Acid Dissociation Constant and pH Relationships
The acid dissociation constant Ka is a crucial parameter in understanding acid-base equilibria. It quantifies the strength of an acid in aqueous solution.
Definition: The acid dissociation constant Ka is the equilibrium constant for the dissociation of an acid in aqueous solution.
For the general acid dissociation reaction:
HA + H2O ⇌ A- + H3O+
Ka = [A−][H3O+] / HA
Vocabulary: pKa is the negative logarithm of Ka: pKa = -log Ka
The relationship between pH and pKa is given by the Henderson-Hasselbalch equation:
pH = pKa + log[A−]/[HA]
This equation is fundamental for understanding buffer solutions and is often used in exercices corrigés acide base terminale s' pdf.
Highlight: When pH = pKa, the concentrations of the acid and its conjugate base are equal: HA = A−
The pKa value determines the predominant species in a solution:
- If pH < pKa: The acid form HA predominates
- If pH > pKa: The conjugate base form A− predominates
Example: For acetic acid CH3COOH, pKa = 4.76. In a solution with pH = 3.76, the acid form will predominate.
Acid-base indicators are compounds whose acid and conjugate base forms have different colors. The color change occurs near the pKa of the indicator.
Definition: A buffer solution is an aqueous system that resists changes in pH when small amounts of acid or base are added.
Buffer solutions are important in many biological and chemical processes. They typically consist of a weak acid and its conjugate base or a weak base and its conjugate acid.
Vocabulary: Amino acids are amphoteric molecules, meaning they can act as both acids and bases. They contain both a carboxyl group −COOH and an amino group −NH2.
Understanding these concepts is essential for solving problems related to force des acides et des bases Terminale exercices and preparing for force des acides et des bases sujet bac.
