For the reaction, $\mathrm{C}_3 \mathrm{H}_{8(\mathrm{~g})}+5 \mathrm{O}_{2(\mathrm{~g})} \longrightarrow 3…
For the reaction,
$\mathrm{C}_3 \mathrm{H}_{8(\mathrm{~g})}+5 \mathrm{O}_{2(\mathrm{~g})} \longrightarrow 3 \mathrm{CO}_{2(\mathrm{~g})}+4 \mathrm{H}_2 \mathrm{O}_{(\mathrm{i})}$
at constant temperature, $\Delta \mathrm{H}-\Delta \mathrm{U}$ is
$\quad-2 \mathrm{RT}$
RT
$\quad-3 R T$
3 RT
Solution
$\begin{aligned}
& \mathrm{C}_3 \mathrm{H}_{8(\mathrm{~g})}+5 \mathrm{O}_{2(\mathrm{~g})} \rightarrow 3 \mathrm{CO}_{2(\mathrm{~g})}+4 \mathrm{H}_2 \mathrm{O}_{(\mathrm{l}} \\
& \Delta \mathrm{ng}_{\mathrm{g}}=3-(1+5)=-3 \mathrm{~mol}
\end{aligned}$ Now,
$\begin{array}{ll}
& \Delta H=\Delta U+\Delta n_g R T \\
\therefore & \Delta H=\Delta U+(-3) R T \\
\therefore & \Delta H-\Delta U=-3 R T
\end{array}$