base Definitions
There are three main classifications of acids and bases1) 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.
Arrhenius Acids and Bases
Arrhenius Acids are compounds that produce hydronium H3O+ (also called “protons” H+) in aqueous soln (water).
hydronium
“proton”
Arrhenius Acids and Bases
Arrhenius Bases are compounds that produce hydroxide OH in aqueous solution (water).
Brønsted-Lowry Acids and Bases
Brønsted-Lowry acids donate (or “give away”) protons H+.
H+ on HCl is donated to water to produce hydronium
hydronium
Brønsted-Lowry Acids and Bases
Brønsted-Lowry bases accept protons H+.
Water (H2O) accepts H+ from HCl(Water acts as a base)
NH3 accepts H+ from water (H2O) (NH3 acts as a base)
Lewis Acids and Bases
A Lewis acid is an electron pair acceptor.
A Lewis base is an electron pair
donor.Lewis acid
Lewis base
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+]
The pH Scale
pH = log [H3O+]
The lower the pH, the more acidic the solution. The higher the pH, the more basic.
acid 14neutra
l0 7 bas
e
Strong versus Weak Acids
Analyze the two solutions shown below.
What is different about them? What is similar?
H3O+
Cl
H3O+
F
HF
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
Strong versus Weak Acids
HCl is a strong acid
HF is a weak acid
Strong and Weak Bases
Bases can be categorized in exactly the same way.
Strong bases dissolve completely into ions in solution
Example:
Strong and Weak Bases
Weak bases only partially dissolve into ions in solution.
Examples:
Conjugate Acid-Base Pairs
When an acid donates a proton, a conjugate base is formed as well.
acid conjugate base for HCl
acid conjugate base for HF
Conjugate Acid-Base Pairs
When a base accepts a proton, a conjugate acid is formed as well.
base
conjugate acid for NH3
Conjugate Acid-Base Pairs
Conjugate acid-base pairs are two related species differing only by a proton (H+)
base
conjugate acid for NH3
acid conjugate base for HF
Neutralization Reactions
An acid and a base react to form water and a “salt”.
acid “salt”
acid “salt”
base
base
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.
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).
Next up, The Autoionization of
Water(Pt 2)