# 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.