Question

For the combustion of benzene, which of the following relations is correct?

• A. $\Delta H=\Delta U-3RT$
• B. $\Delta H=\Delta U-1.5RT$
• C. $\Delta H=\Delta U+3RT$
• D. $\Delta H=\Delta U+1.5RT$

Answer 1

The correct option is B $\Delta H=\Delta U-1.5RT$

Combustion of benzene:
$C_6{H}_6\left(l\right)+\frac{15}{2}{O}_2\left(g\right)\to 6C{O}_2\left(g\right)+3H_{2}O\left(l\right)$
change in moles involved = $(6-\frac{15}{2})=-\frac{3}{2}$
We know,
$\delta H=\delta U+\delta \left(PV\right)$
which can also be written as
$\delta H=\delta U+\delta \left(nRT\right)$
put up value of n we get,
$\delta H=\delta U-\frac{3}{2}\delta \left(RT\right)$