Reactions having named enthalpy changes
The Born-Haber cycle uses reactions that have known enthalpy changes for a variety of elements. It is important to be able to associate the name or symbol for a known enthalpy change with the reaction. These are summarised below.
- Enthalpy changes for conversion of elements to atoms in the gas phase
Enthalpy changes producing gas phase atoms are always endothermic.
For metals the enthalpy of sublimationis equal to the enthalpy change for conversion of one mole of the substance in its standard state to its component atoms in the gas phase.
ΔsubH is for M(s)
M(g)
For nonmetallic elements that exist as diatomic molecules, the enthalpy of atomisation
is equal the enthalpy change for conversion of one mole of substance to its component atoms in the gas phase.
ΔaH is the enthalpy change forX2 (in state corresponding to standard state) 2X(g) - Ionization enthalpies for converting gas phase atoms to gas phase cations
This process is referred to as ionization: E(g) E+(g) + e–(g)
Ionization enthalpies (ΔiH) are always positive because charge separation is required.
- Electron affinities (interconversion between gas phase atom and anion)
Electron affinities (ΔEAH) may be defined as the enthalpy change either for
attaching an electron to an atom: E(g) + e– (g) E– (g) These are exothermic.removing an electron from an anion: E–(g) E(g) + e– (g)These are endothermic.Pay careful attention to the sign of the electron affinity given to know which definition is being used.
- Lattice enthalpies (interconversion of a lattice and its gas phase ions)
Lattice dissociation enthalpy: MX(s) M+(g) + X–(g) ALWAYS endothermicSeparating charge is endothermic.Lattice enthalpy: M+(g) + X–(g) MX(s) ALWAYS exothermic
Pay careful attention to the language used and the sign to see which is relevant for a particular problem.