Question

Using the data provided, calculate the multiple bond energy $\left( kJ mol ^{-1}\right)$ of a $C \equiv C$ bond in $C _{2} H _{2}$. That energy is (take the bond energy of a $C - H$ bond as $350 kJ$ mol $^{-1}$)
$2C (s) \longrightarrow 2C (g) \,\,\,\,\Delta H =1410 kJ$ mol $^{-1}$
$2C (s) \longrightarrow 2C (g) \,\,\,\,\Delta H =1410 kJ$ mol $^{-1}$
$H _{2} (g) \longrightarrow 2H (g) \,\,\,\,\Delta H =330 kJ$ mol $^{-1}$

• A. 1165
• B. 837
• C. 865
• D. 815

Answer 1

Answer: (D)

(i) $2C (s)+ H _{2}(g) \longrightarrow H - C \equiv C - H (g) \,\,\,\, \Delta H =225 kJ$ mol $^{-1}$

(ii) $2C (s) \longrightarrow 2C (g)\,\,\,\,\Delta H =1410 kJ$ mol $^{-1}$

(iii) $H _{2}(g) \longrightarrow 2H (g)\,\,\,\,\Delta H =330 kJ$ mol $^{-1}$

From equation (i):

$225=\left[2 \times \Delta H _{ C ( s ) \longrightarrow C ( g )}+1 \times BE _{ H - H }\right]-\left[2 \times BE _{ C - H }+1 \times BE _{ C = C }\right]$

$225=[1410+1 \times 330]-\left[2 \times 350+1 \times BE _{ C = C }\right]$

$225=[1410+330]-\left[700+ BE _{ C = C }\right]$

$225=1740-700- BE _{ C = C }$

$225=1040- BE _{ C = C }$

$BE _{ CeC }=1040-225=815 kJ$ mol $^{-1}$