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.

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

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

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

Bond enthalpy values = section 11

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

Mg2+(g) +O2-(g) Mg2+(g)+2e-+O(g) Mg(g)+O(g) Mg2+(g) +O-(g) 2nd EA= +753 1st and 2nd IE = 738+1451 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) 602+148+249+738+1451+(-141)+753 -3799 kJ mol-1 ΔHf=-602 MgO(s)

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

Lattice enthalpies depend on the size and charge of the ions

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

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

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