Why heat of adsorption is negative? Discuss the free energy change during adsorption.

Adsorption involves attracting molecules of adsorbate on surface of the adsorbent. Due to this, energy is released and thus heat of adsorption is negative i.e. adsorption is always exothermic. Further physical adsorption involves weak forces of attraction, heat evolved is less whereas chemical adsorption involves strong forces of attraction, and heat evolved is much higher.

Free Energy Change during Adsorption:
For adsorption free energy ∆H is negative. The molecules of the adsorbate are held on surface of the solid adsorbent due to this entropy decreases i.e. ∆S is also negative.

Now we know
As ∆G = ∆H – T∆S
Therefore adsorption will occur only when ∆G is negative and this is possible only if ∆H > T∆S.

Initially this condition is met but as adsorption process continues ∆H value decreases where as T∆S increases and finally ∆H becomes equal to T∆S so that ∆G = 0. This is when we say the state of adsorption equilibrium has been achieved.