1. 15.1 Energy cycles
Representative equations can be used for enthalpy/energy of hydration, ionization, atomization, electron affinity, lattice, covalent bond and solution.
Enthalpy of solution, hydration enthalpy, and lattice enthalpy are related in an energy cycle.

2. Ionization Energy and Electron Affinity
Ionization Energy = the energy (enthalpy) needed to remove one mole of electrons from one mole of ____________atoms
Electron affinity = the energy (enthalpy) change that occurs when _______________________ attracts one mole of electrons
Section 8 Data Booklet
gaseous
one mole of gaseous atoms

3. Lattice Enthalpy
Oppositely charged gaseous ions come together to form an ionic lattice (remember from ionic bonding), which is a very _____________process
The lattice enthalpy expresses the enthalpy change in terms of the endothermic process of one mole of a solid crystal lattice breaking apart into its gaseous ions
exothermic

4. Experimental Lattice Enthalpies and the Born Haber Cycle
Experimental lattice enthalpies cannot be determined directly so an energy cycle based on Hess’s Law is used called the Born-Haber cycle.
The cycle takes place in several steps.
Let’s look at sodium chloride for an example.
starting with parts in elemental form

6. Bond enthalpy values = section 11

7. We are using Hess’s Law to derive a theoretical value for:
Let’s construct the cycle for MgO(s), using section 8 and 11 of data booklet and values below:
We are using Hess’s Law to derive a theoretical value for:
Mg2+(g)+ O2-(g)MgO(s)
ΔH units are all kJ mol -1

8. Mg2+(g) +O2-(g) Mg2+(g)+2e-+O(g) Mg(g)+O(g) Mg2+(g) +O-(g)
2nd EA= +753
1st and 2nd IE =
1st EA= -141
Mg(g)+O(g)
Mg2+(g) +O-(g)
½ ΔE (O=O)= ½ (+498)
Mg(g)+ 1 2 O2(g)
ΔHatom=+148
ΔHlattice=?
Mg(s)+ 1 2 O2(g)
(-141)+753
-3799 kJ mol-1
ΔHf=-602
MgO(s)

9. Theoretical lattice enthalpies can be calculated from the ionic model
Assume the crystal is made from perfectly spherical ions
The energy needed to separate the ions depends on the product of the ionic charges and the sum of their ionic radii
Increase radii = decrease in attraction
Increase ionic charge = increase in attraction

11. Lattice enthalpies depend on the size and charge of the ions

12. Enthalpies of solution
Enthalpies of solution can be calculated by measuring the temperature change of a solution
Interactions between solute and solvent depend on the concentration of the solution
Hydrated = surrounded by water molecules

13. The hydration enthalpy of an ion depends on the attraction between the ions and the polar water molecules
Hydration enthalpies are approximately inversely proportional to the ionic radii

14. The hydration enthalpy of an ion depends on the attraction between the ions and the polar water molecules
Across period 3, hydration enthalpy becomes more exothermic as ionic charge increases and ionic radius decreases