destroyer 120 vs topscore question 50 GEN CHEM

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pistolpete007

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ok on destroyer 120 how come they only take into account one C-H bond in both products and reactants... for example there are clearly 6 C-H bonds so why do they do 6*414


on top score when the product was 2H2O they took all 4 O-H into account and did 4*464

does ne1 know what im talking aobut?

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So I dont know the topscore question but the Destroyer only counts one C-H bond b/c only one of them broke! Im assuming thats not the case in topscroe and they count the number of C-H bonds broken in their problem.
 
I believe its a short cut as the number C-H broken on the reactant side is made up on the product.
 
Destroyer #120 GChem
The Net Energy = (Sum of Delta H Bonds Broken) - (Sum of Delta H Bonds Formed)

1 C-H bond is broken = 414
1 Br-Br bond is broken = 193

1 C-Br bond is formed = 293
1 H-Br bond is formed = 366

(414+193)-(293+366) = (607)-(659) = -52kJ Therefore 52 kJ is released, answer is correct.

It's not the number of bonds there are, it's the amount of bonds broken an formed is what you're trying to keep track of. I don't have topscore so i can't tell you what it says their. But Destroyer is correct.
 
I dont think its a short cut, they are just using the bonds broken which in the C2H6 molecule its just C-H and then that Hydrogen that left is replaced by Br, so the formed bond on there would be between C-Br....

Keep in mind your not breaking every bond in the molecule, only the ones that are interchanged...for example for C2H6 your not breaking every single C-H bond in the molecule your just breaking the C-H bond and forming a C-Br bond in the products...

Also for Br2 its Br--Br so thats 1 bond as well that your breaking and forming HBr.

The easiest way is to draw out the molecules...then I separate them into Products (bonds formed) and Reactants (bonds broken) and figure them out separately then I add up products separately and reactants separately and finally use the equation (broken)-(formed) and plug them in...this basically ensures that you dont mess up with all the numbers!!!
 
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