Chemistry - Thermochemistry Question with Solution | TestHub
The value of heat of combustion per CH2 unit of cyclopropane is -697 kJ/mol. Calculate its strain energy. 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 C₃H₆: Δc H = 3 × (-697) = -2091 kJ/mol.
The enthalpy of combustion (Δc H) can also be expressed using the enthalpies of formation (Δf H) as:
Δc H = 3Δf H(CO2) + 3Δf H(H2O) - Δf H(C3H6)
Therefore, the enthalpy of formation for C₃H₆ is:
Δf H(C3H6) = -1179 - 855 + 2091 = +57 kJ/mol
For atomization, the sum of the enthalpies of formation for 3C(g) + 6H(g) is:
Δf Hsum = 3(715) + 6(218) = 3453 kJ
The energy actually needed to break all bonds in C₃H₆ is:
3453 - 57 = 3396 kJ
For a strain-free molecule with 3 C-C bonds and 6 C-H bonds, the calculated energy is:
3(356) + 6(408) = 1068 + 2448 = 3516 kJ
The strain energy is the difference between the calculated strain-free energy and the actual energy needed:
3516 - 3396 = 120 kJ/mol
The answer is 0120.
