Summary Topic 1 - Lattice Enthalpy

Chemistry
Topic 1: Lattice Enthalpy

Standard lattice enthalpy ΔHlatt (Endothermic – Positive value): The enthalpy change when 1 mole of
an ionic compound is converted into its gaseous ions under standard conditions

A large value for lattice enthalpy shows that there are strong electrostatic forces of attraction
between the oppositely charged ions in the lattice.

Born-Haber Cycles
Standard enthalpy change of Formation, ΔHf (Exothermic – Negative value): The enthalpy change
when one mole of a compound is formed from its elements under standard conditions.
e.g. K(s) + ½ Cl2(g)  KCl(s)

Standard enthalpy change of Atomisation, ΔHat (Endothermic – positive value): The enthalpy change
when one mole of gaseous atoms is formed from the element in its standard state.
e.g. K(s)  K(g)
½ Cl2 (g)  Cl(g)

Standard bond dissociation enthalpy, ΔHBDE (Endothermic – positive value): The enthalpy change
required to break 1 mol of a covalent bond under standard conditions.
e.g. Cl2 (g)  2Cl (g)

First ionisation energy, ΔHI1 (Endothermic – Positive value): The energy change required to convert
one mole of gaseous atoms into gaseous ions with a single positive charge.
e.g. K(g)  K+(g) + e-

Second ionisation energy, ΔH12 (Endothermic – Positive value): The energy change required to
convert one mole of gaseous ions with a single positive charge into gaseous ions with a double
positive charge.
e.g. Ca2+(g)  Ca2+(g) + e-

First Electron affinity, ΔHEA1 (Exothermic – Negative value): The enthalpy change when one mole of
gaseous atoms is converted int gaseous ions with a single negative charge.
e.g. Cl(g) + e-  Cl- (g)

Second Electron affinity, ΔHEA2 (Endothermic – Positive Value): The enthalpy change when one mole
of gaseous ions with a single negative charge is converted into gaseous ions with a double negative
charge.
e.g. O-(g) + e-  O2-(g)
Charges repel and therefore the gaseous ions must take in energy