Intermediate Topic 362 of 1102 Thermodynamics & Energetics
Standard Molar Entropy
Absolute entropy of one mole of substance under standard conditions.
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In-depth explanation of Standard Molar Entropy
The standard molar entropy, S standard, is the absolute entropy of one mole of a substance in its standard state at 298 K, expressed in joules per mole per kelvin. Unlike formation enthalpies, these values are absolute, because the third law sets zero entropy at 0 K. Gases generally have the largest standard molar entropies, followed by liquids and solids, and larger or more complex molecules have higher entropies because they possess more vibrational and rotational states. Tables of standard molar entropies allow the entropy change of any reaction to be computed. They are fundamental to predicting spontaneity through the Gibbs energy relation.
Going deeper: Standard Molar Entropy
Key ideas: Use the tables to rank substances: gases exceed liquids, which exceed solids, and larger molecules exceed smaller ones, with harder, more rigid solids lowest. Diamond has about 2.4 J per mole kelvin, graphite 5.7, and gases roughly 130 to 250.
Common mistakes and connections: A frequent mistake is to assume that elements have zero standard entropy, as they do for enthalpy of formation, but they do not. Another is to mix up units, since entropy is in joules and enthalpy in kilojoules. Standard entropies feed into delta S of reaction and into delta G calculations.
Visualization of Standard Molar Entropy
Standard molar entropies at 298 K: gases exceed liquids, which exceed solids, and larger molecules have higher entropy than simple ones. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Liquid water at 298 K has a standard molar entropy of about 70 J per mole per kelvin, whereas water vapor is about 189.
Frequently asked questions about Standard Molar Entropy
What is Standard Molar Entropy?
Absolute entropy of one mole of substance under standard conditions.
Is Standard Molar Entropy a beginner, intermediate or advanced topic?
Standard Molar Entropy is a intermediate topic in Thermodynamics & Energetics. It builds on the vocabulary of beginner topics in this area.
What should I learn before Standard Molar Entropy?
Study these earlier topics in Thermodynamics & Energetics first: Boltzmann Entropy Formula, Second Law of Thermodynamics, Third Law of Thermodynamics.
What comes after Standard Molar Entropy?
The next topics in Thermodynamics & Energetics are Entropy Changes in Reactions and Entropy of Phase Change.
Can I practise Standard Molar Entropy with quizzes?
Yes. The interactive lesson includes a self-check question on Standard Molar Entropy, and the Thermodynamics & Energetics quiz draws on every topic in this area.
Where Standard Molar Entropy fits in Thermodynamics & Energetics
Standard Molar Entropy is topic 34 of 91 in Thermodynamics & Energetics.