OCR A Level Chemistry

acid

A species that is a proton donor.

activation energy

The minimum amount of kinetic energy that particles need to have in order to react when they collide

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

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.

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 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.

(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 regular 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.

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).

Homologous 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.