We learned in Chapter 5 that any process that violates the first law is impossible. For processes that obey the first law, a theoretical process may be reversible and therefore occur in both directions, but all real process are irreversible and therefore can only occur in one direction.
While the first law can tell us if a process can occur, it cannot tell us the direction of the process. In order to predict the direction of a process we need to introduce a property called entropy and state the Second Law of Thermodynamics.