C2 | Chemistry
Cheltenham College Chemistry Department Lower Sixth List of meanings and definitions for AS Chemistry You need to know and understand the meanings of these words for OCR AS Chemistry You will be asked some as definitions in the exams and you need to be precise and accurate. When revising from the Criteria Checklist, look up the meanings of words from this list. 1/10 C2 | Chemistry
Full word and definitions list for OCR AS Chemistry acid A species that is a proton donor. activation energy The minimum energy required to start a reaction by the breaking of bonds. addition polymer A very long molecular chain formed by repeated addition reactions of many unsaturated alkene molecules (monomers). addition polymerisation The process in which unsaturated alkene molecules (monomers) add on to a growing polymer chain one at a time to form a very long saturated molecular chain (the addition polymer). addition reaction A reaction in which a reactant is added to an unsaturated molecule to make a saturated molecule. adsorption The process that occurs when a gas, liquid or solute is held to the surface of a solid or, more rarely, a liquid. alicyclic hydrocarbon A hydrocarbon with carbon atoms joined together in a ring structure. aliphatic hydrocarbon A hydrocarbon with carbon atoms joined together in straight or branched chains. alkali A type of base that dissolves in water forming hydroxide ions, OH–
(aq) ions. alkanes The homologous series with the general formula: CnH2n+2. alkyl group An alkane with a hydrogen atom removed, e.g. CH3, C2H5; alkyl groups are often shown as ‘R’. amount of substance The quantity whose unit of the mole. Chemists use ‘amount of substance’ as a means of counting atoms. anhydrous A substance that contains no water molecules. anion A negatively charged ion. atom economy atom economy =
molecular mass of the desired product
× 100 sum of molecular masses of all products
atomic orbital A region within an atom that can hold up to two electrons, with opposite spins. atomic (proton) number The number of protons in the nucleus of an atom. 2/10 C2 | Chemistry
average bond enthalpy The average enthalpy change that takes place when breaking by homolytic fission 1 mol of a given type of bond in the molecules of a gaseous species. Avogadro constant, NA The number of atoms per mole of the carbon-­‐12 isotope (6.02 × 1023 mol–1). base A species that is a proton acceptor. biodegradable material A substance that is broken down naturally in the environment by living organisms. Boltzmann distribution The distribution of energies of molecules at a particular temperature, usually shown as a graph. bond enthalpy The enthalpy change that takes place when breaking by homolytic fission 1 mol of a given bond in the molecules of a gaseous species. carbanion An organic ion in which a carbon atom has a negative charge. carbocation An organic ion in which a carbon atom has a positive charge. catalyst A substance that increases the rate of a chemical reaction without being used up in the process. cation A positively charged ion. cis–trans isomerism A special type of E/Z isomerism in which there is a non-­‐hydrogen group and a hydrogen atom on each C of a C=C double bond: the cis isomer (Z isomer) has the H atoms on each carbon on the same side; the trans isomer (E isomer) has the H atoms on each carbon on different sides of the bond. compound A substance formed from two or more chemically bonded elements in a fixed ratio, usually shown by a chemical formula concentration The amount of solute, in mol, per 1 dm3 (1000 cm3) of solution. coordinate bond A shared pair of electrons which has been provided by one of the bonding atoms only; also called a dative covalent bond. covalent bond A bond formed by a shared pair of electrons. cracking The breaking down of long-­‐chained saturated hydrocarbons to form a mixture of shorter-­‐chained alkanes and alkenes. curly arrow A symbol used in reaction mechanisms to show the movement of an electron pair in the breaking or formation of a covalent bond. dative covalent A shared pair of electrons which has been provided by one of the bonding atoms only; also called a coordinate bond. dehydration An elimination reaction in which water is removed from a 3/10 C2 | Chemistry
saturated molecule to make an unsaturated molecule. delocalised electrons Electrons that are shared between more than two atoms. displacement reaction A reaction in which a more reactive element displaces a less reactive element from an aqueous solution of the latter’s ions. displayed formula A formula showing the relative positioning of all the atoms in a molecule and the bonds between them. disproportionation The oxidation and reduction of the same element in a redox reaction. dynamic equilibrium The equilibrium that exists in a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction. E/Z isomerism A type of stereoisomerism in which different groups attached to each carbon of a C=C double bond may be arranged differently in space because of the restricted rotation of the C=C bond. electron configuration The arrangement of electrons in an atom. electronegativity A measure of the attraction of a bonded atom for the pair of electrons in a covalent bond. electron shielding The repulsion between electrons in different inner shells. Shielding reduces the net attractive force from the positive nucleus on the outer-­‐shell electrons. electrophile An atom (or group of atoms) that is attracted to an electron-­‐rich centre or atom, where it accepts a pair of electrons to form a new covalent bond. electrophilic addition A type of addition reaction in which an electrophile is attracted to an electron-­‐rich centre or atom, where it accepts a pair of electrons to form a new covalent bond. elimination reaction The removal of a molecule from a saturated molecule to make an unsaturated molecule. empirical formula The simplest whole-­‐number ratio of atoms of each element present in a compound. endothermic reaction A reaction in which the enthalpy of the products is greater than the enthalpy of the reactants, resulting in heat being taken in from the surroundings (∆H +ve). enthalpy, H The heat content that is stored in a chemical system. (standard) enthalpy change of combustion, ∆Hcө The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard states. 4/10 C2 | Chemistry
(standard) enthalpy change of formation, ∆Hfө The enthalpy change that takes place when one mole of a compound is formed from its constituent elements in their standard states under standard conditions. (standard) enthalpy change of reaction, ∆Hrө The enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states. enthalpy cycle A diagram showing alternative routes between reactants and products which allows the indirect determination of an enthalpy change from other known enthalpy changes using Hess’ law. enthalpy profile diagram A diagram for a reaction to compare the enthalpy of the reactants with the enthalpy of the products. esterification The reaction of an alcohol with a carboxylic acid to produce an ester and water. exothermic reaction A reaction in which the enthalpy of the products is smaller than the enthalpy of the reactants, resulting in heat loss to the surroundings (∆H –ve). fractional distillation The separation of the components in a liquid mixture into fractions which differ in boiling point (and hence chemical composition) by means of distillation, typically using a fractionating column. fragmentation The process in mass spectrometry that causes a positive ion to split into pieces, one of which is a positive fragment ion. functional group The part of the organic molecule responsible for its chemical reactions. general formula The simplest algebraic formula of a member of a homologous series. For example, the general formula of the alkanes is CnH2n+2. giant covalent lattice A three-­‐dimensional structure of atoms, bonded together by strong covalent bonds. giant ionic lattice A three-­‐dimensional structure of oppositely charged ions, bonded together by strong ionic bonds. giant metallic lattice A three-­‐dimensional structure of positive ions and delocalised electrons, bonded together by strong metallic bonds. greenhouse effect The process in which the absorption and subsequent emission of infrared radiation by atmospheric gases warms the lower atmosphere and the planet’s surface. group A vertical column in the Periodic Table. Elements in a group have similar chemical properties and their atoms have the same number of outer shell electrons. 5/10 C2 | Chemistry
Hess’ law If a reaction can take place by more than one route and the initial and final conditions are the same, the total enthalpy change is the same for each route. heterogeneous catalysis A reaction in which the catalyst has a different physical state from the reactants; frequently, reactants are gases whilst the catalyst is a solid. heterolytic fission The breaking of a covalent bond with both of the bonded electrons going to one of the atoms, forming a cation (+ ion) and an anion (– ion). homogeneous catalysis A reaction in which the catalyst and reactants are in the same physical state, which is most frequently the aqueous or gaseous state. homologous series A series of organic compounds with the same functional group, but with each successive member differing by CH2. homolytic fission The breaking of a covalent bond with one of the bonded electrons going to each atom, forming two radicals. hydrated Crystalline and containing water molecules. hydrocarbon A compound of hydrogen and carbon only. hydrogen bond A strong dipole–dipole attraction between an electron-­‐deficient hydrogen atom (O–Hδ+ or N–Hδ+) on one molecule and a lone pair of electrons on a highly electronegative atom (H–O:δ– or H–N:δ–) on a different molecule. hydrolysis A reaction with water or aqueous hydroxide ions that breaks a chemical compound into two compounds. initiation The first step in a radical substitution in which the free radicals are generated by ultraviolet radiation. intermolecular force An attractive force between neighbouring molecules. Intermolecular forces can be van der Waals’ forces (induced dipole–dipole forces), permanent dipole–dipole forces or hydrogen bonds. ion A positively or negatively charge atom or (covalently bonded) group of atoms (a molecular ion). ionic bonding The electrostatic attraction between oppositely charged ions. (first) ionisation energy The energy required to remove one electron from each ion in one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions. (second) ionisation energy The energy required to remove one electron from each ion in one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions. (successive) ionisation A measure of the energy required to remove each electron in turn, 6/10 C2 | Chemistry
energy e.g. the second ionisation energy is the energy required to remove one electron from each ion in one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions. isotopes Atoms of the same element with different numbers of neutrons and different masses. le Chatelier’s principle When a system in dynamic equilibrium is subjected to a change, the position of equilibrium will shift to minimise the change. limiting reagent The substance in a chemical reaction that runs out first. lone pair An outer-­‐shell pair of electrons that is not involved in chemical bonding. mass (nucleon) number The number of particles (protons and neutrons) in the nucleus. mechanism A sequence of steps showing the path taken by electrons in a reaction. metallic bond The electrostatic attraction between positive metal ions and delocalised electrons. molar mass, M The mass mole of a substance. The units of molar mass are g mol–1. molar volume The volume per mole of a gas. The units of molar volume are dm3 mol–1. At room temperature and pressure the molar volume is approximately 24.0 dm3 mol–1. mole The amount of any substance containing as many particles as there are carbon atoms in exactly 12 g of the carbon-­‐12 isotope. molecular formula The number of atoms of each element in a molecule. molecular ion, M+ The positive ion formed in mass spectrometry when a molecule loses an electron. molecule A small group of atoms held together by covalent bonds. monomer A small molecule that combines with many other monomers to form a polymer. nomenclature A system of naming compounds. nucleophile An atom (or group of atoms) that is attracted to an electron-­‐
deficient centre or atom, where it donates a pair of electrons to form a new covalent bond. nucleophilic substitution A type of substitution reaction in which a nucleophile is attracted to an electron-­‐deficient centre or atom, where it donates a pair of electrons to form a new covalent bond. oxidation Loss of electrons or an increase in oxidation number. 7/10 C2 | Chemistry
oxidation number A measure of the number of electrons that an atom uses to bond with atoms of another element. Oxidation numbers are derived from a set of rules. oxidising agent A reagent that oxidises (takes electrons from) another species. percentage yield % yield =
period periodicity actual amount, in mol, of product
×100
theoretical amount, in mol, of product
A horizontal row of elements in the Periodic Table. Elements show trends in properties across a period. €
A regular periodic variation of properties of elements with atomic number and position in the Periodic Table. permanent dipole A small charge difference across a bond resulting from a difference in electronegativities of the bonded atoms. permanent dipole–
dipole force An attractive force between permanent dipoles in neighbouring polar molecules. pi-­‐bond (π-­‐bond) The reactive part of a double bond formed above and below the plane of the bonded atoms by sideways overlap of p-­‐orbitals. polar covalent bond A bond with a permanent dipole. polar molecule A molecule with an overall dipole, taking into account any dipoles across bonds. polymer A long molecular chain built up from monomer units. precipitation reaction The formation of a solid from a solution during a chemical reaction. Precipitates are often formed when two aqueous solutions are mixed together. principal quantum number, n A number representing the relative overall energy of each orbital, which increases with distance from the nucleus. The sets of orbitals with the same n value are referred to as electron shells or energy levels. propagation The two repeated steps in radical substitution that build up the products in a chain reaction. radical A species with an unpaired electron. rate of reaction The change in concentration of a reactant or a product in a given time. redox reaction A reaction in which both reduction and oxidation take place. reducing agent A reagent that reduces (adds electron to) another species. reduction Gain of electrons or a decrease in oxidation number. 8/10 C2 | Chemistry
reflux The continual boiling and condensing of a reaction mixture to ensure that the reaction takes place without the contents of the flask boiling dry. relative atomic mass, Ar The weighted mean mass of an atom of an element compared with one-­‐twelfth of the mass of an atom of carbon-­‐12. relative formula mass The weighted mean mass of a formula unit compared with one-­‐
twelfth of the mass of an atom of carbon-­‐12. relative isotopic mass The mass of an atom of an isotope compared with one-­‐twelfth of the mass of an atom of carbon-­‐12. relative molecular mass, Mr The weighted mean mass of a molecule compared with one-­‐
twelfth of the mass of an atom of carbon-­‐12. repeat unit A specific arrangement of atoms that occurs in the structure over and over again. Repeat units are included in brackets, outside of which is the symbol n. Salt A chemical compound formed from an acid, when a H+ ion from the acid has been replaced by a metal ion or another positive ion, such as the ammonium ion, NH4+. saturated hydrocarbon A hydrocarbon with single bonds only. shell A group of atomic orbitals with the same principal quantum number, n. Also known as a main energy level. simple molecular lattice A three-­‐dimensional structure of molecules, bonded together by weak intermolecular forces. skeletal formula A simplified organic formula, with hydrogen atoms removed from alkyl chains, leaving just a carbon skeleton and associated functional groups. species Any type of particle that takes part in a chemical reaction. specific heat capacity, c The energy required to raise the temperature of 1 g of a substance by 1 °C. spectator ions Ions that are present but take no part in a chemical reaction. standard conditions A pressure of 100 kPa (1 atmosphere), a stated temperature, usually 298 K (25 °C), and a concentration of 1 mol dm–3 (for reactions with aqueous solutions). standard enthalpies See enthalpy. standard solution A solution of known concentration. Standard solutions are normally used in titrations to determine unknown information about another substance. 9/10 C2 | Chemistry
standard state The physical state of a substance under the standard conditions of 100 kPa (1 atmosphere) and 298 K (25 °C). stereoisomers Compounds with the same structural formula but with a different arrangement of the atoms in space. stoichiometry The molar relationship between the relative quantities of substances taking part in a reaction. stratosphere The second layer of the Earth’s atmosphere, containing the ‘ozone layer’, about 10 km to 50 km above the Earth’s surface. structural formula A formula showing the minimal detail for the arrangement of atoms in a molecule. structural isomers Molecules with the same molecular formula but with different structural arrangements of atoms. sub-­‐shell A group of the same type of atomic orbitals (s, p, d or f) within a shell. substitution reaction A reaction in which an atom or group of atoms is replaced with a different atom or group of atoms. termination The step at the end of a radical substitution when two radicals combine to form a molecule. thermal decomposition The breaking up of a chemical substance with heat into at least two chemical substances. troposphere The lowest layer of the Earth’s atmosphere, extending from the Earth’s surface up to about 7 km (above the poles) and to about 20 km (above the tropics). unsaturated hydrocarbon A hydrocarbon containing carbon-­‐to-­‐carbon multiple bonds. van der Waals’ forces Very weak attractive forces between induced dipoles in neighbouring molecules. volatility The ease that a liquid turns into a gas. Volatility increases as boiling point decreases. water of crystallisation Water molecules that form an essential part of the crystalline structure of a compound. 10/10