CHEMICAL ENERGETICS
1.1 Enthalpy change
Enthalpy change (ΔH) refers to the amount of heat energy transferred during a chemical reaction, at
a constant pressure
Enthalpy change of atomisation
The standard enthalpy change of atomisation (ΔHatꝋ) is the enthalpy change when 1 mole of gaseous
atoms is formed from its element under standard conditions (298 K and 1 atm)
The ΔHatꝋ is always endothermic as energy is required to break any bonds between the atoms in the
element into its gaseous atoms – positive value
Na(s) → Na(g) ΔHatꝋ = +107 kJ mol -1
Lattice energy
The lattice energy (ΔHlattꝋ) is the enthalpy change when 1 mole of an ionic compound is formed from
its gaseous ions (under standard conditions)
The ΔHlattꝋ is always exothermic, as when ions are combined to form an ionic solid lattice there is an
extremely large release of energy – negative value
Ionic compound is much more stable than its gaseous ions because of strong electrostatic forces
Mg2+(g) + 2Cl-(g) → MgCl2(s)
1.2 Electron affinity
The first electron affinity (EA1) is the enthalpy change when 1 mole of electrons is added to 1
mole of gaseous atoms, to form 1 mole of gaseous ions each with a single negative charge under
standard conditions
EA1 is exothermic, as energy is released – negative value
EA2 and EA3 are endothermic, as energy is absorbed – positive value
o The incoming electron is added to an already negative ion
o Energy is required to overcome the repulsive forces between the incoming electron and
negative ion
, Factors affecting electron affinity
The greater the attractive forces between the electron and nucleus, the more energy is released -
more exothermic (more negative) – EA value will be
o Nuclear charge: the greater the nuclear charge, the stronger the attractive forces between an
incoming electron and the nucleus – more exothermic
o Distance: the greater the distance between the nucleus and the orbital where the electron is
added, the weaker the force of attraction – less exothermic
o Shielding: the greater the number of shells, the greater the shielding effect and the weaker the
force of attraction – less exothermic
Trends in electron affinity of Group 16 & Group 17
elements
Electron affinities of non-metals become more
exothermic across a period, with a maximum at Group
17 – increasing number of electrons in same shell –
higher attractive forces
The electron affinities become less exothermic as we
go down group 16 & 17 – increasing shielding effect –
lower attractive forces
Fluorine is an exception as it has a small atomic radius
– higher repulsive forces – lower attractive forces –
less exothermic than chlorine
Factors affecting the lattice energy
Ionic radius - as the ionic radius of the ions increases – ions
more spread out - electrostatic force of attraction decrease –
less exothermic the lattice energy is – less stable the ionic
compound is
Ionic charge - as the ionic charge of the ions increases - higher
the charge density –higher hydration energy– more exothermic
the lattice energy – more stable the ionic compound is
1.1 Enthalpy change
Enthalpy change (ΔH) refers to the amount of heat energy transferred during a chemical reaction, at
a constant pressure
Enthalpy change of atomisation
The standard enthalpy change of atomisation (ΔHatꝋ) is the enthalpy change when 1 mole of gaseous
atoms is formed from its element under standard conditions (298 K and 1 atm)
The ΔHatꝋ is always endothermic as energy is required to break any bonds between the atoms in the
element into its gaseous atoms – positive value
Na(s) → Na(g) ΔHatꝋ = +107 kJ mol -1
Lattice energy
The lattice energy (ΔHlattꝋ) is the enthalpy change when 1 mole of an ionic compound is formed from
its gaseous ions (under standard conditions)
The ΔHlattꝋ is always exothermic, as when ions are combined to form an ionic solid lattice there is an
extremely large release of energy – negative value
Ionic compound is much more stable than its gaseous ions because of strong electrostatic forces
Mg2+(g) + 2Cl-(g) → MgCl2(s)
1.2 Electron affinity
The first electron affinity (EA1) is the enthalpy change when 1 mole of electrons is added to 1
mole of gaseous atoms, to form 1 mole of gaseous ions each with a single negative charge under
standard conditions
EA1 is exothermic, as energy is released – negative value
EA2 and EA3 are endothermic, as energy is absorbed – positive value
o The incoming electron is added to an already negative ion
o Energy is required to overcome the repulsive forces between the incoming electron and
negative ion
, Factors affecting electron affinity
The greater the attractive forces between the electron and nucleus, the more energy is released -
more exothermic (more negative) – EA value will be
o Nuclear charge: the greater the nuclear charge, the stronger the attractive forces between an
incoming electron and the nucleus – more exothermic
o Distance: the greater the distance between the nucleus and the orbital where the electron is
added, the weaker the force of attraction – less exothermic
o Shielding: the greater the number of shells, the greater the shielding effect and the weaker the
force of attraction – less exothermic
Trends in electron affinity of Group 16 & Group 17
elements
Electron affinities of non-metals become more
exothermic across a period, with a maximum at Group
17 – increasing number of electrons in same shell –
higher attractive forces
The electron affinities become less exothermic as we
go down group 16 & 17 – increasing shielding effect –
lower attractive forces
Fluorine is an exception as it has a small atomic radius
– higher repulsive forces – lower attractive forces –
less exothermic than chlorine
Factors affecting the lattice energy
Ionic radius - as the ionic radius of the ions increases – ions
more spread out - electrostatic force of attraction decrease –
less exothermic the lattice energy is – less stable the ionic
compound is
Ionic charge - as the ionic charge of the ions increases - higher
the charge density –higher hydration energy– more exothermic
the lattice energy – more stable the ionic compound is
