HL IB Chemistry Definitions
You must learn these. You must be able to recall them word for word!
Relative atomic mass - The relative atomic mass of an element is the weighted average of the
masses of the isotopes relative to 1/12 of the mass of a carbon-12 atom.
Relative molecular mass - The relative molecular mass of a substance is the mass of one
molecule of that substance relative to 1/12 of the mass of carbon – 12 atom.
Mass Number - is equal to the number of protons plus the number of neutrons in an atom of
that element.
Atomic Number - is equal to the number of protons in an atom of that element.
Isotope - are atoms of the same element with different numbers of neutrons.
1st Ionisation Energy - The first ionisation energy is the energy required to remove the most
loosely held electron from one mole of gaseous atoms to produce 1 mole of gaseous ions.
Electronegativity - Electronegativity is a measure of the tendency of an atom to attract a
bonding pair of electrons.
Ligand - An atom, ion or molecule that can donate a pair of electrons to form a dative covalent
bond to a metal ion thereby forming a complex ion.
Exothermic Reaction - An exothermic reaction is one that gives out heat to its surroundings.
Endothermic Reaction - An endothermic reaction is one that takes in heat from its
surroundings.
Standard Conditions - Temperature = 298K, Pressure = 1atm & solutions 1 mol dm-3.
Standard enthalpy change of a reaction - This is the enthalpy change that occurs for a
reaction under standard conditions.
Average Bond Enthalpy - Average enthalpy change per mole when the same type of bond is
broken in the gas phase for many similar substances.
Standard enthalpy change of Formation - The enthalpy change when 1 mole of a compound is
made from its elements in their standard states, under standard conditions.
Standard enthalpy change of Combustion - the enthalpy change when 1 mole of a substance
burns completely in oxygen under standard conditions.
Lattice Enthalpy - the enthalpy change when 1 mole of the ionic lattice is formed from its ions
in the gaseous state under standard conditions.
Electron Affinity - the enthalpy change when 1 mole of electrons is added to 1 mole of
gaseous atoms, under standard conditions (technically, this should be called the 1st electron
affinity)
Rate of Reaction - how fast a reaction takes place.
Activation Energy - is the minimum energy required before a reaction can occur.
Bronsted Lowry Acid - An acid is a proton (hydrogen ion) donor.
Bronsted Lowry Base - A base is a proton (hydrogen ion) acceptor.
Lewis Acid - An acid is an electron pair acceptor.
Lewis Base - A base is an electron pair donor.
Oxidation - is electron loss.
Reduction - is electron gain.
Oxidising Agent - causes something else to be oxidised (to lose electrons). It gains the
electrons that are lost by the substance being oxidised.
Reducing Agent - causes something else to be reduced (to gain electrons). It gives away
electrons to the substance that is being reduced.
Standard Electrode Potential - The e.m.f (potential difference) of a half-reaction as a
reduction relative to the standard hydrogen electrode, under standard conditions.