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

ΔS_system^??= ΣΔS_products^??- ΣΔS_reactants^?

(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^??= (2 x ΔS^?(NH₃)) - (ΔS^?(N₂) + 3 x ΔS^?(H₂))

Worked example: Calculating entropy changes

Answer

ΔS_system^??= ΣΔS_products^??- ΣΔS_reactants^?

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

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