AP Chemistry Unit 8: Complex Acid-Base Equilibria

Buffer

A solution that resists drastic changes in pH when small amounts of strong acid or strong base are added.

Common Ion Effect

The shift in equilibrium caused by the addition of a compound having an ion in common with the dissolved substance.

Le Châtelier's Principle

The principle that states when a system at equilibrium is disturbed, it will shift in the direction to counteract the disturbance.

Henderson-Hasselbalch Equation

An equation for calculating the pH of buffer solutions, given by pH = pK_a + log([A^-]/[HA]).

Buffer Capacity

The amount of acid or base a buffer can neutralize before the pH begins to change appreciably.

Strong Acid

An acid that completely dissociates in solution.

Weak Acid

An acid that partially dissociates in solution.

Strong Base

A base that completely dissociates in solution.

Weak Base

A base that partially dissociates in solution.

Equivalence Point

The point in a titration where the moles of titrant added equals the moles of substance being analyzed.

Endpoint

The point in a titration where the indicator changes color.

Titration

A quantitative analytical method to determine the concentration of an identified analyte by reacting it with a standard solution.

Net Ionic Equation

An equation that shows only the species that actually participate in the reaction.

Net Ionic Equation for Strong Acid + Strong Base

H^+ + OH^- → H2O.

Initial pH in Strong Acid-Strong Base Titration

Very low, determining the initial concentration.

Half-Equivalence Point

The point in a titration where half of the acid has been neutralized, leading to [HA] = [A^-] and pH = pK_a.

Acid-Base Reaction

A chemical reaction in which an acid and a base react to form water and a salt.

pK_a

A measure of the strength of an acid; lower pK_a indicates a stronger acid.

Polyprotic Acid

An acid that can donate more than one proton, like H3PO4.

Common Mistake 1

Assuming the equivalence point is always pH 7.

Common Mistake 2

Mixing up molarity and moles in titrations.

Common Mistake 3

Confusing Ka and Kb values.

Common Mistake 4

Misidentifying the buffer region in a titration curve.

Conjugate Base

The species that remains after an acid donates a proton.

Conjugate Acid

The species that forms when a base accepts a proton.

Spectator Ions

Ions that do not participate in the actual reaction and remain unchanged.

Buffer Composition

A buffer is typically made of a weak acid and its conjugate base, or a weak base and its conjugate acid.

pH Calculation

In weak acid titrations, pH can often be calculated using the Henderson-Hasselbalch equation.