Entropy generally rises with more gas moles, greater freedom of motion and dissolution.
Practise Thermodynamics & Energetics All Thermodynamics & Energetics topics
The entropy change of a reaction is calculated from delta S rxn equals the sum of standard molar entropies of products minus the sum for reactants, each weighted by its stoichiometric coefficient. Entropy tends to increase when the number of gas molecules increases, because gases have far more accessible microstates than solids or liquids. A reaction in which a solid dissolves or a liquid vaporizes also gains entropy from increased freedom of motion. The reverse processes contribute negative amounts, so a reaction that reduces the moles of gas usually loses entropy. These entropy changes are combined with enthalpy changes to determine spontaneity through the Gibbs energy.
Key ideas: Use delta S rxn equals sum of n S of products minus sum of m S of reactants. Quick sign prediction: more moles of gas on the right means positive delta S, and fewer means negative. Count only gases if a reaction has mixed phases, since they dominate.
Common mistakes and connections: Keep the unit conversion in mind when combining with delta H: convert joules to kilojoules before using delta G equals delta H minus T delta S. Forgetting this error produces answers off by a factor of a thousand. This topic is the entropy half of the Gibbs energy calculation.
For N2 + 3H2 giving 2NH3, four moles of gas become two, so delta S is -198 J/K. Fewer gas particles means lower entropy. The interactive lesson shows this as a live animation or chart.
For the Haber reaction, N2 + 3H2 -> 2NH3, the entropy change is about -198 J/K, because four moles of gas become two.
Entropy generally rises with more gas moles, greater freedom of motion and dissolution.
Entropy Changes in Reactions is a intermediate topic in Thermodynamics & Energetics. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in Thermodynamics & Energetics first: Second Law of Thermodynamics, Third Law of Thermodynamics, Standard Molar Entropy.
The next topics in Thermodynamics & Energetics are Entropy of Phase Change and Gibbs Free Energy.
Yes. The interactive lesson includes a self-check question on Entropy Changes in Reactions, and the Thermodynamics & Energetics quiz draws on every topic in this area.
Entropy Changes in Reactions is topic 35 of 91 in Thermodynamics & Energetics.