Chemistry OCR AS Definitions
Definitions for OCR As Level chemistry
Terms in this set (153)
A species that is a proton donor.
The minimum energy required to start a reaction by the breaking of bonds.
A very long molecular chain formed by repeated addition reactions of many unsaturated alkene molecules (monomers).
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).
A reaction in which a reactant is added to an unsaturated molecule to make a saturated molecule.
The process that occurs when a gas, liquid or solute is held to the surface of a solid or, more rarely, a liquid.
A hydrocarbon with carbon atoms joined together in a ring structure.
A hydrocarbon with carbon atoms joined together in straight or branched chains.
A type of base that dissolves in water forming hydroxide ions, OH-(aq) ions.
The homologous series with the general formula: CnH2n+2.
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.
A substance that contains no water molecules.
A negatively charged ion.
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.
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).
A species that is a proton acceptor.
A substance that is broken down naturally in the environment by living organisms.
The distribution of energies of molecules at a particular temperature, usually shown as a graph.
The enthalpy change that takes place when breaking by homolytic fission 1 mol of a given bond in the molecules of a gaseous species.
An organic ion in which a carbon atom has a negative charge.
An organic ion in which a carbon atom has a positive charge.
A substance that increases the rate of a chemical reaction without being used up in the process.
A positively charged ion.
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.
A substance formed from two or more chemically bonded elements in a fixed ratio, usually shown by a chemical formula
The amount of solute, in mol, per 1 dm3 (1000 cm3) of solution.
A shared pair of electrons which has been provided by one of the bonding atoms only; also called a dative covalent bond.
A bond formed by a shared pair of electrons.
The breaking down of long-chained saturated hydrocarbons to form a mixture of shorter-chained alkanes and alkenes.
A symbol used in reaction mechanisms to show the movement of an electron pair in the breaking or formation of a covalent bond.
A shared pair of electrons which has been provided by one of the bonding atoms only; also called a coordinate bond.
An elimination reaction in which water is removed from a saturated molecule to make an unsaturated molecule.
Electrons that are shared between more than two atoms.
A reaction in which a more reactive element displaces a less reactive element from an aqueous solution of the latter's ions.
A formula showing the relative positioning of all the atoms in a molecule and the bonds between them.
The oxidation and reduction of the same element in a redox reaction.
The equilibrium that exists in a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction.
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.
The arrangement of electrons in an atom.
A measure of the attraction of a bonded atom for the pair of electrons in a covalent bond.
The repulsion between electrons in different inner shells. Shielding reduces the net attractive force from the positive nucleus on the outer-shell electrons.
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.
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.
The removal of a molecule from a saturated molecule to make an unsaturated molecule.
The simplest whole-number ratio of atoms of each element present in a compound.
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).
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.
(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.
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.
The reaction of an alcohol with a carboxylic acid to produce an ester and water.
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).
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.
The process in mass spectrometry that causes a positive ion to split into pieces, one of which is a positive fragment ion.
The part of the organic molecule responsible for its chemical reactions.
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.
The process in which the absorption and subsequent emission of infrared radiation by atmospheric gases warms the lower atmosphere and the planet's surface.
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.
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.
A reaction in which the catalyst has a different physical state from the reactants; frequently, reactants are gases whilst the catalyst is a solid.
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).
A reaction in which the catalyst and reactants are in the same physical state, which is most frequently the aqueous or gaseous state.
A series of organic compounds with the same functional group, but with each successive member differing by CH2.
The breaking of a covalent bond with one of the bonded electrons going to each atom, forming two radicals.
Crystalline and containing water molecules.
A compound of hydrogen and carbon only.
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.
A reaction with water or aqueous hydroxide ions that breaks a chemical compound into two compounds.
The first step in a radical substitution in which the free radicals are generated by ultraviolet radiation.
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.
A positively or negatively charge atom or (covalently bonded) group of atoms (a molecular ion).
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 energy
A measure of the energy required to remove each electron in turn, 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.
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.
The substance in a chemical reaction that runs out first.
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.
A sequence of steps showing the path taken by electrons in a reaction.
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.
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.
The amount of any substance containing as many particles as there are carbon atoms in exactly 12 g of the carbon-12 isotope.
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.
A small group of atoms held together by covalent bonds.
A small molecule that combines with many other monomers to form a polymer.
A system of naming compounds.
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.
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.
Loss of electrons or an increase in 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.
A reagent that oxidises (takes electrons from) another species.
100 x actual yield / predicted yield
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.
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.
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.
A molecule with an overall dipole, taking into account any dipoles across bonds.
A long molecular chain built up from monomer units.
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.
The two repeated steps in radical substitution that build up the products in a chain reaction.
A species with an unpaired electron.
rate of reaction
The change in concentration of a reactant or a product in a given time.
A reaction in which both reduction and oxidation take place.
A reagent that reduces (adds electron to) another species.
Gain of electrons or a decrease in oxidation number.
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.
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.
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+.
A hydrocarbon with single bonds only.
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.
A simplified organic formula, with hydrogen atoms removed from alkyl chains, leaving just a carbon skeleton and associated functional groups.
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.
Ions that are present but take no part in a chemical reaction.
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).
A solution of known concentration. Standard solutions are normally used in titrations to determine unknown information about another substance.
The physical state of a substance under the standard conditions of 100 kPa (1 atmosphere) and 298 K (25 °C).
Compounds with the same structural formula but with a different arrangement of the atoms in space.
The molar relationship between the relative quantities of substances taking part in a reaction.
The second layer of the Earth's atmosphere, containing the 'ozone layer', about 10 km to 50 km above the Earth's surface.
A formula showing the minimal detail for the arrangement of atoms in a molecule.
Molecules with the same molecular formula but with different structural arrangements of atoms.
A group of the same type of atomic orbitals (s, p, d or f) within a shell.
A reaction in which an atom or group of atoms is replaced with a different atom or group of atoms.
The step at the end of a radical substitution when two radicals combine to form a molecule.
The breaking up of a chemical substance with heat into at least two chemical substances.
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).
A hydrocarbon containing carbon-to-carbon multiple bonds.
van der Waals' forces
Very weak attractive forces between induced dipoles in neighbouring molecules.
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.
100 x Mr of desired products / sum of Mr of all products
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