Acid-base equilibria

Acid-Base Equilibria

Definition and Concepts

  • An acid is a substance that can donate a proton (H⁺).
  • A base is a substance that can accept a proton.
  • The reaction of an acid and a base is called a neutralisation reaction, which involves the transfer of a proton from the acid to the base.

The pH Scale and pH Calcualtions

  • The pH of a solution is a measure of the concentration of H⁺ ions.
  • It is calculated using the formula: pH = -log[H⁺].
  • The pH scale runs from 0 to 14, where 7 is neutral, less than 7 is acidic and more than 7 is alkaline (basic).
  • The pOH of a solution is a measure of the concentration of OH⁻ ions and can similarly be calculated using the formula: pOH = -log[OH⁻].

Strong and Weak Acids/Bases

  • Strong acids/bases are completely ionised in water, meaning they fully donate/accept protons.
  • Weak acids/bases are only partially ionised in water, meaning they partially donate/accept protons.

Acid-Base Pairs and Conjugates

  • An acid-base pair consists of two substances that differ only by the presence or absence of a proton.
  • The substance that loses a proton becomes the conjugate base, and the one that accepts the proton becomes the conjugate acid.

Buffer Solutions

  • A buffer solution is a solution that maintains a nearly constant pH when small amounts of acid or base are added.
  • Buffer solutions contain large amounts of a weak acid and its conjugate base (or vice versa).
  • The Henderson-Hasselbalch equation, pH = pKa + log([A⁻]/[HA]), is useful to calculate the pH of a buffer solution.

Titration Curves and Indicators

  • A titration curve is a graph showing the change of pH during titration.
  • The midpoint of the steep part of the titration curve represents the point where equivalent amounts of acid and base have been mixed, called the equivalence point.
  • An indicator is a dye that changes colour at the pH of the equivalence point. The choice of indicator depends on the titration being performed.