Thermodynamics and Thermochemistry
15.0 Second Law of Thermodynamic
15.0 Second Law of Thermodynamic
(1) The first law of thermodynamics does not help us to predict the direction of the change. The answer to this problem is provided by the second law of thermodynamics. The statement of the law is developed in terms of the entropy criterion.
(2) The second law of thermodynamics introduces the concept of entropy and its relation with spontaneous processes.
(3) In an isolated system such as mixing of gases, there is no exchange of energy or matter between the system and the surroundings. But due to increase in randomness, there is the increase in entropy. Thus, we can say that for a spontaneous process in an isolated system, the change in entropy is positive. i.e, $\Delta S > 0.$
(4) However, if the system is not isolated, we have to take into account the entropy changes of the system and the surrounding. Then, the total entropy change $\left( {\Delta {S_{total}}} \right)$ will be equal to the sum of the change in entropy of the system $\left( {\Delta {S_{system}}} \right)$ and the change in entropy of the surroundings $\left( {\Delta {S_{surrounding}}} \right)$ i.e.
$$\Delta {S_{total}} = \Delta {S_{system}} + \Delta {S_{surrounding}}$$
(5) For a spontaneous process, $\Delta {S_{total}}$ must be positive, i,e,
$$\Delta {S_{total}} = \Delta {S_{system}} + \Delta {S_{surrounding}} > 0$$
(6) But system and surroundings constitute universe for thermodynamic point of view so that for spontaneous change $$\Delta {S_{universe}} > 0$$
