MATSEC AM Chemistry key terms

The amount of substance containing exactly 6.022 × 10²³ specified entities (atoms, molecules or ions). It links mass, particle number and gas volume in quantitative calculations.

The number of constituent particles in one mole, 6.022 × 10²³ mol⁻¹. It converts between counted particles and amount of substance in stoichiometric calculations.

The mass of one mole of a substance, expressed in g mol⁻¹. Numerically equal to the relative formula mass, it converts between mass and amount of substance.

The simplest whole-number ratio of atoms of each element in a compound. It is found from percentage composition or combustion data, then scaled to the molecular formula using molar mass.

The actual number of atoms of each element in one molecule of a compound. It is a whole-number multiple of the empirical formula, determined from relative molecular mass.

The reactant that is completely consumed first and therefore determines the maximum amount of product formed. Other reactants are in excess and remain after reaction.

The actual mass of product obtained expressed as a percentage of the theoretical maximum calculated from stoichiometry. Losses arise from side reactions, incomplete reaction and handling.

The molar mass of the desired product as a percentage of the total molar masses of all reactants, measuring how efficiently atoms end up in the target product.

The volume occupied by one mole of gas. At standard conditions it is about 22.7 dm³ mol⁻¹ (22.4 dm³ mol⁻¹ at older STP), allowing volumes to be calculated from moles.

The relationship pV = nRT linking pressure, volume, amount of gas, the gas constant R and absolute temperature. Used to calculate any one variable when the others are known.

A volumetric procedure in which a solution of known concentration is added from a burette to a measured volume of analyte until the equivalence point, indicated by a colour change.

A solution of accurately known concentration, prepared by dissolving a weighed mass of pure solute in a volumetric flask and making up to a fixed volume.

The point in a titration at which the amount of titrant added exactly matches the stoichiometric amount required to react with the analyte, determined from the titration curve.

The point at which the indicator changes colour during a titration. It should coincide with the equivalence point, and the chosen indicator determines how closely it does.

A weak acid or base whose colour differs between its protonated and deprotonated forms, changing colour over a specific pH range near the equivalence point of a titration.

The heat energy transferred at constant pressure when molar quantities of reactants react as stated in the equation. Exothermic reactions give negative values; endothermic reactions give positive values.

The enthalpy change when one mole of a compound forms from its elements in their standard states at 298 K and 100 kPa. Values for elements in standard states are zero.

The total enthalpy change of a reaction is independent of the route taken, provided initial and final states are the same. It allows unknown enthalpy changes to be calculated from known ones.

The average energy required to break one mole of a particular covalent bond in the gaseous state. Used in Hess cycles to estimate reaction enthalpies from bonds broken minus bonds formed.

The minimum energy that colliding particles must possess for a reaction to occur. It corresponds to the height of the energy barrier between reactants and the transition state.

The highest-energy arrangement along a reaction pathway, in which bonds are partially broken and formed. It cannot be isolated and sits at the peak of the energy profile.

The change in concentration of a reactant or product per unit time. It depends on concentration, temperature, surface area and the presence of a catalyst.

A substance that increases reaction rate by providing an alternative pathway of lower activation energy, without being consumed overall. It is regenerated at the end of the reaction.

The power to which the concentration of a reactant is raised in the experimentally determined rate equation. Orders may be zero, fractional or negative and must be found by experiment.

The proportionality constant k in the rate equation. It is independent of concentration but changes with temperature according to the Arrhenius equation.

The slowest step in a multi-step reaction mechanism, which limits the overall rate. Species involved in this step appear in the experimental rate equation.

A state in a closed system where forward and reverse reactions occur at equal rates, so concentrations remain constant though both processes continue.

When a system at equilibrium is disturbed by a change in conditions, the position of equilibrium shifts to oppose that change, as far as possible.

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

A species that donates a proton (H⁺) to another species. Its conjugate base is what remains after the proton has been lost.

Two species differing by one proton. The acid donates the proton and the base accepts it, so every Brønsted–Lowry acid has a corresponding conjugate base.

The negative logarithm to base ten of the hydrogen ion concentration in mol dm⁻³. Values below 7 indicate acidity at 298 K; a change of one unit means a tenfold change in concentration.

A solution that resists changes in pH when small amounts of acid or alkali are added. It contains a weak acid and its conjugate base in comparable amounts.

A measure of the tendency of an atom in a bond to attract the shared pair of electrons towards itself. Differences in electronegativity produce polar bonds and dipoles.

A strong intermolecular attraction between a hydrogen atom bonded to N, O or F and a lone pair on a nearby electronegative atom. It raises melting and boiling points.

Attractive forces between separate molecules, including London dispersion forces, permanent dipole–dipole attractions and hydrogen bonds. They are weaker than covalent or ionic bonds.

The property of a molecule that is not superimposable on its mirror image, usually because a carbon atom carries four different groups. Enantiomers rotate plane-polarised light in opposite directions.

Isomerism in which molecules have the same structural formula but different spatial arrangements. It includes geometric (E/Z) isomerism about double bonds and optical isomerism of chiral molecules.

One of a pair of chiral molecules that are non-superimposable mirror images of each other. They have identical physical properties except for the direction they rotate polarised light.

An electron-deficient species that accepts a pair of electrons from an electron-rich centre. It attacks double bonds in electrophilic addition and substitutes into aromatic rings.

An electron-rich species with a lone pair that donates it to an electron-deficient centre. It attacks halogenoalkanes in substitution and carbonyl carbons in addition.

A positively charged carbon intermediate with three bonds and an empty p orbital. Its stability increases from primary to secondary to tertiary, which controls product distribution in addition reactions.

A step-by-step description of how a reaction proceeds, showing bond breaking and forming, the movement of electron pairs and the intermediates formed along the pathway.

A species that accepts electrons from another species, causing it to be oxidised while itself being reduced. Its oxidation number decreases during the reaction.

A reaction in which electrons are transferred from a reducing agent to an oxidising agent, so that oxidation and reduction occur simultaneously with changes in oxidation number.

The charge an atom would have if the compound were fully ionic, assigned from rules such as oxygen being −2 and hydrogen +1 in most compounds. It tracks electron transfer in redox reactions.

The potential of a half-cell measured against the standard hydrogen electrode under standard conditions (298 K, 100 kPa, 1 mol dm⁻³). More positive values indicate stronger oxidising agents.

A device in which a redox reaction is separated into two half-cells, producing a potential difference and driving electrons through an external circuit.

The decomposition of a substance by passing an electric current through it, either molten or in solution. Oxidation occurs at the anode and reduction at the cathode.

A d-block element that forms at least one ion with a partially filled d subshell. Such elements show variable oxidation states, coloured ions and catalytic activity.

An ion or molecule that donates one or more lone pairs to a central metal ion, forming coordinate bonds in a complex. Ligands may be monodentate, bidentate or polydentate.

A species consisting of a central metal ion surrounded by ligands bonded through coordinate bonds. It typically has a characteristic shape and colour.

The repeating pattern of physical and chemical properties of elements across periods and down groups, explained by changes in atomic radius, ionisation energy and shielding.

The energy needed to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions. It depends on nuclear charge, distance and shielding.

A method in which the masses of carbon dioxide and water produced on burning a compound give the moles of carbon and hydrogen, and hence the simplest formula.