Thermodynamics and Thermochemistry - Result Question 78

####3. A process will be spontaneous at all temperature if

(2019 Main, 10 April I)

(a) $\Delta H>0$ and $\Delta S<0$

(b) $\Delta H<0$ and $\Delta S>0$

(c) $\Delta H<0$ and $\Delta S<0$

(d) $\Delta H>0$ and $\Delta S>0$

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Solution:

  1. A process will be spontaneous when its free energy (Gibb’s energy) change will be negative, i.e. $\Delta G<0$.

Spontaneity of a process is decided by the value of $\Delta G$, which can be predicted from the Gibb’s equation, $\Delta G=\Delta H-T \Delta S$ for positive/negative signs of $\Delta H$ and $\Delta S$ at any/higher/lower temperature as:

$\Delta \boldsymbol{H}$ $\Delta \boldsymbol{S}$ Comment on
temperature
$(T)$
$\Delta \boldsymbol{G}$ Comment on the
process
$<0$ $>0$ at any temp. $<0$ spontaneous
$>0$ $<0$ at any temp. $>0$ non-spontaneous
$<0$ $<0$ at lower temp. $<0$ spontaneous
$>0$ $>0$ at higher temp. $<0$ spontaneous


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