Chem: Enthalpy

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Hi guys, for the question below, i don't understand why they use this formula: ∆H = -∑∆H(product bonds) + ∑∆H(reactant bonds)
shouldnt it be +prod-reactant? thanx in advance!

Chemistry Achiever 2

Given the average bond energies, N-H(389 kJ/mol), H-F(565 kJ/mol), and F-F(155 kJ/mol), calculate the average bond energy for N-F.
NH3(g) + 3F2(g) à NF3(g) + 3HF(g) ∆H = -873.81 kJ


A. 936.81 kJ/mol

B. 810.81 kJ/mol

C. 312.27 kJ/mol

D. 270.27 kJ/mol

E. 63 kJ/mol


Correct Answer: D
explanation:
Generally, energy is required to break each reactant bond while the same is evolved in product bond formation.
Enthalpy change,
∆H = -∑∆H(product bonds) + ∑∆H(reactant bonds)
∆H = -[3∆H(N-F) + 3∆H(H-F)] + [3∆H(N-H) + 3∆H(F-F)]
-873.81 = -[3∆H(N-F) + 3(565)] + [3(389) + 3(155)]
-3∆H(N-F) = -810.81
∆H(N-F) = 270.27 kJ/mol

 

[Fariba]

The numbers given to you are the bond energies = the energy required to BREAK a bond in the molecule. So you have to reverse the sign of the products to give you the form ∆H = -∑∆H(product bonds) + ∑∆H(reactant bonds) It is easy to confuse bond energy with bonds formed.


I think I am right. But maybe someone has a better explanation...

 

[Fariba]

wait.... now I'm confused...

thanks a lot! 😀