Chapter 2: Water - The Solvent for Biochemical Reactions

Vocabulary flashcards covering chemical bonding, polarity, non-covalent interactions, aqueous solution properties, acids, bases, pH, pKa, Henderson-Hasselbalch equation, titrations, and biochemical buffers based on Chapter 2.

Electronegativity

A measure of the force of an atom's attraction for electrons it shares in a chemical bond with another atom.

Polar bond

A chemical bond in which electrons are unequally shared, resulting in a higher negative charge density closer to one atom due to a difference in electronegativity.

Nonpolar molecule

A molecule that may contain polar bonds but possesses a geometric symmetry that results in a net dipole moment of zero, such as $\text{CO}_2$.

Ion-dipole interaction

An electrostatic attraction between an charged ion and a polar molecule, such as when ionic compounds like $\text{KCl}$ dissolve in $\text{H}_2\text{O}$.

Dipole-dipole interaction

An electrostatic attraction between the positive dipole of one polar molecule and the negative dipole of another polar molecule, such as ethanol or acetone dissolved in water.

Dipole-induced dipole interaction

A weak electrostatic interaction between a permanent dipole in a polar molecule and a temporary dipole induced in a nearby nonpolar molecule, which generally does not lead to solubility in water.

Hydrophilic

Describing 'water-loving' substances that tend to dissolve in water, such as polar covalent compounds and ionic compounds.

Hydrophobic

Describing 'water-fearing' substances that tend not to dissolve in water, such as nonpolar hydrocarbons, fatty acids, and cholesterol.

Amphipathic molecule

A molecule containing one or more hydrophobic regions and one or more hydrophilic regions, such as sodium palmitate.

Van der Waals interactions

Weak non-covalent attractive forces between atoms or nonpolar molecules caused by instantaneous dipole moments that induce similar temporary dipoles in adjacent atoms or molecules.

Micelle

A spherical arrangement of organic amphipathic molecules in aqueous solution clustered so that their hydrophobic tails are buried inside the sphere and their hydrophilic heads are on the outer surface in contact with water.

Hydrogen bond

A non-covalent attractive interaction between dipoles in which the positive end is a hydrogen atom bonded to a highly electronegative atom (commonly $\text{O}$ or $\text{N}$) and the negative end is an atom with a lone pair of electrons (commonly $\text{O}$ or $\text{N}$).

Acid

A molecule that behaves as a proton donor in a chemical reaction.

Strong base

A molecule that behaves as a proton acceptor in a chemical reaction.

Acid dissociation constant ($K_a$)

A numerical equilibrium constant representing acid strength, defined by the equation $K_a = \frac{[\text{H}^+][\text{A}^-]}{[\text{HA}]}$.

Ion product constant for water ($K_w$)

The equilibrium constant measuring the tendency of water to dissociate into $\text{H}^+$ and $\text{OH}^-$ ions, equal to $10^{-14}$ at $25\,^\circ\text{C}$.

pH

A logarithmic measurement of the hydrogen ion concentration of a solution, defined as $\text{pH} = -\log_{10}[\text{H}^+]$.

p$K_a$

A numerical measure of acid strength defined as $\text{p}K_a = -\log_{10} K_a$, where a smaller value indicates a stronger acid.

Henderson-Hasselbalch equation

An equation connecting the $\text{p}K_a$ and ratio of weak acid to conjugate base to the solution pH: $\text{pH} = \text{p}K_a + \log\left(\frac{[\text{A}^-]}{[\text{HA}]}\right)$.

Titration

An experiment in which measured amounts of an acid (or base) are added incrementally to measured amounts of a base (or acid).

Equivalence point

The point in an acid-base titration at which enough acid has been added to exactly neutralize the base, or vice versa.

Monoprotic acid

An acid capable of releasing one $\text{H}^+$ ion per mole during dissociation.

Diprotic acid

An acid capable of releasing two $\text{H}^+$ ions per mole during dissociation.

Triprotic acid

An acid capable of releasing three $\text{H}^+$ ions per mole during dissociation.

Buffer

A solution consisting of a weak acid and its conjugate base that resists changes in pH upon addition of small to moderate amounts of a strong acid or base.

Buffer capacity

The measure of a buffer solution's resistance to pH change, which increases with higher concentrations of its weak acid and conjugate base.

TRIS

$N\text{-tris[hydroxymethyl]aminomethane}$, a commonly used biological buffer with a $\text{p}K_a$ of $8.3$.