Glossary


Acid (Bronsted Lowry)	A species that donates a proton (H⁺ ion) to another species. The acid donates a proton to become its conjugate base. This is the common way acids are described in A-level Chemistry.
Acid (Lewis)	A species that accepts a pair of electrons.
Activation Energy	Minimum energy required for a reaction to occur. Symbol: E_a .
Acylation	The process of adding an acyl group to an aromatic ring in organic chemistry.
Aliphatic	A type of non-aromatic organic compound that contains only carbon and hydrogen. Straight chain, branched chain and cyclic hydrocarbons (without delocalised electron systems) are aliphatic.
Alkali	A compound that, when dissolved in water, causes hydroxide ions (OH^-) to be released into the solution.
Alkyl group	Functional group in organic chemistry that contains only carbon and hydrogen atoms in hydrocarbon chains (for example, methyl -CH₃).
Alkylation	Organic process of adding an alkyl chain to an aromatic ring in organic chemistry.
Anhydrous	Without water.
Anions	Negatively charged species (formed by reduction).
Amphoteric	A compound that is able to act as an acid and a base in reactions. Aluminium oxide is a common example (Al₂O₃).
Aromatic	A type of organic compound that contains a benzene ring or carbon atoms bonded in a planar ring with delocalised electrons.
Atom	Smallest, indivisible unit of a chemical element. Has a positively charged nucleus (made up of protons and neutrons) surrounded by negatively charged electrons. Approximate size of 10^-10m.
Base (Bronsted Lowry)	A substance that accepts protons.
Base (Lewis)	A substance that can donate a lone pair of electrons.
Buffer solution (acid)	A mixture of a weak acid and its conjugate base that can resist a change to the pH of a solution.
Buffer solution (alkali)	A mixture of a strong base and its conjugate acid that can resist a change to the pH of a solution.
Calorimetry	The process of determining the energy change that occurs in a process by measuring the thermal energy change between the reactants and their surroundings.
Carbocation	A carbon atom with a positive charge, formed as an unstable intermediate in an organic reaction.
Catalyst	A species that increases the rate of a reaction by lowering the activation energy required for the reaction to occur. Catalysts can take part in a reaction, but since they are reformed at some point they are not considered reactants or products.
Cations	Positively charged species (formed by oxidation).
Chromatography	A technique used in analytical chemistry to separate a mixture based upon a substance’s affinity for either the stationary or mobile phase.
Colorimetry	A technique used to measure the absorbance or transmittance of light through a sample to determine its concentration.
Complex ion	Positively charged metal ion surrounded by smaller molecules or ions that act as ligands.
Concentration	The amount (usually moles) of a substance in a given volume of liquid (solution) or space (gases). Units are usually mol dm^-3 or g dm^-3. Both show the ‘amount’ of the substance in 1 dm³ volume.
Condensation Reaction	A reaction in which a bond is made and water (or another small molecule) is released.
Conjugate Acid	The species formed when a base accepts a hydrogen ion from an acid. For example, the conjugate acid of ammonia is NH⁺₄(aq). The base ammonia accepts a H⁺(aq) from an acid to form an ammonium (NH⁺₄) ion – its conjugate acid.
Conjugate Base	The negative species formed when an acid dissociates in water to release a hydrogen ion. For example, the conjugate base of hydrochloric acid is Cl^-(aq). The HCl loses a H⁺(aq) ion, leaving behind Cl^-(aq) – its conjugate base.
Co-ordination Number	The number of co-ordinate bonds a transition metal ion has made with ligands in a complex ion.
Displacement	The transfer of electrons between a more reactive element and a less reactive element, causing the more reactive element to displace (swap places with) the less reactive element.
Electrochemical Cell	A combination of two different half-cells that results in an electrical current flowing between one half-cell and the other.
Electrolysis	Process of using electricity to split a compound, with oxidation of anions occurring at the anode and reduction of cations occurring at the cathode.
Electrolytes	Solutions that are able to conduct electricity because they contain charged ions that are free to move.
Electrode	A solid electrical conductor that enables current to flow into (and out of) a solution or gas mixture.
Electron	A sub-atomic particle with a negative charge (relative charge of -1) and a very small mass. Found in orbitals around the nucleus of an atom.
Electron Configuration	A way of representing how the electrons within an atom or ion are arranged. Lower energy sub-shells and orbitals are always filled first. For example, 1s²2s² for boron.
Endothermic	Reaction or process in which reactants absorb energy from the surroundings. The products have more energy than the reactants. Endothermic processes have positive enthalpy changes.
Enthalpy	A measure of the thermal energy (kilojoules) absorbed or released by a substance when it undergoes a change (at constant pressure. Enthalpy is measured in kilojoules per mole (of the substance undergoing a change). Symbol: H^⦵.
Enthalpy of Combustion (standard)	The enthalpy change that occurs when one mole of substance undergoes complete combustion, measured under standard conditions. Symbol: ΔH^⦵_c.
Enthalpy of Formation (standard)	The enthalpy change that occurs when one mole of a substance is formed from its constituent elements (that make up the substance) in their standard states, measured under standard conditions. Symbol: ΔH^⦵_f.
Lattice Enthalpy	The enthalpy change that occurs when one mole of an ionic lattice is formed from its gaseous ions, measured under standard conditions.
Entropy	A measure of the disorder of a system, and the possible ways particles in a system can be arranged. The level of disorder in a system can be linked to the energy within a system, meaning the units for entropy are joules per mole. Symbol: S^⦵.
Total Entropy (change)	The overall amount of entropy change that occurs in a process, including both the change in entropy of the particles involved and their surroundings. Symbol: ΔS^⦵.
Exothermic	Reaction or process in which reactants release energy into the surroundings. The products have less energy than the reactants. Exothermic processes have negative enthalpy changes.
Free Radical	A highly reactive species with an unpaired electron in an electron orbital. Symbol: X• .
Gibbs Free Energy	The overall energy change of a system that occurs during a process, accounting for both enthalpy and entropy. The Gibbs free energy value for a process indicates whether energy will either be released or absorbed, showing how feasible the process is thermodynamically. Equation: ΔG = ΔH - TΔS.
Half-cell	An electrode placed in an electrolyte with an equilibrium of oxidation and reduction between the electrode and the electrolyte, giving the electrode an electrical potential.
Half-equation	An equation that shows how a species has lost or gained electrons within a reaction.
Hydrocarbon	Organic compound containing only carbon and hydrogen atoms.
Hydrolysis Reaction	A reaction in which a bond is broken by the addition of water.
Intermediate	A temporary species formed during a reaction that is not possible to isolate. Intermediates are often only a way of helping represent the stages that occur during an organic reaction – they are not necessarily physical or tangible species.
Ionic Equation	An equation that shows only the species changing state or reacting in a reaction. Spectator ions are not included.
Ideal Gas	A gas in which all particles do not interact with each other and the sizes of the gas molecules are incredibly small compared to the volume of space they occupy. Allows the assumption that one mole of a gas occupies the same volume of one mole of another gas at the same temperature and pressure (idea gas equation: pV = nRT).
Infra-red Spectroscopy	A technique used in organic analysis to identify functional groups present in an organic substance or mixture.
Inorganic Chemistry	Part of chemistry studying reactions between inorganic compounds and species (that are not organic).
Ion	A charged species formed when an atom (or group of atoms) loses or gains electrons.
Ionisation Energy (first)	The energy required to remove one mole’s worth of electrons from one mole’s worth of a substance in gaseous sate (measured under standard conditions).
Isomers	Compounds that have the same formula and numbers of atoms, but with a different structure.
Kinetics	The study of particles moving and rates of reaction.
K_a (Acid Dissociation Constant)	A number that shows how easily a weak acid dissociates in solution. A high K_a value indicates a weak acid that dissociates more readily than a weak acid with a low K_a value.
K_c (Equilibrium Constant)	A number that shows the ratio of products to reactants in an equilibrium system. A value of 1 indicates equilibrium favours both the forward and reverse reaction equally. A value greater than 1 indicates the forward reaction is favoured more than the reverse reaction; a value less than 1 indicates the reverse reaction is favoured more than the forward reaction.
K_p (Gas Equilibrium Constant)	A number that shows the ratio of products to reactants in a gaseous equilibrium system. K_p uses partial pressures rather than concentrations (as in K_c).
K_w (Ionic Product of Water)	The product (multiplication) of concentrations of H⁺ and OH^- ions in solution at a constant temperature. The value for K_w at 298K is 1 x 10^-14, which can be used to find the concentration of H⁺ ions in an alkaline solution.
Ligand	Species that forms a co-ordinate bond with a transition metal ion.
Monodentate (ligand)	Species that forms one co-ordinate bond with a transition metal ion.
Bi-dentate (ligand)	Species that forms two co-ordinate bonds with a transition metal ion.
Polydentate (ligand)	Species that forms more than two co-ordinate bonds with a transition metal ion.
Lone pair (of electrons)	A pair of electrons in the outer shell of an atom that are not involved in a covalent bond.
Mass Spectrometry	A technique used to find the relative mass of a sample or element.
Mass Spectrometry (high resolution)	A technique used in organic analysis to identify the relative mass of a sample and its fragment ions, giving indications as to the possible structure of the sample.
Mechanism (organic)	A representation of the steps involved in an organic reaction, showing how a reacting species changes and the bonds being broken and/or formed.
Mole	A unit that represents 6.02x10²³ particles. The number of particles in one mole is Avogadro’s constant.
Monomer	A molecule that forms repeating units that bond together to create a polymer.
Carbon NMR (Nuclear Magnetic Resonance) Spectroscopy	A technique used in organic analysis to identify the number of unique carbon environments within an organic substance.
Proton NMH (Nuclear Magnetic Resonance) Spectroscopy	A technique used in organic analysis to identify the number of unique hydrogen environments within an organic substance. Splitting patterns from spin-coupling can also help indicate the structure of a sample.
Nucleophile	An electron-pair donating species that is attracted to positive charges and regions of low electron density in molecules.
Neutron	A sub-atomic particle found in the nucleus of an atom with no charge (relative charge of 0) and a relative mass of 1.
Optical Isomers	Compounds that have the same structure, but are mirror images of each other and rotate plane-polarised light in opposite directions.
Orbital (electron)	Area of space within an atom that an electron can exist in. Orbitals can have different shapes and only two electrons can exist in any one orbital.
Organic Chemistry	The study of molecules that contain covalently bonded carbon atoms. The part of chemistry involved in reactions and processes between organic molecules. Organic compounds are usually found in nature and are mainly made of carbon, hydrogen and oxygen (they may also contain other non-metals such as nitrogen, sulphur and phosphorus).
Organic Synthesis	The process of making an organic compound, often in a series of reactions.
Oxidation	A species losing electrons during a reaction. The addition of oxygen to a compound (and or removal of hydrogen from an organic compound).
Oxidising Agent	A species that enables another species to be oxidised. Oxidising agents can be reduced (forced to gain electrons).
pH	A scale used to show the acidity (concentration of H⁺(aq) ions) and alkalinity (concentration of OH^-(aq) ions) of a solution. The scale is logarithmic, meaning large differences in the concentrations of H⁺ and OH^- ions can be shown using only the numbers 1 to 14. pH is calculated using log₁₀[H⁺].
Physical Chemistry	Part of chemistry studying physical interactions and linking concepts in physics to observations within chemistry.
Polymer	A large molecule formed by the bonding together of repeating units.
Proton	A sub-atomic particle found in the nucleus of an atom with a positive charge (relative charge of +1) and a relative mass of 1.
Racemic	A mixture that contains equal amounts of two optical isomers. Racemic mixtures show no overall optical activity.
Rate of Reaction	A way of describing how fast a reaction is occurring. Rates of reaction can be calculated by measuring how quickly the concentration of a reactant or product changes over a given time.
Reaction	A process in which a substance undergoes a change in its atomic or molecular structure and becomes a new substance.
Addition Reaction	Type of reaction where new groups or atoms are added to a molecule, forming a product that has more atoms than the starting reactant.
Elimination Reaction	Type of reaction where bonded groups or atoms are removed from a molecule, so the number of atoms within the molecule decreases.
Substitution Reaction	Type of reaction where a bonded group or atom in a reactant is ‘swapped’ for a different group or atom from another reactant.
Reactivity	How likely a substance or element is to react, compared to other substances or elements.
Reduction	A species gaining electrons during a reaction. The removal of oxygen from a compound (and or addition of hydrogen to an organic compound).
Reducing Agent	A species that enables another species to be reduced. Reducing agents are oxidised and lose electrons.
Repeating Unit (polymers)	The units that bond together to form a polymer.
Spectator Ion	A species present in a reaction that does not undergo any change.
Standard Electrode Potential	The potential difference measured between a particular half-cell connected to a standard hydrogen electrode.
Stereoisomers	Compounds that have the same formula and bonding arrangement, but with a different arrangement of atoms in space.
Temperature	Value used to describe the amount of thermal energy in a substance. Absolute temperature is measured in Kelvin (K).
Transition Elements	Elements that can form one or more stable ions with partially filled d-orbitals. Many metals found in the d-block of the periodic table are transition elements. Transition metals often form coloured compounds and can act as catalysts.