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)