Acids and Bases

Acid/Base Particulates

  • Strong acids fully dissociate in solution.
  • Weak acids only partially ionize.

Acid Strength

  • Increased oxygen atoms near the O-H bond weaken the attraction of the proton, strengthening the acid.
  • Larger negative ions have weaker attraction for protons, leading to stronger acids and weaker conjugate bases.
  • HI is the strongest binary acid.

Bronsted-Lowery Acids & Bases

  • Water can act as both an acid and a base (amphoteric).
    • H2O⇌H++OH−H_2O \rightleftharpoons H^+ + OH^-
    • 2H<em>2O⇌H</em>3O++OH−2H<em>2O \rightleftharpoons H</em>3O^+ + OH^-
  • Bronsted-Lowry definition:
    • Acid: proton donor.
    • Base: proton acceptor.
  • Conjugate acid-base pairs are formed through proton transfer.

pH of Weak vs. Strong Acids

  • Strong acids have 100% ionization.
  • Weak acids require more moles to achieve the same [H+] as strong acids due to partial ionization.
  • Weak acids need a larger volume of base to reach the equivalence point during titration.

Kw and Temperature

  • As temperature increases, the pH of pure water decreases, but it remains neutral ([H+]=[OH−][H^+] = [OH^-]).
  • At 50°C, neutral pH is 6.63.
  • The dissociation of water is endothermic; increased temperature increases KwK_w.

Acid/Base Reaction Species

  • When dealing with acid/base reactions, consider:
    • Is the species strong, weak, a salt, or a buffer?
    • Strong acids/bases react to completion.

Acid/Base Mixtures and pH

  • At the equivalence point in a titration:
    • Moles of acid equal moles of base.
    • For example, titrating HF with NaOH results in F−F^− which hydrolyzes with water, producing OH−OH^− and a pH > 7:
    • F−(aq)+H2O(l)⇌HF(aq)+OH−(aq)F^−(aq) + H_2O(l) \rightleftharpoons HF(aq) + OH^−(aq)

Titrations

  • Titration curves vary based on the strength of the acid and base:
    • Weak acid with strong base.
    • Weak base with strong acid.
    • Strong acid with strong base.
    • Polyprotic weak acid with strong base.

Titrations and Concentration

  • At the equivalence point, the stoichiometric molar ratio is reached.

Titration Curve Interpretation

  • Equivalence point on a titration curve:
    • Middle of the vertical section.
    • For a weak base titrated with a strong acid, the pH at equivalence point is < 7.

Finding Major Species at Half Equivalence Point

  • At half equivalence point, concentrations of weak acid and its conjugate base are equal.
  • pH equals pKa at this point.
  • Example: Titration of HC<em>2H</em>3O2HC<em>2H</em>3O_2 with NaOH; (pKa=4.7)(pKa = 4.7)
    • [HC<em>2H</em>3O<em>2]=[C</em>2H<em>3O</em>2−]>[H+]>[OH−][HC<em>2H</em>3O<em>2] = [C</em>2H<em>3O</em>2^−] > [H^+] > [OH^−]

How to Build a Buffer

  • The pH of a buffer is determined by the pKa of the weak acid.
  • When conjugate acid and base concentrations are equal, pH = pKa.
  • Choose a conjugate pair with a pKa close to the desired pH, then adjust concentrations.
  • Buffers are effective when sufficient amounts of both conjugate acid and base are present.

Buffer Mechanism

  • Buffers resist pH change by neutralizing added acids or bases.
  • Example: HF/F- buffer
    • Addition of strong base: HF(aq)+OH−(aq)⇌F−(aq)+H2O(l)HF(aq) + OH^−(aq) \rightleftharpoons F^−(aq) + H_2O(l)
    • Addition of strong acid: H<em>3O+(aq)+F−(aq)⇌HF(aq)+H</em>2O(l)H<em>3O^+(aq) + F^−(aq) \rightleftharpoons HF(aq) + H</em>2O(l)
    • Slight pH shift is governed by the hydrolysis equilibrium: HF(aq)+H<em>2O(l)⇌F−(aq)+H</em>3O+(aq)HF(aq) + H<em>2O(l) \rightleftharpoons F^−(aq) + H</em>3O^+(aq)