Acid-Base Equilibria and Measuring Acidity: General Chemistry Study Notes

Measuring Acidity

Introduction to Acidity

Acidity is a fundamental property of aqueous solutions, determined by the concentration of hydronium ions (H3O+). The measurement of acidity is essential for understanding chemical reactivity, biological processes, and environmental chemistry.

Determining the pH and pOH of Solutions

pH Scale

• pH measures the acidity of a solution.

• Hydronium ion (H3O+) is responsible for the solution's acidity.

• pH scale ranges from 0 (most acidic) to 14 (most basic), with 7 being neutral.

• At pH 7: [H3O+] = [OH-]

• Acidic: [H3O+] > [OH-]; Basic: [OH-] > [H3O+]

Formula:

$pH = -\log[H_3O^+]$

pOH Scale

• pOH measures the alkalinity/basicity of a solution.

• Hydroxide ion (OH-) is responsible for the solution's alkalinity.

• pOH scale also ranges from 0 to 14, with 7 being neutral.

Formula:

$pOH = -\log[OH^-]$

Relationship Between pH and pOH

• The sum of pH and pOH in aqueous solution at 25°C is always 14.

Formula:

$pH + pOH = 14.00$

Acid-Base Relationships

Dissociation of Weak Acids

Acid Dissociation Constants

The acid dissociation constant (Ka) is an equilibrium constant that describes the degree of dissociation for an acid. The larger the Ka value, the greater the degree of dissociation.

Dissociation of Weak Bases

Base Dissociation Constants

The base dissociation constant (Kb) is an equilibrium constant that describes the degree of dissociation for a base. The larger the Kb value, the greater the degree of dissociation.

Acid Groups

Classification of Acids by Proton Donation

Type	Description	Examples
Monoprotic Acids	Acids that dissociate only one proton	HCl, HBr, CH3COOH, HClO4
Diprotic Acids	Acids that dissociate two protons	H2SO4, H2CO3
Triprotic Acids	Acids that dissociate three protons	H3PO4, H3AsO4

Acids that dissociate more than one proton (diprotic, triprotic) are collectively known as polyprotic acids.

Polyprotic Acids

Properties and Calculations

• Polyprotic acids generate more than one proton (H+) in solution.

• Calculations are more complex due to multiple dissociation steps.

• Dissociation occurs in a stepwise, sequential order.

• Products of the first dissociation begin the next dissociation reaction.

• Weak polyprotic acids require multiple ICE tables to determine equilibrium concentrations.

Additional info:

• For polyprotic acids, subsequent dissociation steps contribute less to the overall [H3O+] due to much smaller Ka values.

• ICE tables (Initial, Change, Equilibrium) are essential for equilibrium calculations.