NSEC key terms

The amount of substance containing exactly 6.022 × 10²³ specified elementary entities such as atoms, molecules or ions. It links particle counts to measurable masses through molar mass.

The number of elementary entities in one mole, equal to 6.022 × 10²³ per mole. It converts between individual particles and macroscopic amounts in calculations.

The mass of one mole of a substance in grams per mole, numerically equal to its formula mass in atomic mass units. It converts moles to grams and back.

The quantitative relationship between amounts of reactants and products as given by a balanced chemical equation. It allows prediction of masses, moles or volumes consumed and formed.

The reactant that is completely consumed first in a reaction and therefore determines the maximum amount of product formed. Others remain in excess.

The formula showing the simplest whole-number ratio of atoms of each element in a compound. It may differ from the molecular formula by an integer factor.

The formula giving the actual number of atoms of each element in one molecule of a compound. It is an integer multiple of the empirical formula.

The volume occupied by one mole of a gas, equal to 22.4 L at 273.15 K and 1 bar for an ideal gas. It is used for gas-phase stoichiometry.

The equation PV = nRT relating pressure, volume, amount and absolute temperature of a gas through the gas constant R. Real gases approach it at low pressure and high temperature.

The pressure that one component of a gas mixture would exert if it alone occupied the whole volume at the same temperature. Total pressure is the sum of partial pressures.

The ratio of the number of moles of one component to the total number of moles in a mixture. It is dimensionless and its values over all components sum to one.

The number of moles of solute dissolved per kilogram of solvent. Unlike molarity it is independent of temperature because it uses mass rather than volume.

The number of moles of solute per litre of solution. It changes slightly with temperature because solution volume expands or contracts.

The number of gram equivalents of solute per litre of solution. It depends on the reaction context because the equivalent weight of a species can vary.

The mass of a substance that supplies or reacts with one mole of a chosen unit such as hydrogen ions or electrons. It equals molar mass divided by the n-factor.

The partial vapour pressure of each volatile component in an ideal solution equals its mole fraction multiplied by the vapour pressure of the pure component. It underpins colligative property calculations.

A property of dilute solutions that depends on the number of solute particles rather than their identity. Examples are osmotic pressure, boiling point elevation and freezing point depression.

The excess pressure that must be applied to a solution to prevent net flow of solvent across a semipermeable membrane. It equals iCRT for dilute solutions.

The ratio of the observed colligative effect to that expected for the undissociated solute. It exceeds one for dissociating solutes and is below one for associating solutes.

A liquid mixture that boils at constant temperature and distils without change in composition because vapour and liquid have identical composition. It cannot be separated by simple distillation.

Energy cannot be created or destroyed; the change in internal energy of a system equals heat added to it minus work done by it. It defines internal energy as a state function.

The heat content of a system at constant pressure, defined as internal energy plus pressure times volume. Its change equals heat exchanged in a constant-pressure process.

The enthalpy change of a reaction is the same whether it occurs in one step or several steps. It allows calculation of unknown enthalpy changes from known ones.

The enthalpy change when one mole of a compound forms from its elements in their standard states at 298 K and 1 bar. Values for elements in standard states are zero.

A measure of the dispersal or randomness of energy and matter in a system. The second law states that the total entropy of an isolated system never decreases.

A thermodynamic potential defined as enthalpy minus temperature times entropy. A negative change at constant temperature and pressure indicates a spontaneous process.

The tendency of a process to proceed on its own without external work once initiated. It requires a negative Gibbs free energy change at constant temperature and pressure.

A property whose value depends only on the current state of the system, not on the path taken to reach it. Examples are internal energy, enthalpy and entropy.

When a system at equilibrium is disturbed by a change in concentration, pressure or temperature, it shifts in the direction that partly opposes the change. It predicts the direction of equilibrium shift.

The ratio of product concentrations or partial pressures to reactant values, each raised to its stoichiometric coefficient, at equilibrium. Its magnitude indicates how far a reaction proceeds.

The same ratio as the equilibrium constant but computed at any instant using current concentrations. Comparing it with the equilibrium constant predicts the direction of net change.

The negative base-10 logarithm of hydrogen ion activity in a solution. It indicates acidity on a scale where 7 is neutral at 298 K.

A solution that resists large changes in pH when small amounts of acid or base are added. It typically contains a weak acid with its conjugate base or the reverse pair.

A species that donates a proton to another species in a reaction. Its partner after donation is a conjugate base.

The amount of added acid or base a buffer can absorb before its pH changes appreciably. It is greatest when the acid and conjugate base concentrations are comparable.

A number assigned to an atom in a compound reflecting its apparent charge if bonding electrons were fully assigned to the more electronegative atom. It is used to track redox changes.

A reaction in which one species loses electrons and another gains them, so oxidation and reduction occur together. Total electron loss equals total electron gain.

A device that converts chemical energy into electrical energy, or the reverse, through redox reactions at two electrodes. Electrons flow through an external circuit.

The potential of a half-cell measured against the standard hydrogen electrode under standard conditions. Its sign and magnitude indicate the tendency of a species to be reduced.

The maximum potential difference between the two electrodes of a cell when no current flows. It equals the difference of the two electrode potentials under standard conditions.