Overview
This lecture covers the calculations and key concepts related to strong and weak acids and bases, including reaction equations, pH calculations, and usage of equilibrium constants.
Strong Acids and Bases
- Strong acids and bases fully dissociate in water to produce H⁺ and OH⁻ ions.
- The reaction between an acid and a base forms a conjugate base and a conjugate acid.
- The stronger the acid, the weaker its conjugate base, and vice versa for bases.
- Water can act as both an acid and a base (amphoteric).
- The auto-dissociation of water creates hydronium (H₃O⁺) and hydroxide (OH⁻) ions.
- The ionic product of water, Kw, is Kw = [H⁺][OH⁻] = 1 × 10⁻¹⁴ at 24°C.
pH Calculations for Strong Acids and Bases
- pH = –log[H⁺]; for monoprotic strong acids, [H⁺] = acid concentration.
- Example: 0.2 M nitric acid → pH = –log(0.2) = 0.7.
- For strong bases, pOH = –log[OH⁻]; pH = 14 – pOH.
- Example: 0.3 M sodium hydroxide → pOH = –log(0.3) = 0.5; pH = 13.5.
- Alternatively, use Kw to find [H⁺] from [OH⁻] and then calculate pH.
pH and [H⁺] Examples
- To find pH from HCl (0.5 M): pH = –log(0.5) = 0.3.
- To find [H⁺] from pH (pH = 9.6): [H⁺] = 10⁻⁹.⁶ ≈ 2.5 × 10⁻¹⁰ M.
Weak Acids and Bases
- Weak acids only partially dissociate in water; degree of dissociation affects pH.
- The equilibrium constant for weak acid dissociation is Ka = [H⁺][A⁻]/[HA].
- pKa = –log(Ka); lower pKa means a stronger acid.
- pH of a weak acid: pH = ½(pKa – log C), where C is the acid concentration.
- Ka and pKa values for common weak acids are typically provided in data books.
pH Calculations for Weak Acids (Examples)
- For 0.2 M ethanoic acid (pKa = 4.76): pH = ½(4.76 – log 0.2) = 2.73.
- For 0.3 M carbonic acid (pKa = 6.35): pH = 3.44.
- For 0.47 M benzoic acid (pKa = 4.2): pH = 2.27.
Key Terms & Definitions
- Conjugate Base — species formed when an acid loses an H⁺ ion.
- Conjugate Acid — species formed when a base gains an H⁺ ion.
- Kw (Ionic Product of Water) — product of [H⁺] and [OH⁻], equals 1 × 10⁻¹⁴ at 24°C.
- Ka — equilibrium constant for weak acid dissociation.
- pKa — negative logarithm of Ka, indicates acid strength.
- pH — negative logarithm of hydrogen ion concentration.
Action Items / Next Steps
- Practice pH calculations for both strong and weak acids and bases using the provided examples.
- Use your data book to find pKa values for weak acid calculations.
- Review and memorize the formulas for pH, pOH, Ka, and Kw.
- Know which acids and bases are strong or weak for correct equation application.