Water and Aqueous Chemistry

These vocabulary flashcards cover the fundamental properties of water, non-covalent forces, aqueous chemistry, buffer systems, and the thermodynamics of aqueous reactions based on the lecture notes.

sp3 hybridization

The electron structure in the valence shell of the oxygen atom in water, consisting of four sets of electrons (2 sets bonding and 2 sets non-bonding).

Electronegativity

An atom's ability to attract electrons to itself within a covalent bond.

Bent molecular shape

The specific structure of water produced by its electron arrangement, which is an essential structural property combined with electronegativity to yield a polarized molecule.

Dipole arrows

Arrows used by scientists to indicate the direction of higher electron density, pointing toward the more electronegative atom.

Autoionization

A compound’s ability to react with itself to form ions, such as water breaking down into equal parts acid ($H^+$) and base ($OH^-$).

Heat of vaporization

The amount of heat needed to change a liquid into a gas (vapor) for one gram of substance; water's value is $540\,\text{cal/g}$.

Non-covalent forces

Attractive and repulsive forces that occur between two molecules, resulting from partial positive ($\delta+$) and partial negative ($\delta-$) charges.

Dipole-dipole forces

Intermolecular forces common in polar molecules that form when two molecules align their dipoles to generate an attraction.

Hydrogen donor

A hydrogen atom covalently bonded to either a nitrogen, oxygen, or fluorine atom in a hydrogen bond interaction.

Hydrogen acceptor

A pair of non-bonded electrons on a separate oxygen, nitrogen, or fluorine atom that interacts with a hydrogen donor.

Hydrogen bonds

The strongest class of dipole-dipole interactions, requiring both an H donor and an H acceptor on different molecules.

London dispersion or London force

The weakest type of non-covalent interaction characterized by a temporary, transient arrangement of electrons within a molecule.

Covalent bonds

Permanent bonds where atoms share electrons, which are significantly stronger than non-covalent interactions (e.g., an $O-H$ bond is approximately $400\,\text{kcal}$).

Hydrophilic

Molecules that are polar and charged, making them typically soluble in water ("water-loving").

Hydrophobic

Nonpolar molecules, like oil, that do not dissolve in water ("water-hating").

Clathrate

An ordered cage structure that water forms around the hydrophobic portion of nonpolar regions to maximize hydrogen bonds.

Hydrophobic effect

The packing of nonpolar regions to minimize interaction with water, driven by the spontaneous drive of water to bind to itself through hydrogen bonds.

Equilibrium

A condition achieved when the rates of the forward and reverse reactions are equal.

Equilibrium constant ($K_{eq}$)

A unitless number that describes the extent to which a reaction proceeds in the forward direction.

Ion product ($K_w$)

The specific equilibrium constant for water, determined experimentally to be $1.0 \times 10^{-14}$.

pH

A scale ranging from 0 to 14 used to express acid concentration, calculated as $pH = -\log[H^+]$.

Physiological pH

The pH of the cytoplasm for a typical human cell, which is $pH = 7.4$.

Strong acids

Compounds like hydrochloric acid and sulfuric acid that dissolve in water and wholly ionize.

Weak acids or bases

Molecules that dissolve in solution but only partially ionize.

Proton donor

A term for a weak acid because it generates an $H^+$ in its reaction.

Proton acceptor

The corresponding anion produced in a weak acid reaction that can accept a proton to reform the acid.

Acid dissociation constant ($K_a$)

An equilibrium constant indicating the tendency of a weak acid to lose its proton; a larger value indicates a stronger acid.

pKa

The negative log of the $K_a$ value ($pKa = -\log(K_a)$), where a smaller value indicates a stronger acid.

Buffer

An aqueous system that resists changes in pH when small amounts of acid ($H^+$) or base ($OH^-$) are added.

Buffering range

The span of $\pm 1.0$ pH units of a weak acid's pKa within which the buffer works best.

Buffering capacity

The limit to the amount of acid or base a buffer system can neutralize.

Bicarbonate buffer system

A complex buffering system effective in the bloodstream where $CO_2$ reacts with water to form carbonic acid, which then generates bicarbonate ions and protons.

Enthalpy ($H$)

A reflection of the number and kinds of bonds in a chemical system.

Entropy ($S$)

A measure of the randomness or disorder of the components of a chemical system.

Free-energy change ($\Delta G$)

The measure of change in a system's free energy, calculated as $\Delta G = \Delta H - T\Delta S$; a process is spontaneous only if $\Delta G$ is negative.