Calculating Entropy Changes (CIE A Level Chemistry)

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

ΔS_system^ꝋ = ΣS_products^ꝋ - ΣS_reactants^ꝋ

(where Σ = 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^ꝋ = (2 x ΔS^ꝋ(NH₃)) - (ΔS^ꝋ(N₂) + 3 x ΔS^ꝋ(H₂))

Worked example: Calculating entropy changes

Answer

ΔS_system^ꝋ = ΣΔS_products^ꝋ - ΣΔS_reactants^ꝋ

ΔS_system^ꝋ = (2 x 38.20) - (2 x 32.60 + 205.0)

= -193.8 J K^-1 mol^-1