Year 12 Definitions

Mass number

total number of protons and neutrons

Atomic number

number of protons in the nucleus

Isotopes

atoms of the same elements with the same number of protons but a different number of neutrons

First ionisation energy

the energy required to remove one mole of electrons from one mole of gaseous atoms of an element to form one mole of gaseous 1+ ions

Relative atomic mass

weighted average mass of an atom relative to the mass of 1/12th the relative atomic mass of an atom of carbon-12

Relative molecular mass

weighted average mass of a molecule relative to the mass of 1/12th the relative atomic mass of an atom of carbon-12

Relative formula mass

weighted average mass of a formula unit relative to the mass of 1/12th the relative atomic mass of an atom of carbon-12

Avogadro constant

the number of particles in a mole

Number of particles

number of moles x Avogadro's constant

Empirical formula

simplest whole number ratio of atoms of each element in a compound

Molecular formula

actual number of atoms of each element in a compound

Ionic bond

strong electrostatic forces of attraction between oppositely charged ions in a lattice formed by the transfer of electrons

Covalent bond

strong electrostatic forces of attraction between the shared pair of electrons and the positive nuclei

Metallic bond

strong electrostatic forces of attraction between the sea of delocalised electrons and their positive metal ions arranged in a lattice

Electronegativity

the ability of an atom to attract the pair of electrons in a covalent bond

Endothermic

heat energy taken in

Exothermic

heat energy given out

Enthalpy change (ΔH)

heat energy measured under constant pressure

Standard enthalpy of combustion

the enthalpy change when 1 mole of a substance is completely burned in oxygen in standard states under standard conditions, ΔcHθ

Standard enthalpy of formation

the enthalpy change associated with the formation of 1 mol of a compound from its constituent elements, ΔfHθ, in standard states under standard conditions

Enthalpy of neutralisation

enthalpy change when 1 mole of water is formed in a reaction between an acid and alkali under standard condition

Hess's Law

the total enthalpy change for a chemical reaction is independent of the route taken

Mean bond enthalpy

the average value of the bond dissociation enthalpy for a given type of bond taken from a range of different compounds

Activation Energy (Ea)

the minimum kinetic energy that a particle needs in order to react; the energy (enthalpy) difference between the reactants and the transition state

Rate of reaction

change in concentration of a reactant or product per unit time

Catalyst

a substance that increases the rate of a chemical reaction without being changed in chemical composition or amount, by providing an alternative pathway with a lower activation energy

Enthalpy of formation (∆Hf)

the enthalpy change when 1 mole of a compound is formed from its elements in their standard states e.g. Ca(s) + Cl2(g) --> CaCl2(s)

Bond dissociation enthalpy (∆Hdiss)

the enthalpy change when all the bonds of the same type in 1 mole of gaseous molecules are broken e.g. Cl2(g) --> 2Cl(g)

Enthalpy change of atomisation of an element (∆Hat)

the enthalpy change when 1 mole of gaseous atoms is formed from an element in its standard state

Enthalpy change of atomisation of a compound (∆Hat)

the enthalpy change when 1 mole of a compound in its standard state is converted to gaseous atoms

Second ionisation energy (∆Hie2)

the enthalpy change when 1 mole of gaseous 2+ ions is formed from 1 mole of gaseous 1+ ions e.g. Mg+(g) --> Mg2+(g) + e-

First electron affinity (∆Hea1)

the enthalpy change when 1 mole of gaseous 1- ions is made from 1 mole of gaseous atoms e.g. O(g) + e- --> O-(g)

Second electron affinity (∆Hea2)

the enthalpy change when 1 mole of gaseous 2- ions is made from 1 mole of gaseous 1- ions e.g. O-(g) + e- --> O2-(g)

Enthalpy change of hydration (∆Hhyd)

the enthalpy change when 1 mole of aqueous ions is formed from gaseous ions e.g. Na+(g) --> Na+(aq)

Enthalpy change of solution(∆Hsolution)

the enthalpy change when 1 mole of an ionic substance dissolves in enough solvent to form an infinitely dilute solution

Lattice formation enthalpy

the enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions

Lattice dissociation enthalpy

the enthalpy change when 1 mole of a solid ionic compound is completely dissociated into its gaseous ions

Free radical

a chemical species with an unpaired electron

Isomers

molecules with same molecular formula but whose atoms are arranged differently

Isomers

molecules that have the same molecular formula but whose atoms are arranged differently

Stereoisomerism

two (or more) compounds have the same structural formula but they differ in the arrangement of the bonds in space

Petroleum

a mixture consisting mainly of alkane hydrocarbons that can be separated into different fractions

Cracking

breaking long-chain alkanes into smaller hydrocarbons (which can include alkenes)

Nucleophile

lone-pair donor

Electrophiles

lone pair acceptors

Addition polymers

a type of polymer formed by joining small alkenes (monomers) together

Racemate (or racemic mixture)

a mixture that contains equal quantities of each enantiomer of an optically active compound