Question

Fruend and Lang were 2 scientists interested in adsorption.
Once in a discussion Fruend asked "Dude, I can see adsorption is spontaneous. But why is it always exothermic?"
Lang said "___".
Choose Lang's answer among the following options.

• A. $\Delta G>0and\Delta S<0\Rightarrow \Delta H<0$, so it is exothermic
  
  
• B. $\Delta G<0and\Delta S<0\Rightarrow \Delta H<0$, so it is exothermic
  
• C. $\Delta G>0and\Delta S>0\Rightarrow \Delta H>0$, so it is exothermic
  
• D. $\Delta G<0and\Delta S>0\Rightarrow \Delta H>0$, so it is exothermic
  

Answer 1

The correct option is A $\Delta G>0and\Delta S<0\Rightarrow \Delta H<0$, so it is exothermic



To answer this question, let us recall our fundamental equation
$\Delta G=\Delta H-T\Delta S$
We already know and accept the fact that adsorption is spontaneous. So we can conclude that $\Delta G<0$
Now, when the gases get adsorbed on the surface of the solid from their native state, their entropy reduces. An easy way to think about why this happens is that when the gases were in their native state they had more 'randomness'.
But when they are adsorbed on the surface, they are more restricted and are less 'random'.
So, we can say that entropy change $\Delta S<0$
From the equation, if $\Delta G<0and-T\Delta S>0$ since we have negative sign, then we need $\Delta H<0$ and also higher in value than $T\Delta S$