Question

Difference between $△H$ and $△U$ for the combustion of benzene at 300 K is :
$C_6{H}_6\left(l\right)+\frac{15}{2}{O}_2\left(g\right)\to 6C{O}_2\left(g\right)+3H_{2}O\left(l\right)$

• A. 10.48 kJ
• B. 3.74 kJ
• C. 14.86 kJ
• D. 6.73 kJ

Answer 1

The correct option is B 3.74 kJ

Combustion of benzene is represented by following reaction;
$C_6{H}_6\left(l\right)+\frac{15}{2}{O}_2\left(g\right)\to 6C{O}_2\left(g\right)+3H_{2}O\left(l\right)$

The difference in gaseous moles of product and reactant is :
$△n_g=6-\frac{15}{2}=-3/2$
And we have relation,
$△H=△U+△n_{g}RT$
so,
$△H-△U=△n_{g}RT$
$△H-△U=-\frac{3}{2}\times 8.314\times 300$
$=3741.3J=-3.741kJ$

Hence the difference between $△H$ and $△U$ for the combustion of liquid benzene is 3.74 kJ.