Question

what is the sign of delta h and delta s when a gas gets absorbed on the surface of a solid?

Answer 1

Answer: $\Delta H$=-ve and $\Delta S$=-ve

Explanation:-

Endothermic reactions are those in which heat is absorbed by the system and exothermic reactions are those in which heat is released by the system.  $\Delta H$ for Endothermic reaction is positive and  $\Delta H$ for Exothermic reaction is negative.

Entropy is the measure of randomness or disorder of a system. If a system moves from  an ordered arrangement to a disordered arrangement, the entropy is said to decrease and vice versa.  $\Delta S$ is positive when randomness increases and $\Delta S$ is negative when randomness decreases.

When the gas is adsorbed on solid, the randomness of particles decreases and hence  $\Delta S$ is negative.                  

As the adsorption is spontaneous : $\Delta G=-ve$

Using Gibbs Helmholtz equation:

$\Delta G=\Delta H-T\Delta S$

$\Delta G=\Delta H-T(-ve)$

$\Delta G=\Delta H+T(+ve)$

For $\Delta G$ to be negative, $\Delta H$ has to be negative.

$\Delta H=-ve$

Thus the value of  $\Delta H$  is negative and  

$\Delta S$ is negative.