Why is adsorption always exothermic ?

Answer

Adsorption is accompanied by decrease of randomness, i.e. this factor opposes the process, i.e. `DeltaS` is -ve. For the process to be spontaneous, `DeltaG` must be -ve. Hence, according to eqn, `DeltaG=DeltaH-TDeltaS, DeltaG` can be -ve only if `DeltaH` is -ve.

Explanation

(a) `3I^(-)+S_(2)O_(8)^(2-) rarr I_(3)^(-)+2SO_(4)^(2-)`
`(-d[S_(2)O_(8)^(2-)])/(dt)=1.5xx10^(-3)" "(-1)/(3)(Delta[I^(-)])/(Deltat)=(-Delta[S_(2)O_(8)^(2-)])/(Deltat)`
`-1/3(Delta[I^(-)])/(Deltat)=1.5xx10^(-3)" "-(Delta[I^(-)])/(Deltat)=4.5xx10^(-3)`
(b) `-(d[S_(2)O_(8)^(2-)])/(dt)=1/2(d[SO_(4)^(2-)])/(dt)=1.5xx10^(-3)=1/2(d[SO_(4)^(2-)])/(dt)`
`(d[SO_(4)^(2-)])/(dt)=3xx10^(-3)`