Lattice Energy, DUlattice The change when one mole of a solid crystal formed from its ions, atoms, molecules in their gaseous state ,its always negative Relates to the endothermic process MX(s) M+(g) + X-(g) NaCl(s)  Na+(g) + Cl-(g) DHlattice= 769 kJ/mol Endothermic reactions

Factors affecting the lattice enthalpy 1-The greater the charge of the ions, the stronger the electrostatic attraction

Factors affecting the lattice enthalpy 2-smaller the ionic radius, shorter the distance, stronger the electrostatic attraction

Boardworks A2 Chemistry Thermodynamics Example 1 F– O2– N3– high charge increasing lattice formation enthalpy K+ Na+ small size. Teacher notes It should be noted that N3–, O2– and F– are not actually exactly the same size. F– is the smallest because it has the highest nuclear charge, followed by O2–, followed by N3–. Li+ 4

Boardworks A2 Chemistry Thermodynamics Example 2 The size of the ion. The smaller the ion, the larger the enthalpy of hydration. Ion DHhyd (kJ mol–1) Li+ –519 F– –506 increasing size Na+ –406 Cl– –364 K+ –322 Br – –335 The charge on the ion. The larger the charge on the ion, the larger the enthalpy of hydration. Ion DHhyd (kJ mol–1) Fe2+ –1950 Fe3+ –4430

Lattice energy: estimates from an electrostatic model Born landel equation N= Avogadros constant =6.022 *10 M= Madlonuge constant

The lattice energy is estimated in joules per mole of compound

n = Born exponent

Lattice energy: the Born-Haber cycle To determine the lattice enthalpy and the degree of ionic character of a salt (Hess’ law) .

Worked example :the Born-Haber cycle Given that the standard enthalpy of formation at 298 K of CaF2 is -1228 kJ mol-1, determine the lattice energy for CaF2 using appropriate data from the Appendices.

1-standard enthalpy of formation is the change of enthalpy during the formation of 1 mole of the substance from its constituent elements, in their standard states.(usually 1atm , 298.15 K). 2-standard enthalpy of sublimation (atomization), is the heat required to change 1mole of a substance from solid state to gaseous state at usually standard temperature and pressure (STP) 3-standard dissociation energy  is the change of enthalpy  to break 1mole of a bond to give separate atoms