Question

If for a certain reaction $\Delta_{r}H$ is $30\, kJ\, mol^{-1}$ at $450 \,K$, the value of $\Delta_{r}S$ (in $JK^{-1}\, mol^{-1})$ for which the same reaction will be spontaneous at the same temperature is

• A. $70$
• B. $-33$
• C. $33$
• D. $-70$

Answer 1

Answer: (A)

$\Delta \,G=\Delta\,H-T \Delta\,S$
For spontaneous,
$\Delta\,G <\,0$
$\Delta\,H-T\, \Delta\,S<\,O$
$\Delta\,S>\,\frac{\Delta\,H}{T}$
$\Delta\,S\,> \frac{30 \times 10^{3}\,J\,mol^{-1}}{450\,K}$
$\Delta\,S>\,66.6\,J\,mol^{-1}\,K^{-1}$ (Check by options)