VCE Chemistry key terms

The amount of substance containing 6.02 × 10^23 representative particles, used as the chemist's counting unit to relate mass, particle number and gas volume in calculations.

The number of particles in one mole, 6.02 × 10^23 per mole, allowing conversion between particle counts and amounts of substance in stoichiometric problems.

The mass of one mole of a substance in grams per mole, found by summing relative atomic masses, and used to convert between mass and amount of substance.

The weighted average mass of an element's naturally occurring isotopes compared with one-twelfth the mass of a carbon-12 atom, expressed without units on the periodic table.

Atoms of the same element with identical proton numbers but different neutron numbers, giving the same chemical properties but different masses and sometimes different stability.

The simplest whole-number ratio of atoms of each element in a compound, determined from percentage composition or experimental mass data by dividing by the smallest mole ratio.

The actual number of atoms of each element in one molecule of a compound, always a whole-number multiple of the empirical formula and found using molar mass.

The mass percentage of each element in a compound, calculated from the molar mass and the total mass of that element in the formula, used to check purity and derive empirical formulas.

The reactant that is completely consumed first in a chemical reaction, determining the maximum amount of product formed; other reactants are said to be in excess.

The maximum mass or amount of product predicted from stoichiometry when the limiting reagent is fully converted, assuming ideal conditions and no losses.

The actual yield expressed as a percentage of the theoretical yield, reflecting losses from side reactions, incomplete reactions, transfer losses or impure starting materials.

The quantitative relationship between amounts of reactants and products in a balanced equation, based on mole ratios from the coefficients of the equation.

The volume occupied by one mole of a gas, approximately 24.8 L at 25 °C and 100 kPa, used to convert between gas volume and amount of substance.

The relationship pV = nRT linking pressure, volume, amount of gas, temperature in kelvin and the gas constant, used when conditions differ from standard laboratory conditions.

Electrostatic attraction between oppositely charged ions formed when electrons are transferred from a metal to a non-metal, producing a three-dimensional lattice with high melting points.

A bond formed when two atoms share one or more pairs of electrons, typically between non-metals, producing discrete molecules or extended covalent networks.

Attraction between a lattice of positive metal ions and a sea of delocalised valence electrons, explaining conductivity, malleability and lustre of metals.

A regular repeating three-dimensional arrangement of ions, atoms or molecules held together by forces, whose strength affects melting point and hardness.

The tendency of an atom in a bond to attract shared electrons, increasing across a period and decreasing down a group, used to predict bond polarity.

A molecule with an uneven distribution of electron density caused by polar bonds that do not cancel in shape, giving partial charges and stronger dipole interactions.

Relatively weak attractions between molecules, including dispersion forces, dipole–dipole attractions and hydrogen bonding, which govern boiling point and solubility behaviour.

A strong dipole–dipole attraction between a hydrogen atom bonded to oxygen, nitrogen or fluorine and a lone pair on a nearby electronegative atom, raising boiling points markedly.

Electrons not confined to a single bond or atom but spread over a region, allowing conduction of electricity and heat in metals and graphite.

One of two or more structural forms of the same element in the same state, such as diamond and graphite for carbon, which differ in bonding and properties.

A substance with at least one dimension between about 1 and 100 nanometres, where the high surface-area-to-volume ratio produces properties different from the bulk material.

The total exposed area of a solid, which increases per unit mass as particles get smaller and generally increases the rate of reactions involving solids.

The model stating that particles must collide with sufficient energy and correct orientation for a reaction to occur, explaining how temperature, concentration and catalysts affect rate.

The minimum energy that colliding particles must possess for a reaction to proceed, represented as the energy barrier between reactants and products on an energy profile.

A substance that increases reaction rate by providing an alternative pathway with lower activation energy without being consumed, so it appears unchanged at the end.

A reaction that releases energy to the surroundings as bonds form more energy than is needed to break bonds, giving a negative enthalpy change and a temperature rise.

A reaction that absorbs energy from the surroundings because breaking bonds requires more energy than forming new bonds releases, giving a positive enthalpy change and a temperature fall.

The heat energy exchanged with the surroundings at constant pressure during a reaction, given the symbol ΔH and quoted per mole of a specified reactant or product.

The average energy required to break one mole of a particular bond in the gaseous state, used to estimate enthalpy changes from bonds broken minus bonds formed.

An experimental technique measuring heat released or absorbed using q = mcΔT for a known mass of water, from which the enthalpy change per mole can be calculated.

The energy needed to raise the temperature of one gram of a substance by one kelvin, about 4.18 J g⁻¹ K⁻¹ for water, used in calorimetry calculations.

The state in a closed system where forward and reverse reaction rates are equal, so concentrations remain constant although both reactions continue at the molecular level.

When a system at equilibrium is disturbed by a change in concentration, pressure or temperature, the system shifts to partially oppose that change and restore equilibrium.

The ratio of product concentrations to reactant concentrations at equilibrium, each raised to its stoichiometric coefficient, constant at a fixed temperature and indicating reaction extent.

The same ratio as the equilibrium constant but calculated with concentrations at any moment, compared with K to predict the direction of shift.

A proton donor; in water it releases hydrogen ions that are accepted by solvent molecules, forming hydronium ions and lowering pH.

Two species differing by a single proton, such as an acid and its base after donating a proton, linked by the reversible transfer of that proton.

Describing a species that can act as either a proton donor or a proton acceptor, such as water or the hydrogen carbonate ion, depending on the other reactant present.

A logarithmic measure of hydrogen ion concentration, pH = −log₁₀[H⁺], where each unit change means a tenfold change in acidity.

A solution containing a weak acid and its conjugate base that resists pH change when small amounts of acid or base are added, by neutralising them.

A reaction involving simultaneous oxidation and reduction, identified by changes in oxidation number and transfer of electrons between species.

A number assigned to an atom showing its apparent charge, derived from rules such as elements being zero and oxygen usually −2, used to identify redox changes.

An electrochemical cell that converts chemical energy into electrical energy using two half-cells joined by a conductor and salt bridge, driven by a spontaneous redox reaction.

The potential of a half-cell measured against the standard hydrogen electrode under standard conditions, with more positive values indicating a greater tendency to be reduced.

A cell in which an external power supply drives a non-spontaneous redox reaction, used for electroplating and extracting reactive metals from molten compounds.

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

An atom or group of atoms within a molecule that determines its characteristic chemical reactions and influences physical properties such as solubility and boiling point.

One of two or more compounds with the same molecular formula but different arrangements of atoms, leading to different physical and sometimes chemical properties.

A reaction in which two molecules combine to form one product, typical of alkenes where the double bond opens to add atoms across the carbon–carbon bond.

A reaction in which one atom or group in a molecule is replaced by another, typical of alkanes and haloalkanes reacting with halogens or hydroxide ions.

A reaction in which two molecules join with the loss of a small molecule such as water, used to build esters, amides and condensation polymers.

The acid-catalysed condensation of a carboxylic acid with an alcohol producing an ester and water, often with a characteristic fruity odour.

A large molecule made by joining many small monomer units, either by addition across double bonds or by condensation with loss of small molecules.

An analytical technique identifying functional groups from the absorption of infrared radiation at frequencies matching particular bond vibrations.

An analytical technique that ionises molecules and separates the fragments by mass-to-charge ratio, giving molecular mass and structural clues from fragmentation patterns.

A separation technique relying on different rates of movement of components between a mobile phase and a stationary phase, used to identify and quantify substances in mixtures.