Summary AQA A-level thermodynamics notes

ENTHALPY CHANGE OF FORMATION(of a compound)= …is the energy
transferred when 1 mole of a compound is formed from it’s constituent elements
under standard conditions(298K and 100kPa), with all reactants and products being in
their standard states.

ENTHALPY CHANGE OF Enthalpy of sublimation:
ATOMISATION(of an element)= …is the The enthalpy change for a solid turning
enthalpy change when 1 mole of gaseous to gaseous atoms
atoms is formed from the element in it’s It will be numerically the same as
standard state: the enthalpy change of atomisation
Na (s) → Na (g) [atH= +148 kJ/mol]
½ O2 (g) → O (g) [atH= +249 kJ/mol] Na (s) → Na (g) [atH= +148 kJ/mol]


BOND DISSOCIATION ENTHALPY (Bond energy) …standard molar enthalpy change
when one mole of a covalent bond is broken into two gaseous atoms (or free radicals)

Cl2 (g)→ 2Cl (g) dissH= +242 kJ/mol
OR
CH4 (g) → CH3 (g)+ H (g) dissH=+435 kJ/mol


Bond dissociation enthalpy for diatomic molecules…
It’s the same as 2x atH of the element
Cl2 (g)→ 2Cl (g) dissH= +242 kJ/mol

½ Cl2 (g) → Cl (g) atH= +121 kJ/mol




First Ionisation energy enthalpy Second Ionisation energy enthalpy
Enthalpy change required to remove 1 Enthalpy change required to remove 1
mole of electrons from 1 mole of gaseous mole of electrons from 1 mole of gaseous
atoms to form one mole of gaseous ions +1 ions to form one mole of gaseous ions
with a +1 charge with a +2 charge

Mg(g) → Mg+ (g) + e- [IE 1 H] Mg+(g) → Mg2+ (g) + e- [IE 2 H]

, FIRST ELECTRON AFFINITY
Enthalpy change that occurs when 1 mole of gaseous atoms gain 1 mole of electrons to form
1 mole of gaseous ions with a -1 charge

O(g) + e- → O- (g) [ea 1 H]= -141.1 kJ/mol
Refers to single atoms (O) not molecules (O 2)


SECOND ELECTRON AFFINITY
Enthalpy change that occurs when one mole of gaseous -1 ions gains one electron per ion
to produce gaseous -2 ions. Note: the second electron affinity of
oxygen is endothermic because it
O-(g) + e- → O2- (g) [ea 2 H]= +798 kJ/mol takes energy to overcome the
Refers to single atoms (O) not molecules (O2) repulsive force between the negative
ion and electron.


ENTHALPY OF LATTICE FORMATION
Standard enthalpy change when 1 mole of an ionic crystal is formed from it’s constituent
ions in gaseous form. Note: when a lattice forms, new bonds
are formed, resulting in energy being
Na+ (g) + Cl-(g) → NaCl (s) [lattH= -787kJ/mol] given out, so H is always negative
for this process



ENTHALPY OF LATTICE DISSOCIATION Always the
Standard enthalpy change when 1 mole of an ionic crystal lattice form is same value,
separated into it’s constituent ions in gaseous form but
Note: when a lattice is
formation is
broken, bonds are
NaCl (s) → Na+ (g) + Cl- (g) [lattH= +787kJ/mol] negative
broken and this requires
and
energy input, so H is
dissociation
always positive for this
is positive
process


ENTHALPY OF HYDRATION hydH
Enthalpy change when one mole of gaseous ions become aqueous ions
Note: always negative;
X+ (g) + aq → X+ (aq) For Li+ hydH= -519 kJ/mol energy is released as
OR electrostatic attractions are
X- (g) + aq → X-(aq) For F- hydH= -506 kJ/mol formed between the ion and
the polar water molecules
The greater the charge: size ratio of the ion, the surrounding it
greater the attraction for the polar water molecules,
so the greater the value of hydH