SCIENCE · INVESTIGATE AND EXPLAIN

Construct a standard reaction entropy ledger

Calculate reaction entropy using balanced coefficients, direction and extent.

Learning outcome: Calculate reaction entropy using balanced coefficients, direction and extent.

Starting recap

Standard molar entropy is not a formation entropy. Elements can have nonzero S° at ordinary temperatures.

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Try

Use stipulated S° values N2=192, H2=131 and NH3=193 J/(mol K).

Open the experiment

Predict

Does reversing the equation change the species values or the subtraction order?

Your prediction and reason

Experiment

Compare forward and reverse directions at extents 1 and 2. Reconstruct every term before reading the total.

Build a stoichiometric entropy ledger

Worked teaching model · synthetic values · Rounded teaching reference values at one common standard temperature: N2(g) 192, H2(g) 131, NH3(g) 193 J/(mol K). Reaction N2+3H2→2NH3. These values are a stipulated exercise, not newly measured data.

Reaction extent: 1 mol reaction Direction: 0 forward, 1 reverse: 0 case

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Conditional calculations, not measured observations
Quantity	Value	Unit
Products sum	386	J/(mol reaction K)
Reactants sum	585	J/(mol reaction K)
Standard molar reaction entropy	-199	J/(mol reaction K)
Entropy change for extent	-199	J/K

Gas stoichiometry and reference entropy give a negative system change.

Absolute standard molar entropies of elemental gases are not zero. Coefficients multiply species entropy; reaction extent is an additional scale factor.

Predict before changing a control. Save two contrasting captures and explain what is held fixed. Displayed digits are calculation precision, not experimental uncertainty.

Gas stoichiometry and reference entropy give a negative system change.

Absolute standard molar entropies of elemental gases are not zero. Coefficients multiply species entropy; reaction extent is an additional scale factor.

Observation to attach to a capture

Captures preserve recipes and replay their calculated results. They are synthetic model evidence, not physical measurements.

Understand

Count species before subtracting

For a balanced reaction, ΔrS° equals the coefficient-weighted product entropies minus the coefficient-weighted reactant entropies. Standard molar values refer to a mole of each species in its stated phase, at the specified temperature and standard state. A coefficient is not optional decoration.

For N2(g) + 3H2(g) → 2NH3(g), use the exercise values 192, 131 and 193 J/(mol K). The product sum is 2×193 = 386 J/(mol reaction K). The reactant sum is 192+3×131 = 585. Their difference is −199. This matches the qualitative expectation for four gas moles becoming two, but it is the full supplied ledger that sets the numerical result.

Absolute entropy is not a formation convention

A common error is setting S° of N2 and H2 to zero because they are elements. The zero convention for standard formation enthalpies or formation Gibbs energies is a different convention. The third law concerns a perfect crystal at zero kelvin with its specified ground-state conditions. An elemental gas at a positive temperature has many accessible states and a nonzero standard molar entropy.

The values in this investigation are rounded teaching inputs at one common reference temperature. Their arithmetic does not create more measurement precision. Do not mix a liquid entry with a gas equation, or combine entries taken at different temperatures without appropriate corrections.

Reaction definition and extent

Reversing the equation exchanges product and reactant sums, giving +199 J/(mol reverse reaction K). Multiplying the written equation by two doubles its molar reaction entropy. Separately, running the original reaction through an extent of 2 mol gives ΔS = 2×(−199) = −398 J/K. Identify which convention changed before comparing numbers.

This distinction prevents a subtle double count: if you double the equation and also double the extent, you have represented four times the original species changes, not twice. In the lab the equation stays fixed while the extent control scales the total change.

Transfer and explain

For an original packet A(g) → 2B(g), let S°(A)=210 and S°(B)=120 J/(mol K). Calculate ΔrS°=+30 J/(mol reaction K). Reverse the reaction at extent 0.5 mol: ΔS=−15 J/K. Explain why the sign of this system entropy alone does not establish the favored reaction direction. Enthalpy and surroundings matter; the next lesson makes that relationship quantitative.

Source scope and continuation

Original teaching explanation, calculations and practice. Framework alignment: AP Chemistry topic 9.2 in the College Board course framework, effective Fall 2024, checked 30 October 2026. These pages do not reproduce restricted examination questions or establish AP authorization. The investigation is a bounded computational model, not a conducted laboratory.

Return to the preceding Unit 9 lesson when its reasoning is unfamiliar.

Continue to Find when enthalpy and entropy favor change.

Apply

Public practice checks a specific claim. It does not establish independent mastery.

Show your calculation or representation, explain the changed case, and separate the stated model from measured evidence. Calculate reaction entropy using balanced coefficients, direction and extent.

Your explanation and limitations

Continue

Completion requires a saved prediction, at least one capture and a reflection. Presence checks do not grade your reasoning.

Skipping retains experiments and awards no mastery.

Previous: Reason from particle access to entropy change Next: Find when enthalpy and entropy favor change

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