Question

Using bond energy data, calculate heat of formation

of isoprene

5C(s) + 4H2(g) — H,C=C—CH=CH,

CH,

Given : BE for C-H, H-H, C-C, C = C and C(s) →

C(g) respectively as 98.8 kcal, 104 kcal, 83 kcal,

147 kcal, 171 kcal

(1) – 20.6 kcal

(2) 20.6 kcal

(3) 40 kcal

(4) 50 kcal

Answer 1

Answer.....

Explanation:

Answer 2

Answer:

Heat energy data is 20.6 kcal.

So option 2 is the right answer.

Explanation:

Consider a moving successive Hydrogen atoms from Methane molecule.

So to dissociate a single C-H bond requires some energy i.e 105 kcal and this energy is BDE.

DH reaction = Bond energy data used for formation of Bond = Bond energy data used for dissociation of bond.

$= - {2(1 - c) + 2(c = c) + 8(c - 4)} + {5c + 4(4 - 4)}}$

$= - {(2 \times 83 + 2 \times 104 + 8 \times 98.8} + {5 \times 171 + 4 \times 14)}$

= 20.6 kcal