Strong and Weak Acids & Bases: Properties, Conjugates, and Examples

Strong and Weak Acids & Bases

Classification of Acids and Bases

Acids and bases are classified as strong or weak based on their ability to dissociate (ionize) in water. This classification affects their behavior as electrolytes and their chemical reactivity.

• Strong acids and bases: Dissociate completely in water, making them strong electrolytes.

• Weak acids and bases: Dissociate partially in water, making them weak electrolytes.

Electrolytes are substances that produce ions in solution and conduct electricity. Strong acids and bases are strong electrolytes, while weak acids and bases are weak electrolytes.

Properties of Strong and Weak Acids

An acid is a substance that donates a proton (H+) in water. The strength of an acid depends on its degree of ionization.

• Strong Acid: Ionizes completely in water and readily donates a proton.

• Weak Acid: Ionizes partially in water and donates a proton less readily; favors reactants.

Examples:

Strong Acids	Weak Acids
HCl (hydrochloric acid)	HCN (hydrocyanic acid)
HNO3 (nitric acid)	CH3COOH (acetic acid)
H2SO4 (sulfuric acid)	HF (hydrofluoric acid)
HBr (hydrobromic acid)	H2CO3 (carbonic acid)

Equation for strong acid dissociation:

$\text{HA} \rightarrow \text{H}^+ + \text{A}^-$

Equation for weak acid dissociation:

$\text{HA} \rightleftharpoons \text{H}^+ + \text{A}^-$

Properties of Strong and Weak Bases

A base is a substance that accepts a proton (H+) in water. The strength of a base depends on its degree of ionization and affinity for protons.

• Strong Base: Ionizes completely in water and has a high affinity for protons.

• Weak Base: Ionizes partially in water and has a low affinity for protons; favors reactants.

Examples:

Strong Bases	Weak Bases
NaOH (sodium hydroxide)	NH3 (ammonia)
KOH (potassium hydroxide)	CH3NH2 (methylamine)
Ca(OH)2 (calcium hydroxide)	C5H5N (pyridine)
Ba(OH)2 (barium hydroxide)	CH3CH2NH2 (ethylamine)

Equation for strong base dissociation:

$\text{BOH} \rightarrow \text{B}^+ + \text{OH}^-$

Equation for weak base dissociation:

$\text{B} + \text{H}_2\text{O} \rightleftharpoons \text{BH}^+ + \text{OH}^-$

Examples of Acid Solutions

When comparing acid solutions, the strongest acid will have the highest concentration of H+ ions, while the weakest acid will have the lowest. Visual representations often show the degree of dissociation using particle diagrams.

Strength of Conjugate Acids & Bases

Inverse Relationship

There is an inverse relationship between the strength of acids and their conjugate bases.

• A strong acid has a weak conjugate base.

• A weak acid has a strong conjugate base.

Example Reaction:

$\text{HCN} (aq) + \text{H}_2\text{O} (l) \rightleftharpoons \text{H}_3\text{O}^+ (aq) + \text{CN}^- (aq)$

Here, HCN is a weak acid, so CN- is a relatively strong conjugate base.

Comparing Conjugate Strengths

• Stronger acid = weaker conjugate base

• Weaker acid = stronger conjugate base

• Stronger base = weaker conjugate acid

• Weaker base = stronger conjugate acid

Example: Among the acids HBrO4, HCN, HNO3, and HClO4, HCN has the strongest conjugate base because it is the weakest acid.

Practice Problems and Applications

Identifying Strongest and Weakest Bases

• Strongest base: C- (carbanion)

• Weakest conjugate acid: C2H5NH2 (ethylamine)

Reactant and Product Favorability

In aqueous solutions, species that are weak acids or bases favor reactants, meaning the equilibrium lies to the left and the solution contains mostly undissociated species.

• Example: Sr(OH)2 is a strong base and will contain mostly products; weak bases will contain mostly reactants.

Hydroxide Ion Concentration

The concentration of OH- ions in a base solution depends on the strength and concentration of the base.

• Strong bases (e.g., NaOH, KOH) produce higher [OH-].

• Weak bases (e.g., NH3) produce lower [OH-].

Predicting Reaction Direction

The direction of acid-base reactions depends on the relative strengths of acids and bases involved. The reaction favors the formation of the weaker acid and base (i.e., equilibrium lies toward the side with weaker species).

General Rule: The reaction proceeds from stronger acid/base to weaker acid/base.

Summary Table: Strong vs. Weak Acids and Bases

Property	Strong Acid	Weak Acid	Strong Base	Weak Base
Dissociation in Water	Complete	Partial	Complete	Partial
Electrolyte Strength	Strong	Weak	Strong	Weak
Conjugate Strength	Weak base	Strong base	Weak acid	Strong acid
Examples	HCl, HNO3	HCN, CH3COOH	NaOH, KOH	NH3, C5H5N

Additional info: The notes include practice questions and examples to reinforce the concepts of acid/base strength, conjugate relationships, and equilibrium direction. These are essential for understanding acid-base chemistry in GOB Chemistry courses.