Water Chemistry, Non-Covalent Interactions & pH Fundamentals

Water – Centrality to Life

  • Life emerged in water; oceans shielded early biomolecules from harsh UV.
  • Modern organisms are 70−90%70-90\% water; humans: 55−80%55-80\% depending on age, sex, metabolic state.
  • Roles across scales:
    • Molecular: solvent for biochemical reactions; dictates folding & activity of proteins, nucleic acids, membranes.
    • Cellular: major component of cytoplasm, interstitial fluid, blood plasma (≈90%90\% water).
    • Physiological: temperature regulation (sweat, respiration), lubrication (saliva, mucus), shock absorption (CSF, amniotic fluid), transport (blood), waste removal (urine, feces).
  • Thirst mechanism: ↓body H₂O → ↓blood volume → ↑osmolarity → ↓BP → ↑angiotensin II + hypothalamic osmoreceptor firing → dry mouth → thirst → fluid intake restores osmolarity.

Molecular Structure of Water

  • Formula H2O\text{H}_2\text O; oxygen atomic number 8.
  • Electron configuration: 1s22s22p41s^2 2s^2 2p^4 → two lone pairs in 2p.
  • Geometry: distorted tetrahedron; ∠H–O–H≈105∘\angle\text{H–O–H}≈105^{\circ} (vs ideal 109.5∘109.5^{\circ} in sp3sp^3).
  • Electronegativity difference O>H draws shared electrons toward O → partial charges ((\delta^-) on O, (\delta^+) on H) → permanent dipole.

Hydrogen Bonds (H-Bonds)

  • Definition: electrostatic attraction between H covalently bound to a highly EN atom (O, N) and a lone-pair bearing EN atom.
  • Donor: group containing the covalent H; Acceptor: EN atom with lone pair.
  • Linear arrangement (donor–H···acceptor colinear) maximises strength (4-6 kJ mol⁻¹ neutral, 6-10 kJ mol⁻¹ if one participant charged).
  • Water can donate and accept → each molecule can form up to 4 H-bonds.
    • Ice (hexagonal lattice): ≈4 H-bonds/molecule.
    • Liquid: ≈3.4 dynamic H-bonds (lifetime 1–20 ps) due to entropy-driven rearrangement.

Physical Consequences of H-Bonding

  • High b.p. 100∘C100^{\circ}\text C, high m.p. 0∘C0^{\circ}\text C for small MW molecule.
  • High heat capacity & heat of vaporisation → thermal buffer for organisms.
  • High surface tension, cohesion & adhesion (capillarity).
  • Ice less dense than liquid (open lattice) → floats, insulating aquatic life.

Non-Covalent Interactions (Overview)

  1. Ionic (electrostatic) – attraction/repulsion between full charges; important in salt bridges.
  2. Hydrogen bonds – polar but uncharged species; detailed above.
  3. Van der Waals (London + steric repulsion) – universal; weak (~0.4 kJ mol⁻¹ each) but cumulative; dictate steric complementarity (e.g. base stacking in DNA).
  4. Hydrophobic effect – entropy-driven association of non-polar groups in water; not due to mutual attraction but exclusion by H-bonded water.

Hydrophobic Effect & Amphipathic Self-Assembly

  • Water near non-polar surface becomes ordered ((\downarrow S)); system minimises this by clustering hydrophobes → releases ordered H₂O → (\uparrow S), (\downarrow G).
  • Drives:
    • Protein folding (non-polar core).
    • Membrane bilayers & micelles: amphipathic lipids aggregate; polar heads face water, non-polar tails sequestered.
    • Binding of hydrophobic ligands (steroids) into enzyme/receptor pockets.

Colligative vs Non-Colligative Properties

  • Colligative (depend only on solute #, not nature): boiling-point elevation, freezing-point depression, vapor-pressure lowering, osmotic pressure.
  • Non-colligative (depend on solute identity): viscosity, taste, colour, surface tension.

Osmosis & Osmolarity

  • Semi-permeable membrane allows H₂O not solute.
  • Osmotic pressure π\pi: pressure required to prevent net H₂O influx.
  • Cellular environments:
    • Isotonic – equal osmolarity; cell volume stable.
    • Hypertonic – extracellular > intracellular; H₂O leaves, cell shrinks (exosmosis).
    • Hypotonic – extracellular < intracellular; H₂O enters, cell swells/lyses (endosmosis).

Acids, Bases, Conjugates

  • Acid: proton donor (low H affinity) e.g. HCl,CH3COOH\text{HCl}, \text{CH}_3\text{COOH}.
  • Base: proton acceptor e.g. OH−,NH<em>3,CH</em>3COO−\text{OH}^- , \text{NH}<em>3 , \text{CH}</em>3\text{COO}^-.
  • Conjugate pairs differ by one H⁺ (more protonated = acid).

Auto-Ionisation of Water & Ionic Product

H2O⇌H++OH−\text{H}_2\text O \rightleftharpoons \text{H}^+ + \text{OH}^-

  • Equilibrium constant K<em>eq=[H+][OH−][H</em>2O]K<em>{eq}=\dfrac{[\text H^+][\text{OH}^-]}{[\text{H}</em>2\text O]}.
  • At 25∘C25^{\circ}\text C: Keq=1.8×10−16K_{eq}=1.8\times10^{-16}.
  • Molarity of pure water [H<em>2O]=55.5  M[\text{H}<em>2\text O]=55.5\;\text M → ionic productK</em>w=[H+][OH−]=1.0×10−14  (M2)K</em>w=[\text H^+][\text{OH}^-]=1.0\times10^{-14}\;(\text M^2).
  • In neutral water: [H+]=[OH−]=10−7  M[\text H^+]=[\text{OH}^-]=10^{-7}\;\text M.

pH, pOH & Scale

pH=−log⁡[H+]pOH=−log⁡[OH−]\text{pH}=-\log[\text H^+] \qquad \text{pOH}=-\log[\text{OH}^-]

  • Relationship: pH+pOH=14\text{pH}+\text{pOH}=14 (at 25∘C25^{\circ}\text C).
  • Acidic: \text{pH}<7 (e.g. cola ≈ 3).
  • Neutral: pH=7\text{pH}=7 (pure water).
  • Basic: \text{pH}>7 (blood ≈ 7.4; 1 M NaOH ≈ 14).
  • pH < 0 or > 14 possible for strong concentrated acids/bases.

Key Numerical Constants & Equations

  • Kw=1.0×10−14  M2  (25∘C)K_w=1.0\times10^{-14}\;\text M^2\;(25^{\circ}\text C).
  • [H2O]pure=1000 g L−118 g mol−1=55.5 M[\text{H}_2\text O]_{pure}=\dfrac{1000\,\text{g L}^{-1}}{18\,\text{g mol}^{-1}}=55.5\,\text M.
  • π=CRT\pi=CRT (van ’t Hoff osmotic pressure; C = molar concentration, R = 0.0821 L·atm·K⁻¹·mol⁻¹, T = Kelvin).
  • ΔG=ΔH−TΔS\Delta G = \Delta H - T\Delta S links hydrophobic effect ((\uparrow S) lowers (\Delta G)).

Worked Examples

  1. [\text H⁺] = 10−510^{-5} M → pH=5\text{pH}=5; [OH−]=Kw[H+]=10−9 M[\text{OH}^-]=\dfrac{K_w}{[\text H^+]}=10^{-9}\,\text M.
  2. Solution pH = 8.3 ⇒ [H+]=10−8.3=5.0×10−9 M[\text H^+]=10^{-8.3}=5.0\times10^{-9}\,\text M; [OH−]=2.0×10−6 M[\text{OH}^-]=2.0\times10^{-6}\,\text M.
  3. One-liter water mass = 1000 g → 55.5 mol; confirms [H₂O] for equilibrium calculation.

Concept Checks / Exam-Type Q&A

  • Why does water have a higher boiling point than H₂S? • Extensive 3-D H-bond network; requires more energy to disrupt.
  • Which interaction(s) drive lipid micelle formation? • Hydrophobic effect + underlying H-bonding of surrounding water.
  • Predict solubility:
    • Glucose – many –OH groups → highly soluble.
    • Hexane – non-polar C chain → insoluble.
  • Calculate pH of 0.01 M HCl (strong acid): pH=−log⁡(10−2)=2pH=-\log(10^{-2})=2.
  • Protein tertiary structure: hydrophobic core, surface H-bond & ionic interactions = direct application of water chemistry.
  • DNA double helix: base pairing via H-bonds; base stacking via van der Waals.
  • Acid–base balance in physiology (blood pH 7.35–7.45) relies on bicarbonate buffer – covered in next lecture.
  • Pharmacology: drug design targets hydrophobic pockets, exploits enthalpy/entropy trade-offs.
  • Environmental science: ice floating prevents oceans from freezing solid, sustaining marine ecosystems.

Summary Bullets

  • Water’s polarity & 4-way H-bonding make it the ideal biological solvent.
  • H-bond network endows water with anomalously high thermal & cohesive properties vital for life.
  • Four key non-covalent forces (ionic, H-bond, van der Waals, hydrophobic) collectively stabilise macromolecular structure/function.
  • Hydrophobic effect is entropy-driven exclusion by water, not mutual lipid attraction; drives membranes & protein folding.
  • Cell survival demands osmotic balance; isotonic milieu prevents lysis/crenation.
  • Auto-ionisation of water sets KwK_w, forms basis of pH scale; pH=−log⁡[H+]\text{pH}=-\log[\text H^+].
  • Mastery of KwK_w and log rules enables quick pH/ion calculations—high-yield on MCAT, DAT.