Question

Physical Chemistry:

Use a Born-Haber cycle to find an experimentally based value for the lattice enthalpy of sodium bromide (NaBr(s)). The lattice enthalpy corresponds to the enthalpy change for the process NaBr(s) rightarrow Na^+(g) + Br^-(g) Use the following information in doing this problem. delta H degree_f(Na(g)) = 107.32 kJ/mol delta H degree_IE1(Na(g)) = 495.8 kJ/mol delta H degree_f(Br(g)) = 111.88 kJ/mol delta H degree_EA(Br(g)) = -324.6 kJ/mol delta H degree_f(NaBr(s)) = -361.06 kJ/mol The ionization enthalpy (IE_1) and electron affinity enthalpy (EA) are for the processes Na(g) rightarrow Na^+(g) + e^- Br(g) + e^- rightarrow Br^-(g) As discussed in class, the enthalpy of combustion for an organic compound can be used to find the enthalpy of formation for the compound. The energy of combustion for phenanthraquinone (C_14H_8O_2(s)) is, for standard conditions (p = 1.00 bar, T = 25.0 degree C) delta U degree_comb(C_14H_8O_2(s)) = -6457.6 kJ/mol. Give the combustion reaction for phenanthraquinone. Using the value for delta U degree_comb for phenanthraquinone given above, find the value for delta H degree_comb for phenanthraquinone. Find the value for delta H degree_f(C_14H_8O_2(s)), the enthalpy of formation for phenanthraquinone. Use the following values for formation enthalpies in your work. delta H degree_f(CO_2(g)) = -393.51 kJ/mol delta H degree_f(H_2O(1)) = -285.83 kJ/mol

Answer 1

breaking down each of the stages Na (g) Br (g) ionize add electron Na(g) lattice energy Br(g) atomize atmize AHNaBr(s) Na(s)Br2(g) a (s)

Na(s) ---> Na(g) : x
Nag(g) ---> Na+ (g) : y
1/2 Br2(g) ---> Br(g) : z
Br(g) ---> Br-(g) : a

x+y+z+a + lattice energy = energy of NaBr
107.32 + 495.8 + 111.88 - 324.6 + LE = -361.06
LE = -751.46 KJ/mol
Answer: -751.46 KJ/mol

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