Chemistry - Thermochemistry Question with Solution | TestHub

Since, C₂H₆contains one C-C and 6 C-H bonds, thus 620 is the sum of BE of one C-C and 6 C-H bonds. Thus,

$\mathbf{* }$${\text{C}}_2{\text{H}}_6\to \text{C}-\text{C}+6\text{C}-\text{H} \Delta \text{H}=\text{620 kJ/mol}$....... (i)

Similarly,

$\mathbf{* }$${\text{C}}_3{\text{H}}_8\to 2\text{C}-\text{C}+8\text{C}-\text{H} \Delta \text{H}=\text{880 kJ/mol}$.... (ii)

Multiply Eq. (i) by 2 then we get
$\mathbf{* }$$2{\text{C}}_2{\text{H}}_6\to 2\text{C}-\text{C}+12\text{C}-\text{H} \Delta \text{H}=\text{1240 kJ/mol}$.... (iii)

equation (iii)-(i)
$\begin{array}{ccccc}2\text{C}-\text{C}& +& 8\text{C}-\text{H}& =& \text{880 kJ/mol ...... (ii)}\\ 2\text{C}-\text{C}& +& 12\text{C}-\text{H}& =& \text{1240 kJ/mol ...... (iv)}\end{array}$
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$\begin{array}{cccc}& 4\text{C}-\text{H}& =& \text{360 kJ/mol}\\ \therefore & \text{C}-\text{H}& =& \text{90.0 kJ/mol}\end{array}$

On putting the value of C-H, in equation (i) we get

$6\times \text{90.0}+\text{C}-\text{C}=620$

$\text{C}-\text{C}=620-540$

$=80\; kJ/mol$