Honors Chemistry: Strengths of Acids and Bases, pH, and pOH

Vocabulary terms covering self-ionization of water, pH and pOH calculations, and the definitions and constants associated with strong and weak acids and bases.

Self-ionization

The process where water reacts with itself to produce $H_3O^+$ (hydronium) and $OH^-$ (hydroxide) ions.

Amphoteric substance

A substance, such as water, that can act as both an acid and a base.

$$K_w$$

The Ion Product Constant for Water, equal to $[H_3O^+][OH^-] = 1.0 \times 10^{-14}$ at equilibrium.

Alkaline solution

A basic solution where the hydronium ion concentration $[H_3O^+]$ is less than the hydroxide ion concentration $[OH^-]$.

pH

A measure of potential hydrogen ion concentration, calculated as the negative logarithm of the hydronium ion concentration: $pH = -\log[H_3O^+]$.

pOH

The negative logarithm of the hydroxide-ion concentration: $pOH = -\log[OH^-]$.

Concentration

Refers to the number of moles of acid or base in a given volume of solution (e.g., concentrated or dilute).

Strength

Refers to the extent of ionization of particles in a solution (e.g., strong or weak).

Strong Acid

An acid that is completely ionized in aqueous solution, such as $HCl$ or $H_2SO_4$.

Weak Acid

An acid that is only partially ionized in aqueous solution, such as vinegar or lemon juice.

Strong Base

A base that dissociates completely into metal ions and hydroxide ions, such as magnesium hydroxide, calcium hydroxide, or sodium hydroxide.

Weak Base

A base that reacts with water to form hydroxide ions and the conjugate acid of the base, such as ammonia ($NH_3$).

$K_a$ (Acid Dissociation Constant)

A value showing the strength of an acid, calculated as $K_a = \frac{[H_3O^+][A^-]}{[HA]}$. Stronger acids have larger $K_a$ values (typically $> 1$).

$K_b$ (Base Dissociation Constant)

A value that measures the strength of weak bases, calculated as $K_b = \frac{[\text{conjugate acid}][OH^-]}{[\text{base}]}$; a higher $K_b$ indicates greater strength.

Logarithm Scale (pH units)

A scale where a decrease of 1 pH unit represents a $10\times$ increase in acidity (e.g., a change from pH 7 to pH 5 is a $100\times$ increase).