Summarizing the Second and Third Laws of Thermodynamics

Key Concepts and Summary

The second law of thermodynamics states that a spontaneous process increases the entropy of the universe, S_univ > 0. If ΔS_univ < 0, the process is nonspontaneous, and if ΔS_univ = 0, the system is at equilibrium. The third law of thermodynamics establishes the zero for entropy as that of a perfect, pure crystalline solid at 0 K. With only one possible microstate, the entropy is zero. We may compute the standard entropy change for a process by using standard entropy values for the reactants and products involved in the process.

Key Equations

• \(\text{Δ}S\text{°}=\text{Δ}{S}_{298}^{°}=\sum \text{ν}{S}_{298}^{°}\text{(products)}\phantom{\rule{0.2em}{0ex}}-\sum \text{ν}{S}_{298}^{°}\text{(reactants)}\)
• \(\text{Δ}S=\phantom{\rule{0.2em}{0ex}}\cfrac{{q}_{\text{rev}}}{T}\)
• ΔS_univ = ΔS_sys + ΔS_surr
• \(\text{Δ}{S}_{\text{univ}}=\text{Δ}{S}_{\text{sys}}+\text{Δ}{S}_{\text{surr}}=\text{Δ}{S}_{\text{sys}}+\cfrac{{q}_{\text{surr}}}{T}\)

Glossary

second law of thermodynamics

entropy of the universe increases for a spontaneous process

standard entropy (S°)

entropy for a substance at 1 bar pressure; tabulated values are usually determined at 298.15 K and denoted \({S}_{298}^{°}\)

standard entropy change (ΔS°)

change in entropy for a reaction calculated using the standard entropies, usually at room temperature and denoted \(\text{Δ}{S}_{298}^{°}\)

third law of thermodynamics

entropy of a perfect crystal at absolute zero (0 K) is zero