Question

Using bond energy data, calculate the heat of formation of isoprene. Given that bond energy of $C - H =98.8 \,kcal$ $H - H =104\, kcal , C - C =83\, kcal , C = C =147 \,kcal$ and $C _{(s)} \rightarrow C _{(g)}=171 \,kcal$

• A. $28.3 \,kcal$
• B. $20.6 \,kcal$
• C. $283\,kcal$
• D. $220.6 \,kcal$

Answer 1

Answer: (B)

$\Delta H =\Sigma B \cdot E ._{\text {(reactant)}}-\Sigma B . E ._{\text {(product)}}$

$\Delta H =\left[5 B \cdot E \cdot C _{(s) \rightarrow C _{(g)}}+4 B \cdot E \cdot H - H \right]$

$-\left[2 B . E \cdot C - C +2 B \cdot E_{ C = C }+8 B \cdot E \cdot C - H \right. \Delta H =[5 \times 171+4 \times 104]-[2 \times 83+2 \times 147+8 \times 98.8]$

$=1271-1250.4=20.6\, kcal$