ACS key terms

A dimensionless factor that converts a solute's concentration into its effective thermodynamic concentration, accounting for non-ideal interactions in solution. It approaches one in very dilute solutions and deviates as ionic strength or solute concentration rises.

A known amount of a substance added to every sample and calibration standard to correct for variations in injection volume, detector response, or sample preparation. It must be well separated from analytes and absent from the original sample.

A calibration strategy in which increments of the analyte are spiked directly into aliquots of the sample, so matrix effects affect standards and sample equally. The unknown concentration is extrapolated from the linear response plot.

A plot of detector response versus known analyte concentration, used to quantify unknowns. Its slope gives sensitivity, its intercept reveals bias, and its linear range defines the concentrations over which the method is trustworthy.

The smallest analyte amount or concentration that can be reliably distinguished from a blank, typically three times the standard deviation of blank measurements. It is a qualitative capability, distinct from the limit of quantitation.

The ratio of a measured signal to the random fluctuation of the baseline. Higher ratios permit confident detection and quantitation; it can be improved by averaging scans, reducing noise sources, or increasing analyte signal.

A measure of the total concentration of ions in solution, calculated as half the sum of each ion's concentration times the square of its charge. It governs activity coefficients and therefore equilibrium and electrode potentials.

In chromatography, the ratio of time an analyte spends in the stationary phase to time in the mobile phase, corrected for void time. It describes how strongly a compound is retained and is independent of column length.

A conceptual chromatographic segment in which one equilibration of analyte between mobile and stationary phases occurs. The plate number measures column efficiency; more plates give sharper peaks for a given retention time.

A chromatographic measure of separation between two adjacent peaks, based on the difference in their retention times relative to their average peak widths. A value near 1.5 generally indicates baseline separation.

An equation relating the potential of an electrochemical cell to standard potential, temperature, and the reaction quotient of the cell reaction. It underlies potentiometry, pH measurement, and interpretation of redox equilibria.

A species that donates an electron pair to form a new bond with an electron-deficient center. Nucleophilicity depends on charge, polarizability, solvent, and steric accessibility, and does not always track with basicity.

An electron-deficient species that accepts an electron pair from a nucleophile to form a bond. Common examples include carbocations, carbonyl carbons, and halonium ions generated during addition reactions.

A bimolecular nucleophilic substitution in which the nucleophile attacks the substrate from the side opposite the leaving group, passing through a single concerted transition state. The result is inversion of configuration at the reacting carbon.

A positively charged carbon species with only three bonds and an empty p orbital. Stability increases with alkyl substitution and resonance delocalization, and rearrangements such as hydride or alkyl shifts are common.

A predictive guideline stating that in addition of HX to an unsymmetrical alkene, the hydrogen adds to the carbon bearing more hydrogens, placing the electrophile to generate the more stable carbocation intermediate.

The special stability of a planar, cyclic, fully conjugated system containing 4n+2 pi electrons. Such systems satisfy Huckel's rule and exhibit delocalized bonding, altered reactivity, and distinctive magnetic properties.

The geometric property of an object that cannot be superimposed on its mirror image. In molecules it usually arises from a stereocenter bearing four different groups, and enantiomers differ in optical rotation and biological activity.

The fragment that departs with the bonding electrons during a substitution or elimination reaction. Good leaving groups are weak bases, often stabilized by resonance or by being large and polarizable halides.

One of a pair of stereoisomers that are non-superimposable mirror images of each other. They have identical physical properties in achiral environments but rotate plane-polarized light in opposite directions.

A thermodynamic state function combining enthalpy and entropy, whose change at constant temperature and pressure determines whether a process is spontaneous. Negative change indicates spontaneity; zero indicates equilibrium.

The partial molar Gibbs free energy of a component, describing how the system's free energy changes when that component is added at constant temperature, pressure, and other composition. It drives diffusion and phase equilibria.

The ratio of product activities to reactant activities at equilibrium, each raised to its stoichiometric coefficient. It is related to standard free energy change and varies with temperature according to van't Hoff behavior.

The minimum energy that colliding molecules must possess for a reaction to proceed, corresponding to the barrier between reactants and the transition state. It appears in the Arrhenius equation and controls temperature dependence of rate.

The highest-energy configuration along a reaction coordinate, at which bonds are partially broken and formed. It is a fleeting maximum, not an isolable intermediate, and its structure determines reaction rate and selectivity.

The slowest elementary step in a multistep mechanism, which limits the overall reaction rate. Its transition state has the highest energy relative to preceding intermediates, and its kinetics govern the observed rate law.

A thermodynamic measure of energy dispersal or the number of accessible microstates of a system. Positive entropy change favors spontaneity, and it connects statistical mechanics to the second law of thermodynamics.

A property of a solution that depends on the number of dissolved particles rather than their identity. Examples include boiling point elevation, freezing point depression, vapor pressure lowering, and osmotic pressure.

The energy separation of d orbitals when a metal ion sits in a ligand environment, produced by differing electrostatic interactions with the ligand set. Its magnitude explains color, magnetism, and high- or low-spin configurations.

An ion or molecule that donates one or more electron pairs to a central metal to form coordination bonds. Ligands are classified by donor atom and denticity, and they influence metal properties through electronic and steric effects.

The total number of donor atoms directly bonded to a central metal ion in a complex. Common values are four and six, and the number is governed by metal size, charge, and ligand steric demands.

The enhanced thermodynamic stability of complexes formed by polydentate ligands compared with analogous monodentate ligands. It arises largely from favorable entropy of releasing multiple small ligands upon binding.

The gradual decrease in atomic and ionic radii across the lanthanide series due to poor shielding of nuclear charge by 4f electrons. It causes yttrium and heavier transition metals to have unexpectedly similar sizes.

A model describing enzyme-catalyzed rates as a function of substrate concentration, characterized by maximum velocity and the Michaelis constant. It assumes rapid substrate binding equilibrium and a slower catalytic step.

Reduction of enzyme activity by a molecule that interferes with substrate binding or catalysis. Competitive inhibitors raise the apparent Michaelis constant, while noncompetitive and uncompetitive inhibitors lower maximum velocity.

The number of substrate molecules converted to product by one enzyme active site per unit time under saturating conditions. It reflects catalytic efficiency and equals maximum velocity divided by total enzyme active-site concentration.

The negative base-10 logarithm of the acid dissociation constant, expressing the strength of an acid. Lower values indicate stronger acids; the pKa of a functional group predicts its ionization state at a given pH.

A solution that resists pH change on addition of acid or base, usually containing a weak acid and its conjugate base. Capacity is greatest near the pKa and depends on component concentrations.

The average number of monomer units in a polymer chain. It relates number- and weight-average molecular weights and controls properties such as viscosity, glass transition, and mechanical strength.

The ratio of weight-average to number-average molecular weight, quantifying the breadth of a polymer's molecular weight distribution. A value of one indicates a perfectly uniform sample; higher values indicate broader distributions.

The temperature below which an amorphous polymer becomes rigid and glassy rather than rubbery. It reflects segmental mobility and is influenced by chain stiffness, branching, and plasticizer content.

Chemical or physical bonds connecting polymer chains into a three-dimensional network. Increased crosslink density raises stiffness and reduces swelling and solubility, converting a thermoplastic-like material into an insoluble network.