Help with calculating bond energies.

The formula is C3H8 + Br2 -- C3H7Br +HBr

C-H, 414; C-Br, 276; Br-Br, 193; H-Br, 364.

I have bond breaking as 414+193=607
And bond forming as 414+276+364=1054

The answer is -33kj, but I don't understand why. What did I do wrong?

Question

Help with calculating bond energies.

The formula is C3H8 + Br2 --> C3H7Br +HBr

 C-H, 414; C-Br, 276; Br-Br, 193; H-Br, 364.

I have bond breaking as 414+193=607
And bond forming as 414+276+364=1054

The answer is -33kj, but I don't understand why. What did I do wrong?

jfraser · Answer

you have to multiply the bond enthalpy by EACH bond that is broken or formed. THe C-H bond is worth 414kJ/mol, but that's PER BOND. THe C3H8 molecule has 8*C-H bonds that have to break. You hav eto account for all the bonds that break and form to get the DH for the reaction.

anonymous · Answer

Why is it 8 C-H bonds and not 11 or 3?

anonymous · Answer

Ex. c3h8 has 3-C and 8-C. Why do you only use the 8?

jfraser · Answer

because of the lewis structure of propane, C3H8.

anonymous · Answer

oh