Question

Using the data (all values are in $kJ/mol$ at ${25}^{o}C$) given below:

(i) Enthalpy of polymerization of ethylene$=-72$
(ii) Enthalpy of formation of benzene $\left(l\right)=49$
(iii) Enthalpy of vaporization of benzene $\left(l\right)=30$
(iv) Resonance energy of benzene $\left(l\right)=-152$
(v) Heat of formation of gaseous atoms from the elements in their standard states $H=218,C=715$.

Average bond energy of $C-H=415$. Calculate the $B.E$ of $C-C$ and $C=C$.

• A. $C-C=343.67$, $C=C=615.33$
• B. $C-C=615.33$, $C=C=342.67$
• C. $C-C=343.67$, $C=C=959$
• D. None of these

Answer 1

Answer: (A)

$C-C=343.67$, $C=C=615.33$

Given
For the polymerization of ethylene,

$n({CH}_2={CH}_2)\to {(-{CH}_2-{CH}_2-)}_n;$ $\Delta H=-72$

i.e., $B_{C=C}-2B_{C-C}=-72.......\left(i\right)$

For the formation of benzene

$6C_{\left(s\right)}+3H_{2\left(g\right)}⟶C_6{H}_{6\left(l\right)};$ $\Delta H=49......\left(ii\right)$

For the vaporisation of benzene

$C_6{H}_{6\left(l\right)}\to C_6{H}_{6\left(g\right)}$ $\Delta H=30$

R.E of $C_6{H}_6=-152$

For the formation of H atom from hydrogen molecule in standard state,

$\frac{1}{2}{H}_2\to H;$ $\Delta H=218$

For the formation of gaseous C atom

$C_{\left(s\right)}\to C_{\left(g\right)};$ $\Delta H=715$

The average bond energy $B_{C-H}=415$ for equation (2)

$(6\times 715+6\times 218)-(3B_{C-C}+3B_{C=C}+6\times 415-(-152\left)\right)=49+30$

$B_{C-C}+B_{C=C}=\mathrm{959..........}\left(iii\right)$

from equation $\left(i\right)$ and $\left(iii\right)$

The bond energy of $C-C$ bond $B_{C-C}=343.66$ and the bond energy of C=C bond $B_C=C=615.33$