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Question

From the following data :
Enthalpy of formation of CH3CN=87.86 kJ / mol
Enthalpy of formation of C2H6=83.68 kJ / mol
Enthalpy of sublimation of graphite =719.65 kJ / mol
Enthalpy of dissociation of nitrogen =945.58 kJ / mol
Enthalpy of dissociation of H2=435.14 kJ / mol
CH bond enthalpy =414.22 kJ / mol
Calculate the bond enthalpy of ( i ) CC ; ( ii ) CN

A
( i ) 370.58 kJ / mol1 ; ( ii ) 850.2 kJ / mol1
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B
( i ) 343.58 kJ / mol1 ; ( ii ) 891.2 kJ / mol1
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C
( i ) 370.58 kJ / mol1 ; ( ii ) 891.2 kJ / mol1
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D
( i ) 343.58 kJ / mol1 ; ( ii ) 850.2 kJ / mol1
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Solution

The correct option is C ( i ) 370.58 kJ / mol1 ; ( ii ) 891.2 kJ / mol1
Bond enthalpy BECC=? BECN=?
(i)BECC

C2H6(g)2C(graphite)+3H2(g) ΔfH1=83.68kJ/mol

2C(graphite)2C(g) ΔfH2=2×719.65kJ/mol

3H2(g)6H(g) ΔfH3=3×435.14kJ/mol

Adding we get,
C2H6(g)2C(g)+6H(g) ΔfH4=2828.4kJ/mol

ΔfH4=BECC+6BECH

so BECC=[2828.46×414.22]=343.08kJ/mol

(ii)BECN

CH3CN(g)2C(graphite)+32H2(g)+12N2(g) ΔfH1=87.66kJ/mol

2C(graphite)2C(g) ΔfH2=2×719.65kJ/mol

32H2(g)3H(g) ΔfH3=32×435.14kJ/mol

12N2(g)N(g) ΔfH4=12×945.58kJ/mol

Adding we get,
CH3CN(g)2C(g)+3H(g)+N(g) ΔfH5=2476.9kJ/mol

ΔfH5=BECC+3BECH+BECN
So BECN=(2476.93×414.22343.08)=891.2kJ/mol

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