Acids, Bases, Ionization, and pKa: Summary Notes

Acid: Donates $H^+$ , becoming $H_3O^+$ (hydronium ion).
Base: Accepts $H^+$ , reducing $H_3O^+$ concentration (or increasing $OH^-$ ).
pH: Logarithmic measure of $[H^+]$ in solution.
- Expressed as $-log[H^+]$ .
- Pure water: $[H^+] = 10^{-7}$ , pH = 7.
- Acids: High $[H^+]$ , pH < 7.
- Bases: Low $[H^+]$ , pH > 7.
Ionized: Has a charge due to loss or gain of $H^+$ .
Protonated: Has the proton (may be charged or uncharged).
Dissociated: Not attached to the proton.
Conjugate Acid-Base Pairs: Differ by the presence or absence of a proton.

pH: Measures amount of free $H^+$ in solution; general.
pKa: Describes acidity of a specific molecule; the lower the pKa, the stronger the acid.
Weak acids don't completely ionize.
Relationship between pH and pKa:
- When pH = pKa, 50% of the substance is ionized, and 50% is unionized.
- Weakly acidic drug:
  - pH > pKa by 2 units: completely ionized.
  - pH < pKa by 2 units: completely unionized.
- Weakly basic drug: Opposite of acidic drugs.
Henderson-Hasselbalch Equation: $pH = pKa + log \frac{[A-]}{[HA]}$ (acidic drugs) or $pH = pKa + log \frac{[base]}{[salt]}$ (basic drugs).

Factors affecting acid strength:
- Stability of conjugate base (anion).
- Resonance stabilizes the anion, increasing acidity.
Basicity:
- Availability of lone pair electrons determines strength.
- Aromatic amino groups are weaker bases than aliphatic ones.

Definition: A solution that resists changes in pH.
Typically a mixture of an acid and its conjugate base.
Important for maintaining enzyme reactions and cell functions at optimum pH.
How they work:
- Equilibrium between acid and base.
- Adding $H^+$ shifts equilibrium to the left, absorbing $H^+$ . Removing $H^+$ shifts it to the right, releasing $H^+$ .