Entropy Formula
Use standard molar entropies to find DeltaS of a reaction
Reaction Entropy
DeltaS_rxn = sum(n * S°, products) - sum(n * S°, reactants)
S° values are standard molar entropies (J/mol·K). Multiply by stoichiometric coefficients n.
Steps
- Balance the equation.
- Gather S° values for all species (correct phase).
- Multiply S° by stoichiometric coefficients for products and reactants.
- Products minus reactants gives DeltaS_rxn (J/mol·K).
Example: N2 + 3 H2 → 2 NH3(g)
S° (J/mol·K): N2 = 191.5, H2 = 130.6, NH3 = 192.5
Products sum = 2 × 192.5 = 385.0
Reactants sum = 1 × 191.5 + 3 × 130.6 = 583.3
DeltaS_rxn = 385.0 - 583.3 = -198.3 J/mol·K
Entropy decreases (gas moles decrease).
Negative DeltaS_rxn often corresponds to fewer gas molecules in products.
Common Mistakes
Phase mix-ups
Use S° for the correct phase (g, l, s, aq).
Unit confusion
Keep J/mol·K; convert if tables use different units.
Forgetting coefficients
Multiply S° by stoichiometric coefficients before summing.
Sign errors
Always do products minus reactants.
FAQ
Why are elemental S° not zero?
Unlike enthalpy, standard molar entropy of elements is nonzero due to disorder at 298 K.
Can entropy be negative?
Absolute entropy values are positive, but DeltaS for a reaction can be negative.
How does temperature affect S°?
Tables are usually at 298 K; for other temperatures, use heat capacity data or approximations.