AP Chemistry: 8.4 Acid-Base Reactions and Buffers

I. Acid-Base Reactions

A. Neutralization Reactions

• Reactions between an acid and a base, typically forming a salt and water. $$ Acid + Base \rightarrow Salt + Water $$

B. Titration Titration

• A technique used to determine the concentration of an unknown solution (analyte) by reacting it with a solution of known concentration (titrant).
• Equivalence Point: The point in a titration where the moles of acid and base are stoichiometrically equivalent.
• Endpoint: The point in a titration where the indicator changes color, signaling the approximate equivalence point.

C. Strong Acid-Strong Base Titrations

• The pH at the equivalence point is 7.
• The pH curve shows a sharp change around the equivalence point.

D. Weak Acid-Strong Base Titrations

• The pH at the equivalence point is greater than 7. The conjugate base of the weak acid hydrolyzes, producing hydroxide ions.
• Initial pH is determined using an ICE Tables for the weak acid.
• Halfway to the equivalence point, $ pH = pK_a $ .
• Beyond the equivalence point, the pH is determined by the excess strong base.

E. Strong Acid-Weak Base Titrations

• The pH at the equivalence point is less than 7. The conjugate acid of the weak base hydrolyzes, producing hydronium ions.
• Initial pH is determined using an ICE Tables for the weak base.
• Halfway to the equivalence point, $ pOH = pK_b $ or $ pH = 14 - pK_b $ .
• Beyond the equivalence point, the pH is determined by the excess strong acid.

F. Polyprotic Acid Titrations

• Polyprotic acids have multiple ionizable protons. Titration curves show multiple equivalence points, one for each proton. Polyprotic Acids

II. Buffers Buffer Solutions

A. Definition

• A solution that resists changes in pH when small amounts of acid or base are added.
• A buffer typically consists of a weak acid and its conjugate base, or a weak base and its conjugate acid.

B. How Buffers Work

• The weak acid neutralizes added base, and the conjugate base neutralizes added acid.

C. Henderson-Hasselbalch Equation

• Used to calculate the pH of a buffer solution. $$ pH = pK_a + log \frac{[A^-]}{[HA]} $$ * Where $ pK_a = -logK_a $ , $ [A^-] $ is the concentration of the conjugate base, and $ [HA] $ is the concentration of the weak acid.

D. Buffer Capacity

• The amount of acid or base a buffer can neutralize before the pH begins to change significantly.
• Buffer capacity is greatest when the concentrations of the weak acid and conjugate base are equal.

E. Preparing a Buffer

• Choose a weak acid with a $ pK_a $ close to the desired pH.
• Adjust the ratio of $ [A^-]/[HA] $ to achieve the desired pH using the Henderson-Hasselbalch Equation.

F. Effective Buffer Range

• Buffers are most effective within $ \pm 1 $ pH unit of the $ pK_a $ value.