Acid-Base Equilibria

We define acids and bases by the transfer of a proton (H⁺).

• Acid: A compound that donates a proton to another compound.
• Base: A compound that accepts a proton from an acid.

Conjugate Pairs

Every acid-base reaction creates conjugate pairs.

• A conjugate base is what remains after an acid donates its proton.
• A conjugate acid is formed when a base accepts a proton.

Water and the pH Scale

Water can undergo autodissociation, acting as both an acid and a base.

• In pure water, [H₃O⁺] and [OH^-] both equal 1.0 x 10^-7 M.
• pH: Calculated as -log [H₃O⁺].
• pOH: Calculated as -log [OH^-].
• Relationship: pH + pOH = 14

A solution is neutral if pH = 7, acidic if pH < 7, and basic if pH > 7. 

Polyprotic Acids

While monoprotic acids have one acidic hydrogen, polyprotic acids have more.

• Diprotic: Have 2 acidic protons (e.g., H₂SO₄, H₂CO₃).
• Triprotic: Have 3 acidic protons (e.g., H₃PO₄, Citric acid).
• Protons are lost sequentially; the first is always easier to remove than the second.

Buffers

A buffer is a solution that prevents radical pH changes when acids or bases are added.

• Composition: Contains a weak acid and its conjugate base, or a weak base and its conjugate acid.
• How it works: The acid component neutralizes added bases, while the base component neutralizes added acids.
• Biological Importance: Human blood is buffered (pH 7.35-7.45) using a CO₂/bicarbonate system.
  • Acidosis: Blood becomes too acidic due to CO₂ buildup (hypoventilation).
  • Alkalosis: Blood becomes too basic due to rapid CO₂ loss (hyperventilation).

Titrations

A lab technique using a neutralization reaction to find an unknown concentration.

• Titrant: The solution of known concentration in the buret.
• Indicator: A substance (like phenolphthalein) that changes color to signal the completion of the reaction.
• Calculations: Use the volume of titrant used (V_f - V_i) and stoichiometry to determine the unknown molarity.