- The second law of thermodynamics states that the entropy of the universe always increases for a spontaneous process.
- The second law can be represented mathematically as
Entropy can be used to predict the spontaneity of a process using the second law of thermodynamics. The second law of thermodynamics can be stated in a number of equivalent ways. The most common statement of the second law is that “the entropy of the universe is always increasing.” Mathematically, this can be written as
The three possibilities for ΔSuniv indicates whether or not a process is spontaneous (ΔSuniv > 0), reversible (ΔSuniv = 0), or impossible (ΔSuniv < 0). The second law only imposes the limits on the change in entropy of the universe, not the entropy change of the system or the surroundings. This is an important concept because it means that a system can become more ordered (ΔSsys < 0), if such a process introduces a larger positive entropy change in the surroundings (ΔSsurr > 0). This is precisely what happens when liquids freeze into solids, or gases condense into liquids: the entropy of the system decreases, but it is accompanied by a larger increase in the entropy of the surroundings such that the overall entropy of the universe increases.
Interactive: