Acids and Bases Review

Acids and Bases Notes

General Properties of Acids and Bases

• Acids:

  • Have a sour taste.
  • React with metals to produce hydrogen gas and with carbonates to produce carbon dioxide.
  • Examples include acetic acid (vinegar) and citric acid (fruits).

• Bases:

  • Taste bitter and feel slippery.
  • Many soaps are bases.

Definitions of Acids and Bases

• Brønsted Acid: A proton donor.
• Brønsted Base: A proton acceptor.

Conjugate Acid-Base Pairs

• In the reaction: NH3(aq) + HF(aq) \rightleftharpoons NH4^+(aq) + F^-(aq) the pairs are:
  1. NH3 (base) and NH4^+ (conjugate acid)
  2. HF (acid) and F^- (conjugate base).

Acid-Base Properties of Water

• Water can act as both an acid and a base:
  H_2O(l) \rightleftharpoons H^+(aq) + OH^-(aq)

  • This is known as the autoionization of water with an ion product constant:
    K_w = [H^+][OH^-]

  • At 25°C, K_w = 1.0 \times 10^{-14}, indicating:

  • Neutral solution: [H^+] = [OH^-]

  • Acidic solution: [H^+] > [OH^-]

  • Basic solution: [H^+] < [OH^-]

Measuring Acidity: pH and pOH

• pH Scale:
  pH = -\log[H^+]

  • Neutral: pH = 7
  • Acidic: pH < 7
  • Basic: pH > 7

• pOH Scale:
  pOH = -\log[OH^-] and
  pH + pOH = 14

Examples of pH Calculation

1. For a solution of table wine with [H^+] = 3.2 \times 10^{-4} M:
   pH = -\log[H^+] = 3.49.
   After exposure to air, [H^+] = 1.0 \times 10^{-3} M results in pH = 3.00.

2. Rainwater with a pH of 4.82:
   Calculate [H^+]:
   [H^+] = 10^{-pH} = 10^{-4.82} = 1.5 \times 10^{-5} M.

Acid Strength and Ionization

Strong vs Weak Acids

• Strong Acids (e.g., HCl, HNO3):
  • Completely dissociate in water.
  • Equilibrium lies far to the right.
• Weak Acids (e.g., HF):
  • Do not fully dissociate; equilibrium lies to left.

Strong and Weak Bases

• Strong Bases (e.g., NaOH):
  • Fully dissociate in solution.
• Weak Bases (e.g., NH3):
  • Partially ionize to form OH^– ions.

Calculating Approximate Concentrations

• Given [OH^-] = 2.9 \times 10^{-4} M:
  • First calculate pOH = -\log [OH^-]:
    pOH = 3.54.
  • Then find pH = 14 - pOH:
    pH = 10.46.

Ionization Constants

1. Weak Acids:
   • K_a = \frac{[H^+][A^-]}{[HA]}
2. Weak Bases:
   • K_b = \frac{[B^+][OH^-]}{[B]}

Strength of Acid-Base Pairs Table**

• The stronger the acid, the weaker the conjugate base and vice-versa.
• Example:
  • Comparing HClO4 (strong acid) with HF (weak acid).
  • For weak acids, the K_a value provides information on strength.

Acid-Base Properties of Salts

• Salt Solutions:
  • Neutral: involving alkali metals and strong acid conjugates.
  • Acidic: from strong acid and weak base.
  • Basic: from strong base and weak acid.