Question

For which reaction, $\Delta H>\Delta U$? ($\Delta H=$ change in enthalpy, $\Delta U=$ change in internal energy )

• A. $Fe\left(s\right)+5CO\left(g\right)\to Fe\left(CO{)}_5\right(l)$
• B. $C\left(graphite\right)+O_2\left(g\right)\to C{O}_2\left(g\right)$
• C. $H_{2}O\left(I\right)\to H^{+}\left(aq\right)+O{H}^{-}\left(aq\right)$
• D. $CaC{O}_3\left(s\right)\to CaO\left(s\right)+C{O}_2\left(g\right)$

Answer 1

The correct option is A $Fe\left(s\right)+5CO\left(g\right)\to Fe\left(CO{)}_5\right(l)$

$\Delta H=\Delta U+P\Delta V$

$PV=nRT$; So, $P\Delta V=\Delta nRT$

Therefore, $\Delta H=\Delta U+\Delta nRT$

($\Delta n=numberofgaseousmolesofproduct-numberofgaseousmolesofreactant$)

When $\Delta n=-ve$, then the value of $\Delta nRT=-ve$,

As a result $\Delta H>\Delta U$

$A)\Delta n=0-5=-5$

$B)\Delta n=1-1=0$

$C)\Delta n=0-0=0$

$D)\Delta n=1-0=1$