Entropy Change Calculator
Calculate the entropy change (ΔS) for three common processes: phase changes (ΔS = q/T), temperature changes (ΔS = nCp·ln(T₂/T₁)), and ideal mixing (ΔS = -nRΣ(χi·lnχi)). Includes spontaneity assessment.
Entropy (S) is a thermodynamic property that measures the degree of disorder or randomness in a system. The second law of thermodynamics states that the total entropy of an isolated system always increases in a spontaneous process.
Phase Change: ΔS = q_rev/T. For a reversible phase change at constant temperature and pressure, the entropy change equals the heat absorbed divided by the temperature. Vaporization has a large positive ΔS (increased disorder going from liquid to gas). Trouton's rule: ΔS_vap ≈ 85 J/(mol·K) for many liquids.
Temperature Change: ΔS = nCp·ln(T₂/T₁). When a substance is heated or cooled without phase change, the entropy change depends on the heat capacity and the ratio of final to initial temperature. Heating always increases entropy; cooling decreases it.
Ideal Mixing: ΔS_mix = -nRΣ(χi·lnχi). Mixing ideal gases or ideal solutions always increases entropy because the disorder increases when distinguishable particles are combined. This is always positive because χi < 1 and ln(χi) < 0.