NCEA L3 Chemistry key terms

The heat energy exchanged with the surroundings during a reaction at constant pressure, usually measured in kJ per mole. A negative value means energy is released; a positive value means energy is absorbed.

A reaction that releases energy to the surroundings, so the temperature of the surroundings rises and the enthalpy change is negative. Combustion and neutralisation are common examples.

A reaction that absorbs energy from the surroundings, so the surroundings cool and the enthalpy change is positive. Dissolving ammonium salts and thermal decomposition are typical examples.

The minimum energy colliding particles need before a reaction can occur. It is the height of the energy barrier between reactants and the transition state, and a catalyst lowers it.

A substance that increases the rate of a reaction by providing an alternative pathway with lower activation energy, while being chemically unchanged overall. It does not alter the position of equilibrium or the enthalpy change.

The average energy needed to break one mole of a particular covalent bond in the gas phase. Summing bond enthalpies broken minus bond enthalpies formed gives an approximate reaction enthalpy.

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

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

The most stable physical form of an element or compound at standard pressure (100 kPa) and a stated temperature, usually 298 K. Enthalpy values are quoted relative to these conditions.

A measure of the dispersal or spreading of energy among the available microstates of a system. Systems tend toward higher entropy, and the entropy change of the universe determines spontaneity.

A thermodynamic quantity combining enthalpy and entropy: ΔG = ΔH − TΔS. A negative ΔG indicates a spontaneous process at that temperature, while a positive value indicates it is non-spontaneous.

A process that occurs without continuous external input of energy under the stated conditions. It corresponds to a decrease in Gibbs free energy and an overall increase in the entropy of the universe.

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, and is often expressed in mol L⁻¹ s⁻¹.

The model stating that particles must collide with sufficient energy (at least the activation energy) and correct orientation for a reaction to occur. It explains how concentration and temperature affect rates.

The power to which a reactant's concentration is raised in the experimentally determined rate equation. It shows how strongly the rate depends on that reactant and is not deduced from the balanced equation.

An experimentally determined expression linking reaction rate to reactant concentrations, for example rate = k[A]²[B]. The constant k is the rate constant and its units depend on the overall order.

The slowest step in a multi-step reaction mechanism, which limits the overall rate. The rate equation reflects the species involved up to and including this step.

A proposed sequence of elementary steps describing how reactants are converted into products. Mechanisms must be consistent with the observed rate equation and stoichiometry.

A species formed in one step of a reaction mechanism and consumed in a later step, so it does not appear in the overall equation. It is distinct from a catalyst, which is regenerated.

The high-energy arrangement of atoms at the peak of the energy barrier during an elementary step. Bonds are partly broken and partly formed, and it cannot be isolated.

A state in a reversible reaction where forward and reverse rates are equal, so concentrations remain constant while both reactions continue. It requires a closed system.

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. Catalysts do not shift equilibrium position.

The ratio of product concentrations to reactant concentrations at equilibrium, each raised to its stoichiometric coefficient. It is constant at a given temperature and indicates the extent of reaction.

A logarithmic measure of hydrogen ion concentration: pH = −log₁₀[H⁺]. Values below 7 indicate acidic solutions at 25 °C, and each unit represents a tenfold change in [H⁺].

An acid that ionises almost completely in water, such as hydrochloric or nitric acid. Its conjugate base is very weak, and [H⁺] approximates the acid's concentration.

An acid that only partially ionises in water, establishing an equilibrium with its conjugate base. It has a small acid dissociation constant and a pH higher than a strong acid of equal concentration.

The equilibrium constant for the partial ionisation of a weak acid in water. A larger Ka means a stronger acid; pKa is its negative logarithm.

Two species that differ by a single proton (H⁺). When an acid donates a proton it becomes its conjugate base, and when a base accepts a proton it becomes its conjugate acid.

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

A graph of pH against volume of titrant added during a titration. Its shape reveals acid or base strength, and the equivalence point is where stoichiometric amounts have reacted.

The point in a titration where the reactants have been mixed in the exact mole ratio given by the balanced equation. It is not necessarily at pH 7 and is found from the titration curve's steepest region.

The loss of electrons by a species, accompanied by an increase in oxidation number. It is always paired with reduction in a redox reaction.

The gain of electrons by a species, accompanied by a decrease in oxidation number. In electrochemistry it occurs at the cathode.

A bookkeeping value assigned to an atom in a compound, reflecting electron loss or gain relative to the element. Rules based on electronegativity and charge allow it to be calculated.

A reaction involving simultaneous oxidation and reduction, in which electrons are transferred between species. The total number of electrons lost equals the number gained.

An equation showing only the electrons gained or lost by one species in a redox process. Combining two balanced half-equations gives the overall redox equation.

The voltage of a half-cell measured against the standard hydrogen electrode under standard conditions (298 K, 100 kPa, 1 mol L⁻¹). More positive values indicate a greater tendency to be reduced.

The reference half-cell assigned a potential of exactly 0.00 V, consisting of hydrogen gas at 100 kPa bubbling over platinum in 1 mol L⁻¹ H⁺. All standard electrode potentials are measured relative to it.

A device converting chemical energy into electrical energy (galvanic) or electrical energy into chemical change (electrolytic). It contains two electrodes in contact with electrolytes.

The use of an external electrical supply to drive a non-spontaneous chemical reaction in an electrolyte. Oxidation occurs at the anode and reduction at the cathode.

The electrode where oxidation occurs. In a galvanic cell it is the negative terminal, while in an electrolytic cell it is the positive terminal connected to the power supply.

The electrode where reduction occurs. In a galvanic cell it is the positive terminal, while in an electrolytic cell it is the negative terminal.

A molecule with the same molecular formula as another but a different arrangement of atoms or bonds. Types include chain, position, and functional group isomers.

A molecule with the same molecular formula and atom connectivity as another but a different spatial arrangement. Includes geometric (cis-trans) and optical isomers.

The property of a molecule that cannot be superimposed on its mirror image, usually because a carbon atom is bonded to four different groups. Chiral molecules rotate plane-polarised light.

One of a pair of non-superimposable mirror-image stereoisomers of a chiral molecule. The two enantiomers rotate plane-polarised light in opposite directions.

An atom or group of atoms that gives a class of organic molecules its characteristic chemical reactivity. Examples include hydroxyl, carboxyl, amino, and ester groups.

A family of organic compounds with the same functional group, similar chemical properties, and successive members differing by one CH₂ unit. Physical properties change gradually along the series.

A systematic set of rules for naming organic compounds based on the longest carbon chain, functional group priority, and substituent positions. It allows any structure to be named unambiguously.

A reaction in which two molecules combine to form a single product, typically by breaking a double or triple bond. Alkenes and alkynes undergo addition with hydrogen, halogens, and water.

A reaction in which one atom or group in a molecule is replaced by another, leaving the rest of the molecule largely unchanged. Alkanes and haloalkanes undergo substitution.

A reaction in which two atoms or groups are removed from adjacent carbon atoms to form a double bond, often producing an alkene. Haloalkanes with base and alcohols with acid catalyst are common examples.

When a hydrogen halide adds to an unsymmetrical alkene, the hydrogen adds to the carbon with more existing hydrogens, giving the more substituted product. The rule reflects carbocation stability.

A positively charged carbon species with only three bonds, formed as an intermediate in many organic reactions. Tertiary carbocations are more stable than primary ones due to alkyl group donation.

The joining of many small monomer molecules into a large chain molecule. Addition polymerisation opens double bonds, while condensation polymerisation releases a small molecule such as water.

A sugar with a free aldehyde or ketone group that can reduce mild oxidising agents such as Tollens' or Fehling's reagent. Examples include glucose and maltose, but not sucrose.

An amide linkage formed between the carboxyl group of one amino acid and the amino group of another, releasing water. It links amino acids in proteins and peptides.