Water as Solvent and Acid-Base Chemistry — Vocabulary

Vocabulary flashcards covering water as solvent, weak interactions, acid-base chemistry, buffers, and pH concepts from the lecture notes.

Water as solvent

Dissolves salts and charged biomolecules by screening electrostatic interactions; dissolution is often entropically driven.

Dielectric constant (ε)

A dimensionless property indicating solvent polarity; water ≈ 78.5 at 25°C; benzene ≈ 4.6.

Hydrogen bonds

Electrostatic attractions between a hydrogen atom covalently bonded to an electronegative atom and another electronegative atom; key to water’s properties.

Hydrophobic effect

Nonpolar regions cluster in water to maximize water–water hydrogen bonding, increasing entropy by freeing ordered water; drives micelle formation.

Hydrophilic

Describes compounds that dissolve easily in water; typically charged or polar.

Hydrophobic

Describes nonpolar compounds (e.g., lipids, waxes) that do not dissolve well in water.

Amphipathic

Molecules with both polar/charged and nonpolar regions; polar parts interact with water while nonpolar parts cluster away.

Polar biomolecules

Biomolecules with polar groups (e.g., glucose, glycine) that interact favorably with water.

Nonpolar biomolecules

Biomolecules lacking polar groups; tend to be hydrophobic and water-insoluble.

Ionization (of water)

Water self-ionizes to give H+ (proton) and OH−; H+ is hydrated to form H3O+ (hydronium).

Equilibrium constant (Keq)

Represents the position of equilibrium for a reaction; Keq = [C][D]/[A][B] for AB ⇌ CD.

Ka (acid dissociation constant)

Ka = [H+][A−]/[HA] for HA ⇌ H+ + A−; larger Ka means a stronger acid.

pKa

−log(Ka); the pH at which an acid is half-dissociated; lower pKa = stronger acid.

Conjugate acid-base pair

Two species related by proton transfer (e.g., HA/ A−, HB+/B; H2CO3/HCO3−, NH4+/NH3).

Buffer

Aqueous system that resists pH changes when small amounts of acid or base are added; typically a weak acid and its conjugate base.

Buffering region

Flat portion of a titration curve around pKa where [HA] ≈ [A−], pH ≈ pKa.

Henderson–Hasselbalch equation

pH = pKa + log([A−]/[HA]); relates pH, pKa, and buffer composition.

Acetate buffer (acetic acid / acetate)

Buffer system with pKa ≈ 4.76: CH3COOH ⇌ H+ + CH3COO−.

Bicarbonate buffer system

In blood: H2CO3 ⇌ H+ + HCO3−; maintains around pH 7.4; involves CO2 hydration and bicarbonate.

Hydration/CO2 hydration constant (Kh)

Kh is the equilibrium constant for the hydration of CO2 to form H2CO3 in water.

Kcombined

Overall equilibrium constant for CO2 hydration and subsequent reactions; used with pKcombined to relate CO2/HCO3− to pH.

pKcombined

−log(Kcombined); approximately 6.1 at body temperature, used in blood pH calculations.

pH

A measure of hydrogen ion activity in solution; pH = −log[H+].

Kw (ion product of water)

Kw = [H+][OH−]; at 25°C, Kw ≈ 1.0 × 10−14 M2; relates [H+] and [OH−].

Hydronium ion (H3O+)

Protonated water formed when water accepts a proton during ionization.

Osmotic pressure (Π)

Force needed to resist water movement across a semipermeable membrane; Π ≈ i c R T.

Osmolarity

Product i × c; effective solute particle concentration in solution.

Osmosis

Movement of water across a semipermeable membrane driven by osmotic pressure differences.

van der Waals interactions

Weak distance-dependent attractions/repulsions between transient dipoles (London dispersion forces).

van der Waals radius

A measure of the space-filling size of an atom; helps determine how closely atoms can approach.

Proton hopping

Rapid transfer of protons between water molecules via H3O+/H2O networks.

Proton donor/acceptor in buffers

Buffers involve a weak acid (proton donor) and its conjugate base (proton acceptor).

Histidine (pKa)

Amino acid with side chain pKa near 6.0; buffers near neutral pH in enzymes.

Phosphate buffer system

In cells: H2PO4− ⇌ H+ + HPO4^2−; most effective near pH 6.86; works from ~5.9 to 7.9.

Bicarbonate buffer pH dependence

Blood pH near 7.4 depends on [HCO3−] and CO2 (pCO2); buffers adjust with respiration.

pH of blood

Typically around 7.35–7.45; buffered to ~7.4 in healthy individuals.

Acids and bases titration curves

Plot of pH vs added base; midpoint corresponds to pKa for weak acids.

Ka and pKa relationship (stronger acids)

Stronger acids have larger Ka and thus smaller pKa.

Neutral pH

At 25°C, pure water has pH = 7 (equal [H+] and [OH−]).

Amino acid buffer near physiological pH

Some amino acids (e.g., histidine) buffer effectively near pH 7 due to side-chain pKa.

Buffer capacity

Extent to which a buffer resists pH changes; depends on buffer concentration and pKa.

Hypertonic/Hypotonic/Isotonic (contextual)

Descriptions of solutions by osmolarity relative to cell cytosol; hypotonic solutions cause swelling.

Titration midpoint

The point on a titration curve where pH equals pKa; half-neutralization of the acid/base.