Chemistry - Thermochemistry Question with Solution | TestHub
The heat of combustion per CH2 unit of cyclobutane is -686 kJ/mol. Calculate its strain energy (in kJ/mol). Given: the heats of formation of C(g), H(g), carbon dioxide and water are 715, 218, -393 and -285 kJ/mol, respectively, and C-C and C-H bond energies are 356 and 408 kJ/mol, respectively.
Answer:
Solution:
For C₄H₈: Δc H = 4(-686) = -2744 kJ/mol.
Δf H(C₄H₈) = 4(-393) + 4(-285) + 2744 = -2712 + 2744 = +32 kJ/mol.
The energy required to break all bonds in C₄H₈ is calculated as:
4(715) + 8(218) - 32 = 2860 + 1744 - 32 = 4572 kJ.
For a strain-free molecule (with 4 C-C bonds and 8 C-H bonds), the energy would be:
4(356) + 8(408) = 1424 + 3264 = 4688 kJ.
The strain energy is the difference between the energy of a strain-free molecule and the actual energy:
4688 - 4572 = 116 kJ/mol.
The answer is 0116.
