A2 Level Chemistry Definitions

Flashcards to help learn key Chemistry Definitions & Reactions

Standard Enthalpy of Formation (ΔHf°)

The enthalpy change when one mole of a substance is formed from its elements in their standard states.

Standard Enthalpy of Atomization (ΔHat)

The enthalpy change when one mole of gaseous atoms is formed from the element in its standard state.

Ionization Energy (IE)

The enthalpy change when one mole of gaseous atoms loses an electron to form gaseous positive ions.

Electron Affinity (EA)

The energy change when one mole of electrons is gained by one mole of gaseous atoms to become negative gaseous ions.

Lattice Energy/Enthalpy

The energy released when one mole of an ionic compound is formed from its gaseous ions.

Enthalpy of Solution (ΔHsolution)

The enthalpy change when one mole of an ionic solid dissolves in excess water to become aqueous ions.

Enthalpy of Hydration (ΔHhydration)

The enthalpy change when one mole of gaseous ions dissolves in water to give an infinitely dilute solution.

Ion Polarization

The distortion of an electron cloud (typically of an anion) caused by the electric field of a cation.

Charge Density

The concentration of charge in an ion, determined by the ratio of charge to size.

Thermal Stability

A compound's resistance to decomposition when heated, needing higher temperatures for decomposition.

Entropy (S)

A measure of disorder of a system, measured in joules per Kelvin per mole (J K⁻¹ mol⁻¹).

Molar Entropy

The entropy value of a specific substance, measured in joules per Kelvin per mole.

Entropy Change (ΔS)

The difference in entropy between products and reactants in a reaction.

Gibbs Free Energy (G)

A thermodynamic potential that measures the maximum reversible work that may be performed by a thermodynamic system at constant temperature and pressure.

Change in Gibbs Free Energy (ΔG)

Calculated using the formula: ΔG = ΔH - TΔS, where ΔH is enthalpy change, T is temperature in Kelvin, and ΔS is entropy change.

Reaction Kinetics

The study of the speed of chemical reactions and the factors that affect reaction rates.

Rate of a Chemical Reaction

The change in concentration of a reactant or product with respect to time, measured in mol dm⁻³ s⁻¹.

Average Rate

The total change in concentration divided by the total time taken.

Instantaneous Rate

The rate at a specific point in time, measured by the gradient of the tangent on a concentration-time graph.

Initial Rate

The instantaneous rate at time zero (maximum rate).

Rate Equation

A mathematical relationship between the concentrations of reactants and the rate of a reaction.

Order of Reaction

The power to which a concentration term is raised in the rate equation.

Rate Constant (k)

A proportionality constant in the rate equation, dependent on temperature.

Half-Life (t₁/₂)

The time taken for the concentration of a reactant to decrease to half its original value; t₁/₂ = ln(2)/k for first-order reactions.

Reaction Mechanism

A series of elementary steps that make up a complex reaction.

Elementary Step

An individual step in a reaction mechanism, involving the collision of no more than two species.

Intermediate

A species formed during a reaction that is consumed later in the reaction.

Rate-Determining Step

The slowest step in a multi-step reaction mechanism, which controls the overall rate of reaction.

Catalyst

A substance that increases the rate of a reaction without being consumed, providing alternative reaction pathways with lower activation energy.

Catalysis

The process or action of a catalyst.

Homogeneous catalysis

Catalyst is in the same physical state as the reactants

Heterogeneous catalysis

Catalyst is in a different physical state from the reactants

Adsorption

The bonding of reactant molecules to the surface of a solid catalyst.

Desorption

The process where product molecules leave the surface of a solid catalyst.

Solute

The substance that dissolves in a solvent.

Solvent

The liquid in which a solute dissolves

Solution

The mixture formed when a solute dissolves in a solvent.

Saturated Solution

A solution in which no more solute can dissolve; any additional solute will remain undissolved.

Unsaturated Solution

A solution that can dissolve more solute.

Solubility

The concentration of a saturated solution, typically measured in moles per dm³.

Dynamic Equilibrium

In a saturated solution, the rate at which the solid dissolves equals the rate at which ions precipitate back to solid form.

Solubility Product (KSP)

The equilibrium constant for the dissolution of a sparingly soluble ionic compound in water.

Common Ion Effect

When a common ion is added to a saturated solution, the equilibrium shifts backward, causing more solid to precipitate and reducing the solubility of the ionic compound.

Sparingly Soluble Salt

A salt that dissolves only slightly in water, easily forming a saturated solution.

Buffer Solution

A solution that resists changes to its pH when small amounts of acid or alkali are added to it.

Acid Buffer

A mixture of a weak acid and a salt of that weak acid (the conjugate base).

Alkaline Buffer

A mixture of a weak base and a salt of that weak base (the conjugate acid).

Ka (Acid Dissociation Constant)

An equilibrium constant that measures the extent to which an acid dissociates in water; defined as [H+][A-]/[HA].

pKa

The negative logarithm of the acid dissociation constant; pKa = -log(Ka).

Common Ion

An ion that is common to two or more compounds in a solution.

Henderson-Hasselbalch Equation

The mathematical relationship for calculating the pH of a buffer, where pH = pKa + log([salt]/[acid]).

Partition Coefficient (KPC)

The ratio of the concentrations of a solute in two immiscible solvents at equilibrium at a particular temperature; it is unitless.

Immiscible Solvents

Liquids that do not mix with each other and form separate layers when combined.

Solute Extraction

The process of removing a solute from one solvent by using another solvent in which the solute is more soluble.

Successive Extractions

A technique where a solute is extracted using multiple smaller portions of solvent rather than one large portion.

D-block elements

Elements in which the outermost/highest energy electron occupies a d-orbital.

Transition elements

D-block elements that have at least one ion with a partially filled d sub-level.

Complex ion

A transition metal ion dative-covalently bonded to ligands.

Ligand

A molecule or ion that forms a coordinate bond with a metal ion using its lone pair.

Coordination number

The number of bonds the central metal ion forms in a complex.

Monodentate ligand

Ligand that forms one coordinate bond with the central metal ion.

Bidentate ligand

Ligand that forms two coordinate bonds with the central metal ion.

Polydentate ligand

Ligand that forms more than one coordinate bond with the central metal ion.

Hexadentate ligand

Ligand that forms six coordinate bonds with the central metal ion

Cis-trans isomerism

Isomers where similar ligands are at 90° (cis) or 180° (trans) to each other.

Optical isomerism

Isomers that are non-superimposable mirror images of each other.

Stability constant (Kstab)

A measure of the stability of a complex ion relative to the hexaaqua ion.

Arenes (Aromatic Compounds)

Compounds based on benzene (C₆H₆), the simplest arene

Phenol

Benzene with an OH group attached directly to the ring

Phenylamine

Benzene with an NH₂ group attached directly to the ring

Benzoic acid

Benzene with a COOH group attached directly to the ring

Optical isomers

Molecules that have a chiral center and can form mirror images of each other that are not superimposable.

Enantiomers

A pair of molecules that exist in two forms that are mirror images of each other and are not superimposable on one another.

Racemic mixture

A mixture containing equal quantities of the two enantiomers.

Chiral center

A carbon atom bonded to four different groups, which creates the possibility for optical isomerism.

Optical activity

The property of enantiomers to rotate plane-polarized light in opposite directions.

Plane-polarized light

Light waves that vibrate in only one plane, created by passing ordinary light through a polarizer that blocks all other planes of light except one.

Condensation Polymerization

Polymerization where molecules form bonds through condensation reactions, eliminating small molecules.

Addition Polymerization

Polymerization between molecules with carbon-carbon double bonds; only carbon atoms in main chain; no small molecules eliminated

Polyester

Polymers with ester linkages (-COO-) in main chain

Polyamide

Polymers with amide linkages (-CONH-) in main chain

Monomer

Starting molecule used in polymerization

Repeat Unit

Smallest structural unit that repeats in polymer chain

Co-polymer

Polymer made from two or more different monomers

Ester

Functional group with -COO- linkage

Amide

Functional group with -CONH- linkage

Phenol

Hydroxyl group (-OH) attached to benzene ring

Amino acid

Compound containing both an amine (-NH₂) and a carboxylic acid (-COOH) group

Diazonium salt

Compound containing the diazonium group (-N₂⁺)

Electrophilic substitution

Reaction where an electrophile replaces a hydrogen atom in an aromatic ring

Hydrolysis

Reaction with water that breaks chemical bonds

NMR Spectroscopy

Analytical technique that determines the detailed structure of organic molecules without causing the molecule to react with anything.

Chromatography

Technique used to separate and identify components in a mixture.

Stationary Phase

Immobile phase in chromatography that remains fixed in place.

Mobile Phase

Phase in chromatography that moves through or over the stationary phase, carrying the components of the mixture.

Retention Factor (RF)

Ratio of the distance moved by a component to the distance moved by the solvent front.