Alevel chemistry definitions

atomic number

the number of protons in the nucleus of an atom

mass number

the number of protons and neutrons in the nucleus of an atom

isotopes

atoms of the same element with different mass numbers

relative atomic mass

the average mass of the naturally occurring isotope of the element relative to the mass of 1/12 of an atom of carbon-12

ionic bond

electrostatic attraction between oppositely charged ions

covalent bonding

an electrostatic attraction between the positively charged nuclei of the bonded atoms and the shared pair of electrons

nucleophile

molecule/negatively charged ion, possessing a lone pair of electrons, which is attracted to a more positively charged region in a molecule (region with lower electron density) and donates a lone pair to form a dative covelant bond

electronegativity

the ability of an atom to attract electrons in a covenant bond

Rate constant (k)

a proportionality constant that relates the concentration of the reactants to the reaction rate

Reaction order

the power to which the concentration of a reactant is raised in the rate law

overall reaction order

the sum of all the individual reaction orders

First ionisation energy

the energy required to remove 1 electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions under standard conditions

M(g) —> M+ (g) + e-

Second ionisation energy

the energy required to remove 1 electron from each 1+ ion in 1 mole of gaseous 1+ ions to form 1 mole of 2+ ions under standard conditions

M+(g) —> M2+ (g) + e-

Standard enthalpy change of atomisation

energy change when 1 mole of gaseous atoms is formed from the element under standard conditions.

ALWAYS ENDOTHERMIC

½ H2(g) —> H(g)

First Electron Affinity

enthalpy change when 1 e- is added to each atom in 1 mole of gaseous atoms under standard conditions to form 1 mole of gaseous 1- ions

ALWAYS EXOTHERMIC

X(g) + e- —> X- (g)

Second Electron Affinity

enthalpy change when 1 mole of e- is added to each 1- ions in 1 mole of gaseous ions under standard conditions to form 1 mole of gaseous 2- ions

ALWAYS ENDOTHERMIC

X-(g) + e- —> X2- (g)

Lattice enthalpy

energy released when 1 mole if ionic compound is formed from its constituents gaseous ions under standard conditions

ALWAYS EXOTHERMIC

Na+(g) + Cl- (g) —> NaCl(s)

Standard enthalpy change of formation

energy released when 1 mole of ionic compound is formed from its elements under standard conditions with all its reactants and products in their standard states

Na (s) + ½ Cl2(g) —> NaCl (s)

entropy (S\phi )

A measure of the dispersal of energy in a system.

Entropy is greater in a more disordered system, bc the energy can be dispersed more flexibly.

positive change in entropy (/\S +ve)

an increase in entropy - increase in disorder

eg. Increase in number of moles of GAS

negative change in entropy (/\S -ve)

a decrease in entropy - decrease in disorder

eg. decrease in number of moles of GAS

monobasic acid

an acid that can donate 1 H+ per molecule

dibasic acid

can donate 2 H+ ions per molecule

Strong Acid

dissociates in aqueous solutions

eg. HCl or H2SO4 or HNO3

HA(aq) —> H+(aq) + A-(aq)

higher Ka (lower pKa)

weak acid

dissociates only partially in aqueous solutions - equilibrium lies to the left

eg. CH3COOH

lower Ka values (higher pKa)

Strong bases

dissociates completely - equalibrium lies to the right

eg. LiOH or NaOH

weak bases

dissociate partially

eg. Ammonia (NH3) or CH3CH2NH2

Buffer solutions

resists changes in pH when small amounts of acid or alkali are added

made of weak acid and a soluble salt of the weak acid

OR

an excess of weak acid and a strong base

Enthalpy Change of Solution

Enthalpy change when 1 mol of solute is dissolved in water under standard conditions

BOTH EXOTHERMIC OR ENDOTHERMIC

NH4NO3 → NH4+ (aq) + NO3- (aq)

Enthalpy Change of Hydration

Enthalpy change when 1 mole of gaseous ions are completely surrounded by water molecules under standard conditions

EXOTHERMIC

Na+ (g) → Na+ (aq)

Transition metal

A d-block element which forms an ion with a partially-filled d subshell

Ligands

negative ions or neutral molecules which donate lone pairs of e- to bond to a transition metal.

Co-ordinate (dative) covalent bonds

bond formed between ligand and transition metal ion