essential definitions

definitions for the entire a level course OCR A

Isotopes

atoms with the same number of protons and electrons but different numbers of neutrons

relative isotopic mass

the mass of an isotope compared with one-twelfth of the mass of an atom of carbon-12

relative atomic mass

the weighted mean mass of an atom of an element compared with one-twelfth of the mass of an atom of carbon-12

hydrated

when a crystalline compound contains water molecules

anhydrous

containing no water molecules

water of crystallisation

water molecules that are bonded into a crystalline structure of a compound

acid

a species that releases H + ions in aqueous solution

base

a species that neutralises an acid to form a salt

salt

the product of a reaction in which the H + ions from the acid are replaced by metal or ammonium ions

neutralisation

the reaction between an acid and a base to produce a salt

oxidation

loss of electrons or an increase in oxidation number

reduction

gain of electrons or a decrease in oxidation number

atomic orbital

a region around the nucleus that can hold up to two electrons, with opposite spins

ionic bonding

the electrostatic attraction between positive and negative ions

covalent bonding

the electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms

dative covalent bonding

a shared pair of electrons in which the bonded pair has been provided by one of the bonding atoms only

electronegativity

a measure of the attraction of a bonded atom for the pair of electrons in a covalent bond

first ionisation enthalpy

The energy required to remove one electron from each atom in one mole of gaseous atoms of an element to form one mole of gaseous 1 + ions

metallic bonding

The electrostatic attraction between positive metal ions and delocalised electrons

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

giant covalent lattice

A three-dimensional structure of atoms, bonded together by strong covalent bonds

exothermic

a reaction resulting in heat loss to the surrounding

endothermic

a reaction resulting in heat being taken from the surroundings

activation enthalpy

the minimum energy required to start a reaction by the breaking of bonds

standard enthalpy of formation

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 of combustion

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

the average bond enthalpy

average enthalpy change when one mole of a given type of bond is broken in gaseous molecules

standard enthalpy of neutralisation

the enthalpy change when an acid reacts with a base to form one mole of H 2 O(l), under standard conditions, with all reactants and products in their standard states

standard conditions/standard states

100kPa, 298K / physical states under standard conditions

catalyst

a substance that increases the rate of a chemical reaction without being used up in the process (provides an alternative route with lower activation enthalpy)

homogeneous catalyst

catalyst and reactants are in the same physical states

heterogeneous catalyst

catalyst and reactants are in different physical states

dynamic equilibrium

the equilibrium that exists in a closed system when the rate of forward reaction is equal to the rate of the reverse reaction and concentrations do not change.

Saturated

Containing single bonds only

Unsaturated

Contains a multiple carbon-to-carbon bond

Structural isomers

Compounds with the same molecular formula but different structural formulae

Stereoisomers

Compounds with the same structural formula but with a different arrangement of the atoms in space

Homologous series

A series of organic compounds with the same functional group but with each successive member differing by CH 2

radical

A species with an unpaired electron

nucleophile

An atom or group of atoms that is attracted to an electron-deficient carbon atom where it donates a pair of electrons to form a new covalent bond.

electrophile

An atom or group of atoms that is attracted to an electron-rich centre where it accepts a pair of electrons to form a new covalent bond.

homolytic fission

The breaking of a covalent bond with one of the bonded electrons going to each atom (forming two radicals)

heterolytic fission

The breaking of a covalent bond with both of the bonded electrons going to one atom (forming a positive and negative ion)

sigma-bond

A bond formed by the overlap of one orbital from each bonding atom, on a line directly between the bonding atoms.

pi-bond

A bond formed by the sideways overlap of two p orbitals

Bronsted-Lowry acid

A species that is a proton, H + , donor

Bronsted-Lowry base

A species that is a proton, H + , acceptor

buffer

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

lattice enthalpy

The enthalpy change when one mole of an ionic compound is formed from its gaseous ions under standard conditions

standard enthalpy change of solution

The enthalpy change when one mole of a compound is completely dissolved in water under standard conditions

standard enthalpy change of hydration

The enthalpy change when one mole of gaseous ions dissolves in water to form one mole of aqueous ions under standard conditions

standard enthalpy change of atomisation

The enthalpy change when one mole of gaseous atoms forms from the element in its standard states.

entropy

The term used for the dispersal of energy and disorder within a chemical system

oxidising agent

A reagent that oxidises (takes electrons from) another species

reducing agent

A reagent that reduces (gives electrons to) another species

ligand

A molecule or ion that can donate a pair of electrons to a transition metal ion

bidentate

Can donate two pairs of electrons forming two coordinate bonds