Absolute Entropy and Entropy Change
Quick Notes
- Entropy (S) is a state function, which means the change in entropy (ΔS) depends only on the initial and final states of a system, not on the path taken between them.
- Every substance has an absolute entropy value (S°), typically in units of J/mol·K.
- We can calculate the standard entropy change of a reaction using:
ΔS° = Σ S°(products) − Σ S°(reactants).
Full Notes
Every substance, in a given state, has an absolute entropy value (S°), typically in units of J/mol·K.
As reactants turn into products in a reaction, the total entropy of reactants compared to products changes, meaning the entropy of the system changes.
We can use the standard entropy values (S°) for each substance in a reaction (usually given in tables) to determine the entropy change, ΔS, that occurs.
Calculating Entropy Change
- Multiply each substance’s S° value by its stoichiometric coefficient from the balanced equation.
- Subtract the sum of entropies of the reactants from the sum for the products.
Don’t forget the molar ratios of everything in the equation and keep workings very clear. Also, make sure you are using a substance’s S° value for the correct state (for example, H2O(l) has a different S° to H2O(g).
Calculate ΔS for the following reaction
Reaction: C2H6(g) + 3.5 O2(g) → 2 CO2(g) + 3 H2O(g)
Standard molar entropy values (in J/mol·K):
S°(C2H6) = 229.2, S°(O2) = 205.0, S°(CO2) = 213.7, S°(H2O, g) = 188.8
- Sum S° of products
= [2 × 213.7] + [3 × 188.8] = 427.4 + 566.4 = 993.8 J/mol·K - Sum S° of reactants
= [1 × 229.2] + [3.5 × 205.0] = 229.2 + 717.5 = 946.7 J/mol·K - ΔS° = products − reactants
993.8 − 946.7 = +47.1 J/mol·K
Conclusion: The entropy increases by 47.1 J/mol·K during this reaction.
Summary
- We can use standard entropy values (S°) to calculate entropy change.
- Always include stoichiometric coefficients when summing S° values.
- A positive ΔS° suggests increased disorder; negative ΔS° means the system became more ordered.