1. ENERGETICS IB Topics 5 & 15 PART 3 :Energy Cycles

2. Enthalpy of atomization, Definition: the standard enthalpy change when one mole of gaseous atoms is formed from the element in its standard state under standard conditions. For diatomic molecules this is equal to half the bond dissociation enthalpy Ex: ½ Cl 2 (g) + 121 kJ  Cl (g) X(standard) + ΔH at → X(g) Ex: C (s) + heat  C(g)

3. This slide is missing in the notes!!! Standard Heat of formation: ΔH° f Definition: enthalpy change that occurs when one mole of the substance is formed from its elements in their standard states M (s) + ½ X 2(g) → MX (s) )

4. ELECTRON AFFINITY, Definition: the enthalpy change when an electron is added to an isolated atom in the gaseous state. X(g) + e - → X - (g)

5. Ionization energy, IE Definition: the enthalpy needed to remove an electron from an isolated atom in the gaseous state. X(g) + IE → X + (g) + e -

6. LATTICE ENTHALPY, energy needed to turn a crystalline solid into its gaseous ions or energy released when gaseous ions turn into a crystalline solid MX(s)  M + (g) + X - (g)

7. LATTICE ENTHALPY, The sign of the lattice enthalpy indicates whether the lattice is being formed (-) or broken (+). The size of the lattice enthalpy depends both on the size of the ions and on the charge carried by the ions.

8. LATTICE ENTHALPY, Cation size ____________ LiClNaClKCl Lattice Enthalpy (kJ mol -1 ) 846 771701 increasing

9. LATTICE ENTHALPY, anion size ____________ NaClNaBrNaI Lattice Enthalpy (kJ mol -1 ) 771 733684 increasing

10. Makes sense… the larger the ion, the easier to separate

11. LATTICE ENTHALPY, Charge on cation ____________ NaClMgCl 2 Lattice Enthalpy (kJ mol -1 ) 771 2493 increasing

12. LATTICE ENTHALPY, Charge on anion ____________ MgOMgCl 2 Lattice Enthalpy (kJ mol -1 ) 2493 3889 increasing

13. Makes sense… the greater the difference in charge, the stronger the attraction and the harder it is to separate ions

14. How do we determine the lattice enthalpy of an ionic compound? LATTICE ENTHALPY, Experimental Lattice enthalpies cannot be determined directly. An energy cycle known as The Born-Haber Cycle is used.

15. Born Haber Cycles =energy cycles for the formation of ionic compounds. Δ H at Δ H IE Δ H EA M (s) + ½ X 2 (g) MX (s) M (g) + X (g) M + (g) + X - (g) M (s) + ½ X 2 (g) MX (s) M (g) + ½ X 2 (g) M (g) + X (g) M + (g) + X (g) M + (g) + X - (g) Δ H lat Δ H f ΔH lat = ΔH at.M +ΔH at.X +Δ IE + Δ EA -Δ H f

16. Example: The enthalpy change for the formation of sodium chloride can be considered to occur through a series of separate steps. Na(s) + ½Cl 2 (g) Na + Cl - (s) Na + (g) + Cl - (g) Na(g)Cl(g) ΔH lattice NaCl According to Hess Law : ΔH° f (NaCl) = ΔH° atom (Na) + ΔH° IE (Na) + ΔH° atom (Cl) + ΔH° EA (Cl) - ΔH° Lattice (NaCl ) ΔH° Lattice (NaCl ) =ΔH° atom (Na) + ΔH° IE (Na) + ΔH° atom (Cl) + ΔH° EA (Cl) -ΔH° f (NaCl)

17. Born-Haber Cycle for formation of NaCl (Blue = Red) ΔH° Lattice (NaCl ) =ΔH° atom (Na) + ΔH° IE (Na) +ΔH° atom (Cl) + ΔH° EA (Cl) - ΔH° f(NaCl)

18. To form gaseous ions from solid MgO

19. SOLUTIONS: Lattice Enthalpy :endothermic process of turning a crystalline solid into its gaseous ions. ✗ Enthalpy of hydration: energy released when one mole of gaseous ions is dissolved to form an infinitely dilute solution of one mole of aqueous ion. ✗ Enthalpy of solution: energy change that takes place when an ionic compound is dissolved – It can be positive or negative. TO FORM A SOLUTION ( + or – ΔH ) : 1- ions must separated (positive lattice enthalpy) 2- ions must be hydrated (negative hydration enthalpy) What do we omit? ✗

20. Enthalpy of hydration: energy released when one mole of gaseous ions is dissolved to form an infinitely dilute solution of one mole of aqueous ion. M n+ (g) → M n+ (aq)

21. Trends : The values become less exothermic as the groups are descended and the ionic radius increases: lesser attraction between water and the ions The values become more exothermic as the ionic charge increases and the ionic radius decreases: stronger attraction between water and the ions

22. Hydration Enthalpy versus 1/ R The hydration enthalpies of the ions are approximately inversely proportional to the ionic radii for group 1 and 7 (17)

23. TO FORM A SOLUTION ( + or – ΔH solution ) : 1- ions must separated (positive lattice enthalpy) 2- ions must be hydrated (negative hydration enthalpy)