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Are you studying for your IB exams and need to learn key definitions? This post outlines the top 100 IB Chemistry definitions so that you will be well-prepared to tackle the exam!
IB Chemistry Definitions
Core Definitions (SL & HL)
Stoichiometric Relationships
Relative Atomic Mass (Ar): The weighted average mass of an atom of an element compared to one-twelfth of the mass of a carbon-12 atom.
Relative Molecular Mass (Mr): The weighted average mass of a molecule compared to one-twelfth of the mass of a carbon-12 atom.
Molar Mass (M): The mass of one mole of a substance, typically expressed in units of g mol-1.
Mole: The SI unit for the amount of substance, containing exactly 6.02 × 1023 elementary entities (Avogadro's constant).
Empirical Formula: The simplest whole-number ratio of atoms of each element present in a compound.
Molecular Formula: The actual number of atoms of each element present in a single molecule of a compound.
Limiting Reactant: The reactant that is completely consumed first in a chemical reaction, thereby determining the maximum theoretical yield of products.
Theoretical Yield: The maximum quantity of product that can be obtained from a chemical reaction, assuming complete conversion of the limiting reactant.
Molar Volume: The volume occupied by one mole of any gas under specified conditions of temperature and pressure (e.g., 22.7 dm3 mol-1 at STP).
Solute: The smaller component in a solution, which is dissolved by the solvent.
Solvent: The component of a solution that is present in the greatest amount and dissolves the solute.
Concentration: The amount of solute dissolved per unit volume of solution, typically expressed in mol dm-3 or g dm-3.
Atomic Structure
Mass Number (A): The total number of protons plus neutrons in the nucleus of an atom.
Atomic Number (Z): The total number of protons present in the nucleus of an atom, which uniquely determines the identity of the element.
Isotopes: Atoms of the same element that contain the same number of protons but a different number of neutrons, resulting in different physical properties but identical chemical properties.
Continuous Spectrum: A spectrum that shows all the wavelengths, frequencies, and colors of visible light without any gaps or breaks.
Line Spectrum: A spectrum that consists of sharp, discrete, and isolated colored lines at specific wavelengths, produced by excited electrons falling down to lower energy levels.
Orbital: A region of space around an atomic nucleus where there is a high probability (usually 90% or greater) of finding an electron.
Periodicity
First Ionization Energy: The minimum energy required to completely remove one mole of electrons from one mole of gaseous atoms in their ground state to form one mole of gaseous +1 ions.
Electronegativity: The relative measure of the ability of an atom to attract a shared pair of electrons in a covalent bond toward itself.
Electron Affinity: The energy change that occurs when one mole of electrons is added to one mole of gaseous atoms to form one mole of gaseous -1 ions.
Period: A horizontal row of elements in the periodic table, representing the number of occupied electron shells.
Group: A vertical column of elements in the periodic table, containing elements with the same number of valence electrons and similar chemical properties.
Periodicity: The repeating, predictable trends in the physical and chemical properties of elements as atomic number increases across a period.
Chemical Bonding & Structure
Ionic Bond: The electrostatic attraction between oppositely charged ions (cations and anions) formed via the transfer of electrons.
Covalent Bond: The electrostatic attraction between a shared pair of electrons and the positively charged nuclei of the bonding atoms.
Metallic Bond: The electrostatic attraction between a lattice of positive metal ions (cations) and a delocalized sea of mobile valence electrons.
Dative (Coordinate) Covalent Bond: A specialized type of covalent bond where both of the shared bonding electrons are donated entirely by a single atom or species.
Resonance Structures: Two or more valid Lewis structures that can be drawn for a single molecule or polyatomic ion, where the true electron distribution is a hybrid of these structures.
Volatilization: The process where a liquid or solid turns into a vapor or gas, which happens easily in substances with weak intermolecular forces.
Energetics / Thermochemistry
Standard Enthalpy Change of Reaction (ΔHθ): The heat energy exchanged during a chemical reaction under standard conditions (100 kPa pressure, 298 K temperature) with all reactants and products in their standard states.
Standard Enthalpy Change of Combustion (ΔHcθ): The enthalpy change that occurs when one mole of a compound is completely burned in excess oxygen under standard conditions.
Standard Enthalpy Change of Formation (ΔHfθ): The enthalpy change that occurs when one mole of a compound is formed from its constituent elements in their standard states under standard conditions.
Average Bond Enthalpy: The energy required to break one mole of a specific type of covalent bond in gaseous molecules into gaseous atoms, averaged over a wide range of similar compounds.
Exothermic Reaction: A chemical reaction that releases heat energy to its surroundings, resulting in a negative enthalpy change (ΔH < 0).
Endothermic Reaction: A chemical reaction that absorbs heat energy from its surroundings, resulting in a positive enthalpy change (ΔH > 0).
Specific Heat Capacity (c): The amount of heat energy required to raise the temperature of one gram of a substance by one Kelvin (or one degree Celsius).
Chemical Kinetics
Activation Energy (Ea): The minimum amount of kinetic energy that colliding reactant particles must possess in order for a chemical reaction to occur.
Rate of Reaction: The change in the concentration of a reactant or a product per unit of time, typically measured in mol dm-3 s-1.
Catalyst: A substance that increases the rate of a chemical reaction by providing an alternative reaction pathway with a lower activation energy, without being chemically consumed in the process.
Collision Theory: A model stating that for a chemical reaction to occur, reactant particles must collide with each other, possess a minimum activation energy, and be oriented in the correct geometry.
Equilibrium
Dynamic Equilibrium: A state in a reversible reaction where the rate of the forward reaction equals the rate of the reverse reaction, causing the concentrations of reactants and products to remain constant.
Le Chatelier’s Principle: A principle stating that if a system at equilibrium is subjected to a change in conditions, the system will shift its equilibrium position to counteract and minimize that change.
Reaction Quotient (Q): A mathematical ratio of product concentrations to reactant concentrations at any given moment in time, used to determine which direction a reaction will proceed to reach equilibrium.
Acids & Bases
Brønsted–Lowry Acid: A chemical species that acts as a proton donor during a chemical reaction.
Brønsted–Lowry Base: A chemical species that acts as a proton acceptor during a chemical reaction.
Amphiprotic: A specific type of substance that can act as either a Brønsted–Lowry acid by donating a proton or a Brønsted–Lowry base by accepting a proton.
Amphoteric: A broad term for any substance that can react chemically as either an acid or a base.
Strong Acid: An acid that completely dissociates and ionizes into its constituent ions when dissolved in an aqueous solution.
Weak Acid: An acid that only partially dissociates and ionizes into its constituent ions in an aqueous solution, establishing an equilibrium state.
Conjugate Acid-Base Pair: Two chemical species that transform into each other by the gain or loss of a single proton.
Redox Processes
Oxidation: The process involving the loss of electrons, an increase in oxidation state, or the gain of oxygen by a chemical species.
Reduction: The process involving the gain of electrons, a decrease in oxidation state, or the loss of oxygen by a chemical species.
Oxidizing Agent: A substance that oxidizes other species by accepting their electrons, causing itself to be reduced in the process.
Reducing Agent: A substance that reduces other species by donating electrons to them, causing itself to be oxidized in the process.
Oxidation State: A value assigned to an atom in a compound that represents the hypothetical charge it would carry if all bonds were completely ionic.
Anode: The electrode in an electrochemical cell where oxidation takes place.
Cathode: The electrode in an electrochemical cell where reduction takes place.
Voltaic (Galvanic) Cell: An electrochemical cell that uses a spontaneous redox reaction to convert chemical energy into electrical energy.
Electrolytic Cell: An electrochemical cell that uses an external source of electrical energy to drive a non-spontaneous redox reaction.
Organic Chemistry
Homologous Series: A series of organic compounds that share the same general formula, possess similar chemical properties, show a gradual graduation in physical properties, and differ by a -CH2- structural unit.
Structural Isomers: Compounds that share the exact same molecular formula but have different structural arrangements and connectivity of atoms.
Functional Group: An atom or a specific group of atoms within an organic molecule that determines its characteristic chemical properties and reactivity.
Saturated Hydrocarbon: A hydrocarbon molecule that contains only single carbon-carbon bonds, maximizing the number of hydrogen atoms bound to the skeleton.
Unsaturated Hydrocarbon: A hydrocarbon molecule containing at least one carbon-carbon double bond, triple bond, or aromatic ring.
Measurement & Data Processing
Random Error: An unpredictable, statistical variation in measurements that causes data to fluctuate above and below the true value, which can be minimized by repeating trials.
Systematic Error: A reproducible inaccuracy in data that shifts all measurements in the same direction away from the true value, typically caused by poorly calibrated equipment or flawed experimental design.
Precision: A measure of how closely a series of experimental measurements agree with one another, regardless of whether they are close to the accepted true value.
Accuracy: A measure of how close an experimental value or average measurement is to the internationally accepted, true literature value.
Index of Hydrogen Deficiency (IHD): A mathematical value calculated from a molecular formula that indicates the total number of rings and/or pi bonds present in an organic molecule.
HL-Only Definitions
Atomic Structure & Transition Metals
Heisenberg’s Uncertainty Principle: A fundamental quantum principle stating that it is impossible to simultaneously determine both the exact position and the exact momentum of an electron with absolute certainty.
Ligand: A neutral molecule or an anion that contains at least one lone pair of electrons used to form a coordinate covalent bond with a central transition metal ion.
Transition Metal: An element that forms at least one stable ion containing an incomplete d-subshell of electrons.
Periodicity & Chemical Bonding
Formal Charge: The hypothetical charge assigned to an individual atom in a Lewis structure, calculated as valence electrons minus non-bonding electrons minus half of bonding electrons.
Sigma (σ) Bond: A covalent bond formed by the direct head-on overlap of atomic orbitals along the internuclear axis, containing a single electron density cloud.
Pi (π) Bond: A covalent bond formed by the sideways parallel overlap of unhybridized p-orbitals above and below the internuclear axis.
Hybridization: The process of mixing distinct atomic orbitals on a single atom to generate a set of new, energetically equivalent hybrid orbitals optimized for bonding.
Energetics & Kinetics
Lattice Enthalpy: The enthalpy change that occurs when one mole of a crystalline ionic compound is completely broken down into its constituent gaseous ions under standard conditions.
Standard Enthalpy Change of Atomization: The enthalpy change that occurs when one mole of gaseous atoms is formed from an element in its standard state under standard conditions.
Standard Enthalpy Change of Hydration: The enthalpy change that occurs when one mole of gaseous ions is completely dissolved in water to form an infinitely dilute aqueous solution.
Standard Enthalpy Change of Solution: The enthalpy change that occurs when one mole of an ionic substance dissolves completely in excess water under standard conditions.
Entropy (S): A thermodynamic measure of the degree of disorder, randomness, or the distribution of energy among particles within a chemical system.
Spontaneous Reaction: A reaction that can occur naturally without the continuous input of external energy, driven by a negative change in Gibbs Free Energy (ΔGθ< 0).
Order of Reaction: The power to which a specific reactant's concentration is raised in the experimentally derived rate equation, indicating how the reaction rate changes relative to that concentration.
Rate Constant (k): The constant of proportionality in the experimental rate equation that is unique to a specific reaction and is highly dependent on temperature.
Rate-Determining Step: The slowest individual elementary step in a multi-step reaction mechanism, which dictates the overall rate of the chemical reaction.
Equilibrium, Acids & Bases
Lewis Acid: An electron-pair acceptor that forms a coordinate covalent bond with an incoming electron-pair donor.
Lewis Base: An electron-pair donor that utilizes its lone pair of electrons to form a coordinate covalent bond with an electron-pair acceptor.
Buffer Solution: A solution containing a mixture of a weak acid and its conjugate base (or a weak base and its conjugate acid) that resists changes in pH when small amounts of acid or alkali are added to it.
Acid Base Indicator: A weak acid or a weak base where the conjugate acid form and the conjugate base form exhibit visibly different colors in solution.
Equivalence Point: The exact point during a titration reaction where the added titrant is chemically equivalent to the substance being analyzed in the sample.
Redox
Standard Hydrogen Electrode (SHE): A universal reference electrode utilizing a platinum electrode immersed in 1 mol dm-3 H+ ions with hydrogen gas bubbling at 100 kPa and 298 K, assigned an absolute standard electrode potential of 0.00 V.
Standard Electrode Potential (Eθ): The potential difference generated when a particular half-cell is connected to the Standard Hydrogen Electrode under standard conditions.
Organic Chemistry
Nucleophile: An electron-rich species containing a lone pair of electrons that actively attacks areas of low electron density (positive centers) in organic molecules.
Electrophile: An electron-deficient species (often carrying a formal or partial positive charge) that accepts an electron pair from an electron-dense site during an organic reaction.
Stereoisomers: Compounds that have the same molecular formula and structural connectivity but differ fundamentally in the spatial, three-dimensional arrangement of their atoms.
Enantiomers: A pair of optical isomers that form non-superimposable mirror images of each other and possess the unique ability to rotate plane-polarized light in opposite directions.
Chiral Center: A carbon atom within an organic molecule that is bonded to four completely different atoms or functional groups, giving rise to optical activity.
Cis-trans Isomers: A type of stereoisomerism that occurs due to restricted rotation about a carbon-carbon double bond or a ring structure, causing groups to be on the same side or opposite sides.
Diastereomers: Stereoisomers that are not mirror images of each other and have different physical and chemical properties.
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