Entropy Change Calculations
- The standard entropy change (ΔSsystemθ) for a given reaction can be calculated using the standard entropies (Sθ) of the reactants and products
- The equation to calculate the standard entropy change of a system is:
ΔSsystemθ = ∑Sproductsθ - ∑Sreactantsθ
(where ∑ = sum of)
- For example, the standard entropy change for the formation of ammonia (NH3) from nitrogen (N2) and hydrogen (H2) can be calculated using this equation
N2 (g) + 3H2 (g) ⇋ 2NH3 (g)
ΔSsystemθ = (2 x ΔSθ(NH3)) - (ΔSθ(N2) + 3 x ΔSθ(H2))
Worked example
Calculate the entropy change of the system for the following reaction:
2Mg (s) + O2 (g) → 2MgO (s)
- Sθ [Mg (s)] = 32.60 J K–1 mol–1
- Sθ [O2 (s)] = 205.0 J K–1 mol–1
- Sθ [MgO (s)] = 38.20 J K–1 mol–1
Answer:
- ΔSsystemθ = ΣΔSproductsθ - ΣΔSreactantsθ
- ΔSsystemθ = (2 x 38.20) - (2 x 32.60 + 205.0)
- ΔSsystemθ = –193.8 J K-1 mol-1
Exam Tip
Use the stoichiometry of the equation and the correct state of the compounds when calculating the entropy change of a reaction
