In the quest to identify a property that may reliably predict the spontaneity of a process, a promising candidate has been identified: entropy. Scientists refer to the measure of randomness or disorder within a system as entropy. High entropy means high disorder and low energy. To better understand entropy, think of a student’s bedroom. If no energy or work were put into it, the room would quickly become messy. It would exist in a very disordered state, one of high entropy. Energy must be put into the system, in the form of the student doing work and putting everything away, in order to bring the room back to a state of cleanliness and order. This state is one of low entropy.

Processes that involve an increase in entropy of the system (ΔS > 0) are very often spontaneous; however, examples to the contrary are plentiful. By expanding consideration of entropy changes to include the surroundings, we may reach a significant conclusion regarding the relation between this property and spontaneity. In thermodynamic models, the system and surroundings comprise everything, that is, the universe, and so the following is true:

ΔS_univ = ΔS_sys + ΔS_surr

The second law of thermodynamics states that all spontaneous changes cause an increase in the entropy of the universe. A summary of the relation between the entropy and spontaneity of the process is given in the table 1.

Table 1: Relationship between the entropy and the spontaneity of the process.

ΔSuniv > 0	spontaneous
ΔSuniv < 0	nonspontaneous (spontaneous in opposite direction)
ΔSuniv = 0	at equilibrium

This text is adapted from Openstax, Biology 2e, Section 6.3: The Laws of Thermodynamics and Openstax, Chemistry 2e, 16.3 The Second and Third Laws of Thermodynamics.