Chapter 8

Section 8.4

Strength of Acids and Bases

• We use this scale to measure the strength of an acid or base.

• pH can use the concentration of hydronium ions or hydrogen ions

• pH can be measured with a pH meter or an indicator with a wide color range.

The pH Scale

The pH scale is a way of expressing the strength of acids and bases. Instead of using very small numbers, we just use the NEGATIVE power of 10 on the Molarity of the H+/H3O+ (or OH-) ion.

Under 7 = acid 7 = neutralOver 7 = base

The pH scale

pH Scale

pH of Common SubstancespH of Common Substances

Acid – Base Concentrations

pH = 3

pH = 7

pH = 11

OH-

H3O+OH-

OH-H3O+

H3O+

[H3O+] = [OH-] [H3O+] > [OH-] [H3O+] < [OH-]

acidicsolution

neutralsolution

basicsolution

con

cen

trati

on

(m

ole

s/L

)

10-14

10-7

10-1

Strong Acids and Bases

• Acids that ionize completely are called strong acids.

• The strength of an acid is determined by the amount of ionization.

• Bases that dissociate almost completely into its ions are called strong bases.

• HCl – hydrochloric acid

• HNO3 – nitric acid

• HClO4 – perchloric acid

• H2SO4 – sulfuric acid

List of Strong Acids

• NaOH – Sodium Hydroxide

• Ca(OH)2 - Calcium Hydroxide

• KOH – Potassium Hydroxide

• LiOH – Lithium Hydroxide

List of Strong Bases

• An acid that only slightly ionizes in a water solution is called a weak acid.

• Weak acids have only a small percent of acid molecules that donate their hydrogen, and most remain the same.

• Weak acids have a higher pH than a strong acid of the same concentration. (Remember concentration is he amount of solute dissolved in a given amount of solution)

• A base that dissociates only slightly in a water solution is a weak base.

• In these reactions the equilibrium favors the reactants over the products, so few ions form in the solution.

Weak Acids and Bases

• C2H4O2 – Acetic Acid

• CH2O2 – Formic Acid

• NH3 - Ammonia

• NH4OH - Ammonium hydroxide

List of Weak Acids and Bases

• Buffers are solutions in which the pH remains relatively constant, even when small amounts of acid or base are added

• Buffered solutions contain either:• A weak acid and its salt• A weak base and its salt

Buffers

• The operation of a buffer depends on a shift in equilibrium to counteract any changes in pH.

• Buffers are used in products and processes where the pH of the solution needs to be controlled.

• Examples: Shampoo, Medicine, and Fermentation

• Our blood is a buffer, its pH is about 7.4

Buffers

• Substances whose molecules dissociate into ions when they are placed in water.

• CATIONS (+) ANIONS (-)

• Electrolyte is a substance that when dissolved in water conducts electricity

• Strong electrolyte = completely ionized or dissociated; strongly conductive solution

• Weak electrolyte = partially ionized or dissociated; somewhat conductive solution

Electrolytes

• Many salts (ionic compounds) are strong electrolytes

• Strong Acids are strong electrolytes

• Strong Bases are strong electrolytes

• In contrast, weak acids, weak bases are weak electrolytes.

Electrolytes