NEB Class 12 Chemistry key terms

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

The number of moles of solute dissolved per litre of solution, expressed in mol/L. It changes slightly with temperature because solution volume expands or contracts.

The number of moles of solute per kilogram of solvent. Unlike molarity it is temperature independent, so it is preferred in colligative property calculations.

Gram equivalents of solute per litre of solution. It depends on the equivalent factor of the species, which changes with the reaction involved.

The simplest whole-number ratio of atoms of each element in a compound. It is derived from percentage composition and may differ from the actual molecular formula.

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

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

The actual mass of product obtained expressed as a percentage of the theoretical mass predicted by stoichiometry. It is always less than or equal to 100 percent in practice.

The quantitative study of the amounts of reactants and products in a balanced chemical equation, based on mole ratios.

The volume occupied by one mole of a gas. At standard temperature and pressure it is 22.4 litres for an ideal gas.

The relation PV = nRT linking pressure, volume, amount, and absolute temperature of a gas through the universal gas constant R.

The pressure a single gas in a 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 used in Raoult's law.

Properties of dilute solutions that depend only on the number of solute particles, not their identity. Examples include boiling point elevation and osmotic pressure.

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 actual number of particles produced by a solute in solution to the number of formula units dissolved. It accounts for dissociation or association.

For an ideal solution, the partial vapour pressure of each volatile component equals its mole fraction multiplied by the vapour pressure of the pure component.

A liquid mixture whose vapour has the same composition as the liquid, so it boils at constant temperature and cannot be separated by simple distillation.

When a system at equilibrium is disturbed by a change in concentration, pressure, or temperature, it shifts in the direction that partially opposes the change.

The ratio of product concentrations to reactant concentrations, each raised to its stoichiometric coefficient, at equilibrium. Its value depends on temperature.

The negative logarithm to base ten of hydrogen ion concentration in mol/L. It measures acidity on a scale usually from 0 to 14.

A solution that resists large changes in pH when small amounts of acid or base are added, usually made from a weak acid and its conjugate base.

The heat content of a system at constant pressure. Its change, ΔH, equals the heat absorbed or released in a reaction at constant pressure.

The total enthalpy change of a reaction is the same regardless of the route taken, provided initial and final states are identical. It follows from enthalpy being a state function.

A measure of the disorder or randomness of a system. It increases in spontaneous processes and is central to the second law of thermodynamics.

The thermodynamic quantity G = H − TS whose decrease at constant temperature and pressure predicts a spontaneous process. A negative ΔG indicates spontaneity.

An equation expressing reaction rate in terms of reactant concentrations raised to experimentally determined powers. It cannot be written from the balanced equation alone.

The sum of the exponents of concentration terms in the experimental rate law. It is determined experimentally and may differ from the stoichiometric coefficients.

The minimum extra energy that colliding reactant molecules must possess for a successful reaction. It determines how fast the rate changes with temperature.

The time required for half of a reactant to be consumed in a reaction. For a first-order reaction it is constant and independent of initial concentration.

A substance that increases reaction rate by providing an alternative pathway of lower activation energy, without being consumed in the overall reaction.

The potential difference between an electrode and its surrounding solution, measured relative to the standard hydrogen electrode. It indicates tendency to gain or lose electrons.

A device that converts chemical energy into electrical energy, or the reverse, through redox reactions at two electrodes connected by an electrolyte.

An equation relating cell potential to standard potential and reaction quotient: E = E° − (RT/nF) ln Q. It explains how concentration affects cell voltage.

The regular repetition of chemical and physical properties of elements when they are arranged in order of increasing atomic number.

The minimum energy required to remove the most loosely bound electron from an isolated gaseous atom. It generally increases across a period and decreases down a group.

The relative tendency of an atom in a molecule to attract shared electrons toward itself. It increases across a period and decreases down a group.

A d-block element whose atom or common ion has a partially filled d subshell. Such elements show variable oxidation states and form coloured compounds.

An ion or molecule that donates one or more electron pairs to a central metal ion, forming a coordinate bond in a complex.

A charged species consisting of a central metal ion bonded to surrounding ligands. Its shape and stability depend on coordination number and ligand field.

A specific group of atoms within an organic molecule that determines its characteristic chemical reactions and physical properties.

A family of organic compounds with the same functional group and general formula, in which successive members differ by one CH₂ unit.

The existence of compounds with the same molecular formula but different structural arrangements or spatial orientations, leading to different properties.

The property of a molecule that is not superimposable on its mirror image, usually due to a carbon atom bonded to four different groups.

The systematic set of rules for naming chemical compounds, ensuring each structure has a unique and internationally understood name.

A species that donates an electron pair to an electron-deficient centre, attacking positively charged or partially positive atoms in substitution and addition reactions.

An electron-deficient species that accepts an electron pair from a nucleophile. Common examples are carbocations and polarised carbonyl carbons.

In addition of a hydrogen halide to an unsymmetrical alkene, the hydrogen adds to the carbon bearing more hydrogens, giving the more stable carbocation intermediate.

A quantitative procedure in which a solution of known concentration is added gradually to a measured volume of analyte until the reaction is complete.

A weak acid or base that changes colour over a specific pH range, marking the endpoint of a titration.

A separation technique in which components of a mixture distribute differently between a stationary phase and a moving mobile phase.

A separation method based on differences in boiling points, in which a liquid is vaporised and then condensed to purify it.

The study of interaction between matter and electromagnetic radiation, used to identify substances from their absorption or emission patterns.

The mass of a substance that supplies or reacts with one mole of hydrogen ions, or one mole of electrons in redox reactions.