Question

$\Delta H \,\&\, \Delta S$ for a reaction are $+30.0 \,kJ\,mol ^{-1}$ and $0.06\, kJ\,K ^{-1} \,mol ^{-1}$ at $1\, atm$ pressure. The temperature at which free energy change is equal to zero and nature of the reaction below this temperature are

• A. $500{ }^{\circ} C$ and non-spontaneous
• B. $227^{\circ} C$ and non-spontaneous
• C. $400{ }^{\circ} C$ and spontaneous
• D. $127^{\circ} C$ and spontaneous

Answer 1

Answer: (B)

Given
$\Delta H=+30.0 \,kJ \,mol ^{-1} $
$\Delta S=0.06 \,kJ\,K ^{-1}\, mol ^{-1} $
$p=1 \,atm $
$\Delta G=0$
We know that,
$\Delta G=\Delta H-T \Delta S $
$\Rightarrow 0=\Delta H-T \Delta S$
if, we put temperature $227^{\circ} C$
or $227+273=500\, K$
In equation then the value of free energy is obtained will be zero.
Hence, $\Delta G=30-500 \times 0.06$
$\Delta G=30-30=0$
and nature of reaction will be non-spontaneous.