Question

For adsorption of a gas:

A

$\mathrm{ฮ}H<0,\mathrm{ฮ}S<0$

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B

$\mathrm{ฮ}H<0,\mathrm{ฮ}S>0$

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C

$\mathrm{ฮ}H>0,\mathrm{ฮ}S>0$

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D

$\mathrm{ฮ}H=0,\mathrm{ฮ}S>0$

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Solution

The correct option is A $\mathrm{ฮ}H<0,\mathrm{ฮ}S<0$Explanation for the correct options:A) $\mathbit{ฮ}\mathbit{H}\mathbf{}\mathbf{<}\mathbf{}\mathbf{0}\mathbf{,}\mathbf{}\mathbit{ฮ}\mathbit{S}\mathbf{}\mathbf{<}\mathbf{}\mathbf{0}$According to Thermodynamics, $\mathrm{ฮ}G=\mathrm{ฮ}H-T\mathrm{ฮ}S$$\mathrm{ฮ}H<0$, As the residual forces on the surface decrease, this indicates that there is a decrease in surface energy which will appear in form of heat. Thus, adsorption is an exothermic process. $\mathrm{ฮ}S<0$ , when the gas particles are adsorbed on the surface, the movement of the particles is very difficult as the particles stick with the surface of the adsorbent. This leads to a decrease in the entropy of the gas particles. So, $\mathrm{ฮ}S<0$ is negative.Explanation for the incorrect options: B) $\mathbf{}\mathbit{ฮ}\mathbit{H}\mathbf{}\mathbf{<}\mathbf{}\mathbf{0}\mathbf{,}\mathbf{}\mathbit{ฮ}\mathbit{S}\mathbf{}\mathbf{>}\mathbf{}\mathbf{0}$As the entropy of adsorbed gas is positive $\mathrm{ฮ}S>0$ ,the gas will not be adsorbed on the surface.C) $\mathbit{ฮ}\mathbit{H}\mathbf{}\mathbf{>}\mathbf{}\mathbf{0}\mathbf{,}\mathbf{}\mathbit{ฮ}\mathbit{S}\mathbf{}\mathbf{>}\mathbf{}\mathbf{0}$Adsorption is an exothermic reaction as $\mathrm{ฮ}H>0$ the reaction becomes endothermic. As the entropy of adsorbed gas is positive $\mathrm{ฮ}S>0$ , the gas will not be adsorbed to the surface. Thus, the adsorption process will not take place.D) $\mathbit{ฮ}\mathbit{H}\mathbf{}\mathbf{=}\mathbf{}\mathbf{0}\mathbf{,}\mathbf{}\mathbit{ฮ}\mathbit{S}\mathbf{}\mathbf{>}\mathbf{}\mathbf{0}$In this $\mathrm{ฮ}H=0$, so heat will be absorbed on the surface. As the entropy of adsorbed gas is positive $\mathrm{ฮ}S>0$,the gas will not be adsorbed on the surface.Hence option (A) is correct, $\mathbit{ฮ}\mathbit{H}\mathbf{}\mathbf{<}\mathbf{}\mathbf{0}\mathbf{,}\mathbf{}\mathbit{ฮ}\mathbit{S}\mathbf{}\mathbf{<}\mathbf{}\mathbf{0}$

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