help meee! chemistry! experts help!?

Question

Using average bond energy values estimate ?H for the following gas phase reaction.


2CH2=CHCH3 + 2NH3 + 3O2 produces-> 2CH2=CHCN + 6H2O

please tell me how u arrived at htat answer.i've tried sO many times i keep getting it wrong! thanx!

Answer 1

OK, the key here is to note what types of bonds you have on each side of the equation, and the amount of energy in those bonds.

Reactants:
12 C-H bonds (6 * 2) - 413 kJ/mol * 12 = 4956
2 C=C bonds (1 * 2) - 614 = 1228
2 C-C bonds (1 * 2) - 348 * 2 = 696
6 N-H bonds (3 * 2) - 391 * 6 = 2346
3 O-O bonds (1 * 3) - 145 * 3 = 435
Total bond energy: 9661 kJ/mol

Products:
2 C=C bonds (1 * 2) - 614 = 1228
2 C-C bonds (1 * 2) - 348 * 2 = 696
2 C---N bond (1 * 2) (triple bond) - 890 * 2 = 1780
6 C-H bonds (3 * 2) - 413 kJ/mol * 6 = 2478
12 O-H bonds (6 * 2) - 366 * 12 = 4392
Total: 10574 kJ/mol

Thus, the ΔH is 9661 kJ/mol - 10574 kJ/mol = -913 kJ/mol

Note: The CN bond in the product must be a triple bond - because nitrogen has a valence of 3, and isn't bonded to anything but the Carbon.

The quick alternative, is to only write down the bonds that change during the reaction:

Reactants:
6 C-H bonds (6 * 2) - 413 kJ/mol * 6 = 2478
6 N-H bonds (3 * 2) - 391 * 6 = 2346
3 O-O bonds (1 * 3) - 145 * 3 = 435
Total bond energy: 5259

Products:
2 C---N bond (1 * 2) (triple bond) - 890 * 2 = 1780
12 O-H bonds (6 * 2) - 366 * 12 = 4392
Total: 6172

5259 - 6172 = -913 kJ/mol