Enthalpy G chem question

[Corky Cacas]

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isn't the answer is D.
i have the correct answer as C, but i believe it is wrong.
Delta H or rxn = Delta H of product - Delta H of reactant

Bond Bond Enthalpy (KJ/mol)
H-H 435
Cl-Cl 243
H-C 414
H-Cl 431
C-Cl 331

CH4 + Cl ------> CH3Cl + H ?

A. 275KJ
B. 109KJ
C. 83 KJ
D. -83KJ
E. -109

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[shawny8351]

i think the correct answer is C.

change in energy = energy input(bond broke) - energy output (bond formed)

in this case, only C-H bond broke and only C-Cl bond formed.

414-331 = 83


please let me know if i did this right.

 

[osims]

yea thats right, but its deltaH of reaction= (energy of bonds broken) + (energy of bonds formed). When forming bonds there is a release of energy and therefore the C-Cl bond energy would be negative.

 

[sushi320]

I am pretty sure that when you break bonds the reaction is Negative cause there is a release of energy, and when forming bonds its positive because you have to input energy. I could be wrong, but that is how i have always learned it.

 

[sushi320]

Actually is the opposite of what i said.

http://www.chem.queensu.ca/people/faculty/Mombourquette/FirstYrChem/Molecular/BondEne/index.htm

I guess just remember it as, when bonds are formed energy is released cause the electrons are more stable and so the molecule loses excess energy.

so its -331+414. so 83 is correct.

 

[Corky Cacas]

i think the correct answer is C.

change in energy = energy input(bond broke) - energy output (bond formed)

in this case, only C-H bond broke and only C-Cl bond formed.

414-331 = 83


please let me know if i did this right.

yes. the answer is 83. you are right.

 

[joonkimdds]

The answer is 83. I will bet all my properties including playstation3 + wii.

 

[Corky Cacas]

The answer is 83. I will bet all my properties including playstation3 + wii.

i can not argue with you. yes the answer is 83.

 

[joonkimdds]

i can not argue with you. yes the answer is 83.

ROFL, very similar questions were on my OCHEM exam, and final exam so I know this one for sure. I took ochem last semester(final exam was less than 1 month ago 😀)