Entropy Calculations (College Board AP® Chemistry)
Study Guide
Written by: Alexandra Brennan
Reviewed by: Stewart Hird
Entropy Calculations
Entropy changes are much smaller in magnitude compared to enthalpy changes, leading to measurements in joules rather than kilojoules.
The complete unit for entropy is J C-1 mol-1
The standard entropy change (ΔS°) for a reaction can be determined by utilizing the standard entropies (S°) of the reactants and products.
The equation to calculate the standard entropy change of a system is:
ΔS°reaction = ΣS°products - ΣS°reactants (where Σ signifies the sum of).
Consider the standard entropy change for the formation of ammonia (NH3) from nitrogen (N2) and hydrogen (H2)
N2 (g) + 3H2 (g) ⇋ 2NH3 (g)
ΔS°= (2 x ΔS°(NH3)) − (ΔS°(N2) + 3 x ΔS°(H2))
Unlike enthalpy of formation for elements, entropy for elements is not zero
Worked Example
Calculate the entropy change of the system for the following reaction:
2Mg (s) + O2 (g) → 2MgO (s)
S°[Mg (s)] = 32.60 J C-1 mol-1
S°[O2 (g)] = 205.0 J C-1 mol-1
S°[MgO (s)] = 38.20 J C-1 mol-1
Answer:
ΔS° = ΣΔS°products - ΣΔS°reactants
ΔS°= (2 x ΔS°(MgO)) − (2 x ΔS°(Mg) + ΔS°(O2))
ΔS° = (2 x 38.20) - (2 x 32.60 + 205.0)
ΔS° = -193.8 J C-1 mol-1
A negative value indicates that there has been a decrease in entropy
We also knew this from the equation by the fact that a solid is produced from a solid and a gas
Examiner Tips and Tricks
Don't forget to use the stoichiometry of the equation when calculating the entropy change of a reaction!
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