AS Chemistry Definitions

• Isotope


  atoms of the same element with different numbers of neutrons and different masses
• Ion

  a positively or negatively charged atom or group of atoms, where the number of electrons is different to the number of protons
• relative isotopic mass

  the mass of an isotope compared to the mass of 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 to one twelfth of the mass of an atom of carbon-12
• mole

  the amount of any substance containing as many elementary particles as there are carbon atoms in exactly 12g of carbon-12
• empirical formula

  the formula that shows the simplest whole number ratio of atoms in each element present in a compound
• molecular formula

  a formula that shows the number and the type of atoms of each element present in a compound
• standard solution

  a solution of a known concentration
• acid

  proton donor
• base

  proton acceptor
• alkali

  a type of base that dissolves in water to produce OH- ions
• water of crystallisation

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


  loss of electrons
• reduction

  gain of electrons
• reducing reagent

  reagent that reduces another species
• salt

  the product of a reaction where the H+ ions in an acid are replaced by metal or ammonium ions
• oxidising agent


  a reagent that oxidises another species
• disproportionation

  a redox reaction which the same element is both oxidised and reduced
• atomic orbital

  a region of space around the nucleus that can hold up two electrons with opposite spins
• sub-shell

  a group of orbitals of the same type within a shell
• shell

  a group of atomic orbitals with the same principle quantum number
• electronegativity

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


  the heat energy stored in a system
• activation energy

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

  pressure of 100kPa, temperature of 298K, in a concentration of 1 mol dm-3
• standard enthalpy change of reaction

  the enthalpy change that accompanies a reaction in the molar quantities expressed in the equation under standard conditions with all reactants in their standard states
• standard enthalpy change of combustion

  the enthalpy change where one mole of substance reacts completely with oxygen under standard conditions with all reactants and products in their standard states
• standard enthalpy change of formation

  the enthalpy change where one mole of a compound is formed from its constituent elements in standard states under standard conditions
• standard enthalpy change of neutralisation

  the enthalpy change that accompanies the reaction of an acid and a base to form one mole of water under standard conditions with all reactants in their standard states
• average bond enthalpy

  energy required to break one mole of a specific bond by homolytic fission in molecules in a gaseous state
• 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
• catalyst


  a species that lowers the activation energy of a reaction by providing an alternate route but is not used up in itself
• dynamic equilibrium

  en equilibrium in a closed system where the rate of forward reaction equals that of the reverse reaction leaving constant concentrations of products and reactants
• le chatelier’s principle

  when a system in equilibrium is subjected to a change, the system readjusts to minimise the effect of the change
• homogenous equilibrium


  an equilibrium in which all species have the same physical state
• hydrocarbon


  a compound of hydrogen and carbon only
• saturated hydrocarbon

  contains single bonds only
• unsaturated hydrocarbon

  contains a multiple carbon-carbon double bond
• aliphatic hydrocarbon


  carbons joined together in straight or branched chains
• alicyclic hydrocarbon


  carbons joined together in a ring which is not aromatic