Calculating Entropy Changes (Cambridge International A-Level Chemistry)

Entropy Change Calculations

• The standard entropy change (ΔS_system^θ) for a given reaction can be calculated using the standard entropies (S^θ) of the reactants and products
• The equation to calculate the standard entropy change of a system is:

ΔS_system^θ = ∑S_products^θ - ∑S_reactants^θ

(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)

ΔS _system^θ = (2 x ΔS^θ(NH₃)) - (ΔS ^θ(N ₂) + 3 x ΔS^θ(H₂))

Worked Example Calculate the entropy change of the system for the following reaction:

2Mg (s) + O₂(g) → 2MgO (s)

• S^θ****[Mg (s)] = 32.60 J K^–1 mol^–1
• S^θ****[O₂ (g)] = 205.0 J K^–1 mol^–1
• S^θ****[MgO (s)] = 38.20 J K^–1 mol^–1

Answer:

• ΔS_system^θ = ΣΔS_products^θ - ΣΔS_reactants^θ
• ΔS_system^θ = (2 x 38.20) - (2 x 32.60 + 205.0)
• ΔS_system^θ = –193.8 J K^-1 mol^-1

Exam Hint Use the stoichiometry of the equation and the correct state of the compounds when calculating the entropy change of a reaction