The Second Law of Thermodynamics

Entropy, abbreviated as S, is the thermodynamic measure of disorder or randomness in a system. Systems with higher entropy have greater disorder than those with lower entropy. For example, an unfolded amino acid chain has higher entropy than a properly folded protein because the linear chain is more flexible and disorganized than the tightly packed structure.

The second law of thermodynamics states that entropy of an isolated system always increases, meaning everything becomes more disordered without external input. Since isolated systems rarely occur naturally, thermodynamics examines entropy changes of the entire universe, including both the system and its surroundings. This universal perspective explains why disorder naturally increases over time.

A process occurs spontaneously when the entropy of the universe increases, meaning it has a ΔSuniv greater than zero. Conversely, a process where entropy decreases or has a negative ΔSuniv is not spontaneous and requires energy input to occur. The total entropy change includes both the system and surroundings: ΔSuniv = ΔSsys + ΔSsurr.

Entropy change in a system alone does not reliably predict spontaneity; processes with increased system entropy are often spontaneous but have many exceptions. However, when considering entropy changes in both the system and surroundings together, the total entropy change of the universe accurately predicts spontaneity. This expanded perspective reveals that spontaneous processes always increase universal entropy.

A messy room exists in a highly disordered state with high entropy because no energy is invested in maintaining organization. An organized room has low entropy because energy must be continuously applied through work to maintain order and cleanliness. This analogy demonstrates that disorder naturally increases without external energy input, illustrating the second law of thermodynamics in everyday life.

Entropy changes help determine whether endergonic and exergonic reactions in the cell occur spontaneously. When the total entropy of the universe increases, a reaction proceeds spontaneously regardless of whether it absorbs or releases energy. Understanding entropy allows cells to couple unfavorable reactions with favorable ones to drive necessary biological processes forward.

At thermodynamic equilibrium, the entropy of the universe remains constant with ΔSuniv = 0, meaning no net change occurs in the system or surroundings. At this point, the system has reached maximum disorder for its given conditions, and no spontaneous processes occur in either direction. Equilibrium represents a stable state where forward and reverse processes balance perfectly.