Chemistry - Thermochemistry Question with Solution | TestHub
The standard molar enthalpies of formation of BrF3(g) and BrF5(g) are -258 kJ and -436 kJ, respectively. BrF5(g) is square pyramidal with all Br-F bonds equivalent, while BrF3(g) is T-shaped with different Br-F bond lengths. The axial Br-F bonds in BrF3 are equivalent to the Br-F bonds in BrF5. Calculate the equatorial Br-F bond strength (in kJ/mol) in BrF3. Other data: Br2(l) → Br2(g); ΔH = 30 kJ. F2(g) → 2F(g); ΔH = 155 kJ. Br2(g) → 2Br(g); ΔH = 193 kJ.
Answer:
Solution:
Enthalpy to get one Br(g) from Br2(l): 30 + 193/2 = 111.5 kJ; per F(g): 77.5 kJ. BrF5: atomisation = 111.5 + 5(77.5) + 436 = 935 kJ, so each Br-F = 187 kJ/mol. BrF3: atomisation = 111.5 + 3(77.5) + 258 = 602 kJ. Two axial bonds: 2(187) = 374. Equatorial = 602 - 374 = 228 kJ/mol. The answer is 0228.
