Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 12.12 Enthalpy of atomisation, electron affinity, lattice energy-Study Notes - New Syllabus

12.12 Definitions: Atomisation, Electron Affinity and Lattice Energy

These thermodynamic terms are essential for understanding energy changes involved in the formation of ionic compounds and are widely used in Born–Haber cycles. Each term describes a specific step involving atoms, ions, or electrons.

(i) Standard Enthalpy Change of Atomisation (\( \mathrm{\Delta_{at}H} \))

The standard enthalpy change of atomisation is the enthalpy change when one mole of gaseous atoms is formed from an element in its standard state under standard conditions.

Key Features

• Always forms gaseous atoms.
• The element must be in its standard state (e.g. solid metal, diatomic gas).
• Always endothermic (\( \mathrm{\Delta H > 0} \)) because energy is required to break bonds.

Examples

\( \mathrm{Na(s) \rightarrow Na(g)} \)

\( \mathrm{\frac{1}{2}Cl_2(g) \rightarrow Cl(g)} \)

For diatomic elements, bond breaking is involved, so energy is required to dissociate the molecule.

(ii) Electron Affinity

Electron affinity is the enthalpy change when one mole of gaseous atoms each gains one electron to form one mole of gaseous negative ions.

Key Features

• Occurs in the gas phase.
• Involves addition of an electron to an atom.
• The first electron affinity is usually exothermic (\( \mathrm{\Delta H < 0} \)).
• The second electron affinity is always endothermic due to electron–electron repulsion.

Example

\( \mathrm{Cl(g) + e^- \rightarrow Cl^-(g)} \)

This process releases energy because the incoming electron is attracted to the nucleus.

(iii) Lattice Energy

Lattice energy is the enthalpy change when one mole of an ionic solid is formed from its gaseous ions under standard conditions.

Key Features

• Defined as an exothermic process (\( \mathrm{\Delta H < 0} \)).
• Involves formation of a solid ionic lattice from gaseous ions.
• Large energy is released due to strong electrostatic attraction between oppositely charged ions.

Example

\( \mathrm{Na^+(g) + Cl^-(g) \rightarrow NaCl(s)} \)

The magnitude of lattice energy depends on ionic charge and ionic radius.

Key Comparisons

• Atomisation involves formation of gaseous atoms (endothermic).
• Electron affinity involves addition of electrons to gaseous atoms (usually exothermic).
• Lattice energy involves formation of an ionic solid from gaseous ions (exothermic).