Question

Use bond energies as found in Table 9.1 of Tro, Fridgen and Shaw and estimate the energy change ∆ H o (in kJ/mol) for the following reaction:

H—C ≡ C—H + 2 I 2 → CHI 2 —C HI 2

The C-I bond energy is 213 kJ/mol.

Answer 1

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SOLUTION:

For the given reaction:

H-C=C - H+2(1 - 1) + CHI2 - CHI,

Bond Type:	No.of Bonds	Bond energy: (kJ/mol)
C - H	4	411
C= C	1	835
	2	148
C - C	1	346
C - 1	4	213

We know that, $\Delta H^{o}$ of this reaction is given by,

ΔΗ° = ΣΒ.Ε(reactants) – ΣΒ.Ε(products)

AH° = [2.B.E(C-H) + B.E(C = H) +2.B.E(I - 1)]–[B.E(C-C)+ 4.B.E(C-1) + 2B.E(C – H)]

.

AH = [2.(411) + (835) +2.(148)] kJ/mol-[(346)+4.(213)+2(411)] kJ/mol

.

:. AH = 1953 kJ/mol - 2020 kJ/mol

.

:: AH = -67 kJ/mol

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