Define Entropy

For a reversible process at equilibrium, the change in entropy may be expressed as

Considering entropy (S) as a thermodynamic parameter the criterion for the spontaneity of any process is

Adsorption of a gas on the surface of solid is generally accompanied by decrease in entropy, still it is a spontaneous process. Explain.

Which of the following reactions is asssociated with the most negative change in entropy?

Calculate the entropy change (J//mol K) of the given reaction. The molar entropies (J//K-mol) are given in brackets after each substance: 2PbS(s)[19.2]+3O_(2)(g)[205.1] to 2PbO(s)[66.5]+2O_(2)(g)[248.2]