Lattice Enthalpy15.2.1

15.2.1 – Define and apply the terms lattice enthalpy and electron affinity.

Q – Which one of these are you already familiar with?

First ionization energy (ΔHi˚) Earlier we learned that metals and non-

metals tend to form ionic compounds. Q – Who loses and who gains

electrons? ΔHi˚ refers to the energy needed to

form the positive ion.Na(g) Na+(g) + e-(g) ΔHi˚= +496 kJ/mol

Q – Why is this value >0? Q – Which element would have a larger

ΔHi˚ Na or Mg?

First electron affinity (ΔHe˚) On the other hand, the non-metal

elements in ionic compounds attract electrons.

ΔHe˚ refers to the enthalpy change when one mole of gaseous atoms attracts one mole of electrons.Values can be found in Table 7 of the IB

Data Booklet.Cl(g) + e-(g) Cl-(g) ΔHe˚= -349 kJ/mol

Q – Is this process endo or exothermic?

Lattice Enthalpies So what if we put those two processes

together?Na(g) + Cl(g) Na+(g) + Cl-(g)ΔH˚= ΔHe˚ + ΔHi˚ = -349 + 496 = +147 kJ/mol

We can see that e- transfer process is endothermic, yet we know that the formation of salt is highly exothermic. Something else must be at play.

Q - In reality, what do those oppositely charged gas ions do?

Lattice Enthalpies The oppositely charged gas ions

come together to form an ionic lattice (framework).

Q – Is this process endo or exothermic?

This process is highly exothermic as there is a strong attraction between the ions. Na+(g) + Cl-(g) NaCl(s) ΔH˚= -790 kJ/mol

Generally, most endothermic reactions do not readily occur (like the transfer of electrons noted before) but this last step explains the readiness of metals and non-metals to form

ionic compounds.

Lattice Enthalpies The lattice enthalpy ΔHlat˚ expresses this

enthalpy change in terms of the reverse endothermic process.It can also be described as the enthalpy related to

the formation of gaseous ions from one mole of a solid crystal breaking into gaseous ions.

NaCl(s) Na+(g) + Cl-(g) ΔHlat˚= +790 kJ/mol

Review of ΔH variables

ΔH˚ = ? ΔHlat˚ = ?

ΔHi˚ = ?

ΔHe˚ = ?

ΔHc˚ = ?

ΔHf˚ = ?

HW

Work on your calorimetry IA