A2 definitions OCR

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Acid–base pair  

A pair of two species that transform into each other by gain or loss of a proton.

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 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 to form 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; any alkyl group is often shown as ‘R’.

Amount of substance  

The quantity whose unit is 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

 

Atomic (proton) number  

The number of protons in the nucleus of an atom.

Atomic orbital  

A region within an atom that can hold up to two electrons with opposite spins.

Average bond enthalpy  

The average enthalpy change that takes place when breaking by homolytic fission 1mol of a given type of bond in the molecules of a gaseous species.

Avogadro’s constant, NA  

The number of atoms per mole of the carbon-12 isotope
(6.02 × 1023
mol–1).

Biodegradable substance  

A substance that is broken down naturally in the environment by other living organisms.

Biodegradable polymer  

A polymer that breaks down completely into carbon dioxide and water.

Boltzmann distribution  

A diagram showing the distribution of energies of the molecules at a particular temperature.

Bond dissociation enthalpy  

The enthalpy change that takes place when breaking by homolytic fission 1mol of a given bond in the molecules of a gaseous species.

Brønsted–Lowry acid  

A species that is a proton, H+, donor.

Brønsted–Lowry base  

A species that is a proton, H+, acceptor.

Buffer solution  

A system that minimises pH changes on addition of small amounts of an acid or a base.

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.

Chemical shift  

A scale that compares the frequency of an NMR absorption with the frequency of

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