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The heat of formation of ethane is -19.4...

The heat of formation of ethane is `-19.46 kcal`. Bond enegries of `H-H, C -H` and `C- C` bonds are `104.18, 99.0` and `80.0 kcal`, respectively. Calculate the heat of atomisation of graphite.

Text Solution

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`Delta_(f)H` ethane `(C_(2)H_(6))=-19.5` kcal/mol
`2C(s)+3H_(2)(g)rarr C_(2)H_(6)(g) , " " Delta_(f)H=-19.5` kcal/mol
Let the heat of atomisation is x kcal/mol
`{:("Bond Breaking":(Delta_(r )H_(1)),"Bond Formation": (Delta_(r )H_(2))),(2[C(s)rarr C(g), + x],1[C(g)+C(g)rarr C-C(g), - 80]),(3[H_(2)(g)rarr 2H(g), +104.2],6[C(g)+H(g)rarr C-H(g),-99]),(Delta_(r )H_(1)=2x+312.6,Delta_(r )H_(2)=(-80)+(6xx-99)=-674):}`
From Hess's Law : `" " Delta_(f)=Delta_(r )H_(1)+Delta_(r )H_(2)`
`-19.5=(2x+312.6)-674 rArr " " x=171` kcal.
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