PRC Chemist key terms

The SI base unit for amount of substance, defined as exactly 6.02214076 × 10^23 elementary entities. It converts countable particles into weighable masses through molar mass.

The mass of one mole of a substance in grams per mole, numerically equal to its formula mass in atomic mass units. Used to convert between mass and moles in calculations.

The quantitative relationship between amounts of reactants and products in a balanced chemical equation. It uses mole ratios from coefficients to predict yields and required quantities.

The reactant that is completely consumed first and therefore caps the maximum amount of product formed. Identified by comparing available moles against the equation's mole ratios.

The maximum mass or amount of product predicted from stoichiometry when the limiting reagent is fully converted. Actual yield divided by theoretical yield, times 100, gives percent yield.

The number of constituent particles in one mole, fixed at 6.02214076 × 10^23 per mole. It links microscopic particle counts to macroscopic laboratory measurements.

A logarithmic scale of hydrogen ion activity, defined as the negative base-10 logarithm of the hydrogen ion concentration in dilute aqueous solution. Values below 7 are acidic at 25 °C.

An aqueous system that resists pH change on addition of small amounts of strong acid or base, typically a weak acid with its conjugate base. Capacity depends on concentrations and ratio.

A relationship giving pH from the acid dissociation constant and the ratio of conjugate base to weak acid concentrations. It is the standard tool for designing and checking buffer systems.

An equilibrium constant for the loss of a proton from a weak acid, written as the product of hydrogen ion and conjugate base concentrations divided by undissociated acid concentration. Larger values mean stronger acids.

A bookkeeping charge assigned to an atom in a compound by distributing shared electrons to the more electronegative partner. Tracking changes identifies oxidation and reduction in redox reactions.

The potential of a half-cell measured against the standard hydrogen electrode under standard conditions of unit activity and defined temperature. Combining half-cell values predicts cell voltage and reaction spontaneity.

A relation expressing cell potential as the standard potential minus a term proportional to the logarithm of the reaction quotient. It accounts for concentration effects on electrochemical driving force.

A volumetric procedure in which a standard solution is added until the analyte has reacted completely, as signaled by an endpoint. The delivered volume and known concentration give the analyte amount.

The stage in a titration where the moles of titrant added exactly match the stoichiometric requirement of the analyte. It is a theoretical point, detected experimentally via an indicator or instrument.

The observable point at which an indicator changes color or an instrument signals completion during a titration. Ideally it coincides with the equivalence point, but a small titration error may remain.

A highly pure, stable, non-hygroscopic reagent used to prepare a solution of accurately known concentration or to standardize another solution. Purity and reproducibility are essential.

A solution whose concentration is known with high accuracy, either prepared from a primary standard or standardized against one. It is the reference against which unknowns are measured.

An electroanalytical technique measuring the potential of an indicator electrode at near-zero current to infer analyte activity. Common applications include pH measurement and ion-selective electrodes.

A relationship stating that absorbance is proportional to molar absorptivity, path length, and analyte concentration. It underpins quantitative ultraviolet-visible spectrophotometry and calibration curves.

A plot of instrument response against known analyte concentrations, used to determine an unknown by interpolation. Good practice requires blank correction and checking linearity and detection limits.

The lowest analyte concentration that can be reliably distinguished from background noise, typically three times the standard deviation of blank measurements divided by calibration slope. It differs from the limit of quantification.

The ratio of the measured analyte signal to the random background fluctuation. Higher ratios improve precision and lower the smallest concentration that can be reliably reported.

A separation method in which components distribute differently between a mobile phase and a stationary phase, causing them to migrate at different rates. Variants include gas and liquid chromatography.

The elapsed time between sample injection and the appearance of a component's peak maximum in chromatography. It supports qualitative identification when compared with standards under identical conditions.

A measure of how completely two adjacent chromatographic peaks are separated, calculated from retention times and peak widths. Adequate resolution is needed for accurate quantitation of each component.

An analytical technique measuring how much light a sample absorbs or transmits at selected wavelengths. Absorbance data are converted to concentration via the Beer–Lambert law.

A specific grouping of atoms within an organic molecule that largely determines its chemical behavior and reactivity. Examples include hydroxyl, carbonyl, carboxyl, amino, and ester groups.

A species that donates an electron pair to form a new bond, often an anion or a molecule with a lone pair. Nucleophilicity depends on charge, polarizability, and solvent.

A species that accepts an electron pair to form a new bond, typically electron-deficient such as a carbocation or a polarized carbonyl carbon. It is attacked by nucleophiles in many organic reactions.

A reaction in which one atom or group is replaced by another. In organic chemistry, nucleophilic substitution at saturated carbon proceeds by SN1 or SN2 mechanisms.

A reaction that removes two atoms or groups from adjacent positions, forming a double or triple bond. It often competes with nucleophilic substitution under basic or heated conditions.

The geometric property of a molecule that is not superimposable on its mirror image, usually due to a carbon bearing four different substituents. Chiral molecules rotate plane-polarized light and exist as enantiomers.

One of a pair of stereoisomers that are non-superimposable mirror images of each other. They have identical physical properties in achiral environments but differ in optical rotation and biological activity.

The branch of chemistry dealing with the three-dimensional arrangement of atoms in molecules and its effect on properties and reactions. It covers chirality, geometric isomerism, and conformational analysis.

A thermodynamic state function combining internal energy with the product of pressure and volume. Changes in enthalpy at constant pressure equal the heat exchanged with surroundings.

A state function measuring the dispersal of energy and the number of accessible microstates in a system. The second law requires total entropy of an isolated system to increase for spontaneous change.

A state function defined as enthalpy minus the product of temperature and entropy. A negative change at constant temperature and pressure indicates a thermodynamically spontaneous process.

The state in which forward and reverse reaction rates are equal, so concentrations remain constant over time. The equilibrium constant expresses the ratio of product to reactant activities at that state.

A ratio of product activities to reactant activities, each raised to its stoichiometric coefficient, at equilibrium. Its value at a given temperature indicates the extent of reaction.

A rule stating that a system at equilibrium shifts to partly offset a change in concentration, pressure, or temperature. It qualitatively predicts the direction of response to disturbance.

The change in concentration of a reactant or product per unit time. It depends on concentrations, temperature, catalysts, and the activation energy of the pathway.

The minimum energy that colliding molecules must possess to convert into products. It appears in the Arrhenius equation and is lowered by catalysts without changing the equilibrium position.

An equation expressing reaction rate as a function of reactant concentrations and a rate constant, with experimentally determined orders. It cannot generally be inferred from overall stoichiometry alone.

A biological catalyst, usually a protein, that accelerates a specific reaction by lowering activation energy. Activity depends on pH, temperature, substrate concentration, and cofactors.

A model describing enzyme-catalyzed reaction velocity as a function of substrate concentration, characterized by the Michaelis constant Km and maximum velocity Vmax. It assumes rapid equilibrium binding and steady-state catalysis.

A non-protein component required for an enzyme's catalytic activity, such as a metal ion or an organic coenzyme. It may bind transiently or tightly to the enzyme.

A molecule containing both an amino group and a carboxylic acid group, typically with a side chain that determines its properties. Twenty standard amino acids are linked by peptide bonds to form proteins.

An amide linkage formed between the carboxyl group of one amino acid and the amino group of another, releasing water. Its partial double-bond character gives the backbone rigidity.

The hierarchical organization of a protein: primary sequence, local secondary motifs, overall tertiary fold, and quaternary assembly of subunits. Each level contributes to function and stability.

The cytoplasmic pathway that converts glucose to pyruvate through a sequence of enzyme-catalyzed steps, producing ATP and NADH. It functions both with and without oxygen.

A nucleotide that stores and transfers chemical energy through hydrolysis of its terminal phosphate bond. It powers many cellular processes and is regenerated by catabolism.

A substance that increases reaction rate by providing a lower-energy pathway without being consumed overall. It changes kinetics but not the equilibrium constant or thermodynamics.

The amount of desired product obtained from a process, expressed as a fraction or percentage of the theoretical maximum. It is a key economic and environmental performance measure.

A separation method based on differences in boiling point, in which vapor enriched in the more volatile component is condensed. Variants include simple, fractional, and vacuum distillation.

A technique in which vapor from a boiling mixture is condensed and returned to the vessel, maintaining constant volume during prolonged heating. It is common in organic synthesis and industrial columns.

A separation process transferring a solute from one phase into another in which it is more soluble, often an aqueous phase and an immiscible organic solvent. Repeated extraction improves recovery.

A design philosophy aiming to reduce or eliminate hazardous substances and waste across chemical processes. Its principles include atom economy, safer solvents, and energy efficiency.

The fraction of reactant atoms incorporated into the desired product, calculated from molar masses in a balanced equation. High atom economy reduces by-product waste.