Acid-Base Equilibria

Introduced to this in AP Chemistry

Common Ion Effect §

Common ion effect - The shift in equilibrium caused by the addition of a compound having an ion in common with the solute

The presence of a common ion decreases the ionization of a weak acid or a weak base

Example §

CH${}_3$COONa and CH${}_3$COOH:

CH${}_3$COONa (s) $\to$ Na${}^{+}$ (aq) + CH${}_3$COO${}^{-}$ (aq)

CH${}_3$COOH (aq) $\leftrightarrow$ H${}^{+}$ (aq) + CH${}_3$COO${}^{-}$ (aq)

CH${}_3$COO${}^{-}$ (aq) is a common ion

Buffer Solution §

Buffer Solution- Resists a change in pH when H${}^{+}$ or OH${}^{-}$ ions are added

Buffer Solution consists of:

1. A weak acid AND its conjugate base
2. A weak base AND its conjugate acid

A pair MUST be present otherwise it’s not a buffer solution

Example §

CH${}_3$COOH (aq) $\leftrightarrow$ H${}^{+}$ (aq) + CH${}_3$COO${}^{-}$ (aq)

Adding a strong acid causes a shift left and the ratio of conjugates slightly changes

Applying the ideology of LeChatelier’s Principle

Always consider ratio of conjugates when doing buffer solution

If ratio of conjugates the pH changes