# thermo

The heat of combustion of acetylene is 312 kcal . If heat of formation of CO2 and H2O are 94.38 and 68.38 kcal respectively , calculate C triple bond C energy . Given that heat of atomization of C and H are 150 and 51.5 kcal respectively and C-H bond energy is 93.64 kcal.

now i think i am making a calculation error here... :-\

i have added twice the heat of C-H bond , bond energy of C triple bond C ( let it be x ) , twice the heat of formation of CO2 and heat of formation of H2O and also the two heat of atomizations and equated it to 312 kcal.........but not getting the right answer......where am i wrong?? ??? ???

30
Ashish Kothari ·

edit:

C2H2 + 2.5O2â†’ 2CO2 + H2O

Î”Hc=-312kcal (Assuming it..since you have not given negative signs for any of them)

Î”Hc = Î£Î”HÂ°f(P) - Î£Î”HÂ°f(R)

= -(2*94.38 + 68.38) - ( x + 2*150 + 51.5 - 2*93.64) [ 2C(g) + H2(g) â†’ C2H2(g) ]

= -257.14 - x - 351.5 + 187.28

Î”HCâ‰¡C = -109.36 kcal

1
kunl ·

Very easy question answer is coming as 160.86 kcal/mol
[in the above answer i m assuming by heat of formation of CO2 and H2O are 94.38 and 68.38 kcal u meant to say that heat of formation of CO2 and H2O are -94.38 and -68.38 kcal]

plz correct the problem accordingly[1]

30
Ashish Kothari ·

kunl can you please post the solution.

6
AKHIL ·

ya coz i m not gettin the ans as 160......

1
kunl ·

simply use so-called Laplace Law and Hess Law!

6
AKHIL ·

hess's law to theek hai yeh thermo mein laplace law kab aaya???:P

1
kunl ·

jab se chem mein thermo aya[1]

btw cn u post solution to the first problem here as no one except u seems to get the answer mentioned as 1 L in book
http://www.targetiit.com/iit-jee-forum/posts/gaseous-state-18718.html

39
Pritish Chakraborty ·

I believe he's referring to -:
"Lavoisier and Laplaceâ€™s law : The energy change accompanying any transformation is equal and opposite to energy change accompanying the reverse process"

1
kunl ·

ya wahi wala....m weak at remembering these idiotic laws(especially the name of founder's :P....but still i m patting my back for remembering half of it[3])

6
AKHIL ·

can u just show what u did??coz i think i m doin a real stupid calculation mistake..