Question

The catalytic activity and colloidal nature of a substance are surface phenomenon. Both these properties depend upon the property of adsorption. Adsorption may be physisorption or chemisorption. Adsorption is spontaneous and always leads to decrease in entropy along with evolution of heat. Chemisorption is irreversible with temperature, unilayer, specific and directional. Adsorbate molecules adsorb on catalyst surface and thus, lowers the energy of activation of reaction to provide a new pathway for reaction. In colloidal state dispersed phase particles possess the adsorption characteristics at the interface.
Select the correct statements :
1. Adsorption is spontaneous at all the temperatures.
2. Gases having high critical temperature possess more tendency for adsorption.
3. An adsorbent possesses more tendency for adsorption if it is in colloidal state.
4. Chemical adsorption first decreases with increase in temperature and then increases.
5. Water molecules are adsorbed in $CaC{l}_2(s)$.

• A. $1,2,3,4,5$
• B. $2,3,4$
• C. $1,2,3,4$
• D. $2,3$

Answer 1

Answer: (B)

1. Adsorption decreases with increase in temperature but
$\Delta G=-$ ve at high temperature.
$\Delta H=-\text{ ve and }\Delta S=-\text{ ve, thus }T\Delta S$
$=-\text{ ve }$
$\therefore \Delta G=\Delta H-T\Delta S=-\text{ ve }-(-\text{ ve })=$ less negative
2. Gases having high critical temperature are easily liquefiable due to higher forces of attractions among molecules and thus, also show more adsorption.
3. Adsorption extent of an adsorbent is more if its surface area is more.
4. Chemisorption requires energy of activation.
5. $H_{2}O$ is absorbed on $CaC{l}_2$ and nor adsorbed.