Water acids based and buffer solutions

Here is the complete, high-yield summary of the Water, Acids, Bases, and Buffer Solutions lecture using standard text formatting throughout.

1. Molecular Structure & Physical Properties of Water

  • Chemical Structure & Geometry:

    • Consists of two hydrogen atoms and one oxygen atom arranged in a bent shape centered on a tetrahedron.

    • Bond Angle: 104.5° (narrower than a perfect tetrahedron's 109.5° due to strong repulsion between the two lone pairs of electrons on oxygen).

    • O-H Bond Length: ~0.096 nm.

  • Polarity & Hydrogen Bonding:

    • High electronegativity of oxygen creates partial negative charges on oxygen and partial positive charges on hydrogen, giving water a permanent dipole moment.

    • Liquid/solid water can form up to 4 hydrogen bonds with neighboring molecules.

    • Hydrogen bonds are ~10x stronger than Van der Waals interactions, accounting for water's unusually high melting and boiling points.

  • Solvent Characteristics:

    • Hydrophilic Interactions: Excellent solvent for salts, polar organic molecules (sugars, simple alcohols), and gases (O2, CO2).

    • Hydrophobic Effect: Nonpolar molecules (fats, oils, alkanes) and inorganic minerals (silicates, metal oxides) are insoluble, driving protein folding and membrane assembly.

2. Chemical Properties, Ionization & Calculations

  • Self-Ionization Equilibrium: 2 H2O <-> H3O+ + OH-

    • Free H+ ions exist as hydronium ions (H3O+) in solution.

    • In pure water at 25°C, [H+] = 1 x 10^-7 M, yielding a neutral pH = 7.0 (pH = -log10[H+]).

  • Reactivity & Conductance:

    • Pure water has low electrical conductivity, which increases upon adding ionic solutes.

    • Electrolysis splits water into H2 and O2 (requires 285.8 kJ/mol energy input).

    • Water acts as an amphoteric (acidic and basic) and nucleophilic reagent.

  • Molarity of Pure Water Calculation: Molarity = Moles of H2O / Volume in Liters = (1000 g / 18 g/mol) / 1 L = 55.5 M

3. Acids, Bases & Biological pH

  • Strength of Acids and Bases:

    • Strong Acids/Bases: Completely dissociate in water (e.g., HCl -> H+ + Cl-; NaOH -> Na+ + OH-).

    • Weak Acids/Bases: Partially dissociate in water (e.g., acetic acid, carbonic acid, organic fluids).

  • Physiological pH Maintenance:

    • Intracellular pH: ~6.8 (cytosol ~7.0).

    • Extracellular / Blood pH: 7.4.

    • Stomach Environment: Highly acidic (pH 1 to 2).

4. Buffers & Buffer Mechanisms

  • Composition: A buffer consists of a mixture of a weak acid and its conjugate base (or a weak base and its conjugate acid).

  • Mechanism of Action: Resists pH changes upon addition of small amounts of strong acid (H+) or strong base (OH-).

    • Acidic Buffer (e.g., Acetic Acid / Acetate): Added OH- + CH3COOH -> CH3COO- + H2O

    • Basic Buffer (e.g., Ammonia / Ammonium): Added H3O+ + NH3 -> NH4+ + H2O Added OH- + NH4+ -> NH3 + H2O

  • Titration Curves & Maximum Buffering Capacity:

    • Buffering Region: Extends +/- 1 pH unit on either side of the acid's pKa.

    • Maximum Buffering Capacity: Achieved at the midpoint of the titration curve where [HA] = [A-], meaning pH = pKa.

5. Key Quantitative Equation: Henderson-Hasselbalch

pH = pKa + log([Conjugate Base] / [Weak Acid]) = pKa + log([A-] / [HA])

  • Derivation stems from Ka = ([H+][A-]) / [HA].

6. Major Biological Buffer Systems

  1. Bicarbonate Buffer System (Blood):

    • Primary buffer regulating extracellular blood pH (~7.4).

    • Reaction network: H+ + HCO3- <-> H2CO3 <-> H2O + CO2 (exhaled)

    • Lungs eliminate excess CO2 gas to prevent blood acidosis.

  2. Protein Buffers (Intracellular Fluid):

    • High concentrations of cellular proteins contain weak acid/base side chains.

    • Histidine Residues: Side chain pKa ~ 6.0, making histidine-containing proteins exceptionally effective buffers near physiological pH (6.0-7.4).

  3. Clinical Correlate - Antacids:

    • Contain basic carbonate/bicarbonate ions to neutralize excess stomach acid (H+) and relieve heartburn.