Practice: For the following reaction at 27 °C, calculate ∆S°rxn, ∆Ssurr, and ∆Stot. Determine if reaction is favorable.
Fe2O3 (s) + 3 H2 (g) → 2 Fe (s) + 3 H2O (g) ∆Hrxn = 98.8 kJ
Calculations dealing with entropy take into account the effects of our chemical reaction and its surroundings.
Entropy Calculations & Theory
Concept #1: The Total Entropy change of the Universe takes into account the contributions of the chemical reaction and its surroundings.
Example #1: Calculate the total entropy change for a reaction with ∆Ssurr = 2.7 J/K and ∆Sºrxn = - 450.0 kJ/K.
Is this reaction spontaneous?
Concept #2: The entropy of the surroundings is based on the enthalpy of your reaction and its temperature
Example #2: Determine change in entropy of the universe for the following reaction at 32ºC.
2 A (g) + 5 B (s) → AB (g) + 2 C (g) ∆Hrxn = -140 kJ , ∆Srxn = 3.6 J/K
Concept #3: Like enthalpy, the entropy of a chemical reaction can be calculated from standard molar values.
Example #3: Calculate ∆Sºrxn for the following reaction at 25ºC.
2 NO (g) + O2 (g) → 2 NO2 (g) ∆Hrxn = -114.14 kJ
