9.2 Absolute Entropy & Entropy Change

01Calculating ΔS°

Unlike ΔH°f, every substance has a positive absolute entropy S° (elements included). ΔS°rxn = Σ nS°(products) − Σ nS°(reactants).

Units: J/(mol·K) — convert to kJ before combining with ΔH.

S° of elements is NOT zero.

Worked example: A → 2B, S°(A) = 100, S°(B) = 70 → ΔS = 140 − 100 = +40 J/K.

Key takeaways

Watch outForgetting to convert J to kJ in ΔG = ΔH − TΔS is the most common lost point.

Quick check

1.S° of O₂(g) at 25 °C is…

2.S°(products) 300, reactants 200 J/K. ΔS°?

3.ΔS = 150 J/K equals…