Acids and Bases

Ka < 1

• HA retains protons

• left side of equilibrium favored

• HA is not a strong acid

Ka > 1

• HA coughs up proton

• Right side of equilibrium favored

• HA is a strong acid

Autoionization of water

Kw = KaKb = [H3O+][OH-]

• Kw = 1.0 × 10^-14

Low pKa means ______.

the substance is acidic

High pKa means _______.

the substance is more base-like

Negative pKa reflects a ______.

strong acid

Common Strong acids

• HCl

• HBr

• HI

• H2SO4

• HNO3

• HClO4

HCl

hydrochloric acid

• strong acid

HBr

hydrobromic acid

• strong acid

HI

hydroiodic acid

• strong acid

HNO3

nitric acid

• strong acid

HClO4

perchloric acid

• strong acid

Common Weak acids

• H3PO4

• H2SO3

• HCN

• CH3COOH

• HF

H3PO4

Phosphoric acid

• weak acid

H2SO3

Sulfurous acid

• weak acid

HCN

Hydrocyanic acid

• weak acid

CH3COOH

acetic acid

• weak acid

HF

Hydrofluoric acid

• weak acid

Solvent leveling effect

Water is leaving the playing field

• the relative acid strength of the 6 strong acids has been “leveled” due to the strength of the solvent (base).

How do we measure the relative strengths of the strong acids?

Use a solvent (the base) that is less basic aka more acidic

Main HSAB Principle

• Hard bases tend to combine with hard acids

• Soft bases tend to combine with soft acids

General Features of Soft Acids aka metals

• High electronegativity (only high for metals = about 1.9 - 2.5)

• large size

• low charges (0, +1, +2)

• Form a triangle around the most electronegative metal in the periodic table aka gold; heavily shaded areas = the softest acids around gold

General Features of Soft Bases aka non-metals

• Moderate electronegativity (higher than most metals)

• large size of non-metals

General Features of Hard Bases (oxygen and fluorine)

• Very high electronegativity (3.44, 3.98)

• Smallest size of non-metals

General Features of Hard acids (includes bulk of periodic table)

• Low electronegativity (0.7 - 1.6)

• Small size

• High charges (greater or equal to +3)

How to use HSAB Principle to simple Chemical Equilibria

1. Determine charges

2. Identify Lewis acids aka metals

3. Go to periodic table → what is closest to the triangle of soft acids? close = soft; far = hard

4. Identify Lewis bases aka non-metals

5. Compare electronegativities (hard base = high; soft base = low)

6. Label reactant and products (HA, SB, SA, or HB)

7. Determine which side of reaction is favored → remember HA and HB combine and vice versa

When using HSAB for chemical equilibria, for polyatomic anions ______.

determine the electronegativity of the Lewis base donor atom

Bronsted-Lowry acid

donates a proton (H⁺) to another species

Bronsted-Lowry base

species that accepts a proton

Lux-Flood acid

Oxide ion acceptor

Lux-Flood base

Oxide ion donor

Solvent System Concept

Applies to solvents that can autodissociate/autoionize

Acid Solvent System Concept definition

Cation resulting from the autodissociation of the solvent

Base Solvent System Concept definition

Anion resulting from the autodissociation of the solvent

For the solvent system concept, solutes that serve to increase the concentration of acids in this system are _______.

also considered acids; those that increase the concentration of bases are considered bases

• A solution does not need to be just a liquid phase.