If for a certain reaction _ r H is 30 ;kJ ;mol ^ -1 at 450 ;K , t… - Vidyadip

MCQ

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

A
$-70$
✓
$70$
C
$-33$
D
$33$

✓

Answer

Correct option: B.

$70$

b
For spontaneous reaction, $\Delta_{r} G$ must be less than zero.

So, $\Delta_{r} G=\Delta_{r} H-T \Delta_{r} S<0$

or, $\Delta_{r} S >\frac{\Delta_{ r } H }{ T }$

$>\frac{30,000}{450}=66.67 J$

$\therefore$ For reaction to be spontaneous, the value of $\Delta_{r} S$ must be greater than 66.67 J

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