Absolute entropy and entropy change | Applications of thermodynamics | AP Chemistry | Khan Academy

TL;DR

Entropy is a measure of the disorder in a system, and the third law of thermodynamics states that at absolute zero, the entropy of a pure crystalline substance is zero. Standard molar entropy is a measure of the absolute entropy of a substance at a specific temperature and pressure.

Transcript

• [Instructor] Entropy can be measured on an absolute scale, which means there is a point of zero entropy. And that point is reached for a pure crystalline substance when the temperature is equal to zero kelvin or absolute zero. At zero kelvin, the entropy of the pure crystalline substance, S, is equal to zero. We can think about why the entropy is... Read More

Key Insights

• 0️⃣ The third law of thermodynamics states that at absolute zero temperature, the entropy of a pure crystalline substance is zero.
• 🪗 The number of microstates, or possible arrangements, determines entropy according to Boltzmann's equation.
• ❓ Standard molar entropy is a measure of the absolute entropy of a substance at a specific temperature and pressure.
• ✋ Gases generally have higher standard molar entropies than liquids, which have higher entropies than solids.
• 🥺 An increase in the number of atoms or molar mass generally leads to an increase in entropy.
• 💱 Standard molar entropies can be used to calculate the change in entropy for a chemical reaction.
• 💱 A positive change in entropy indicates an increase in disorder or randomness in the system.

Questions & Answers

Q: What is the third law of thermodynamics?

The third law of thermodynamics states that at absolute zero temperature, the entropy of a pure crystalline substance is zero.

Q: How does increasing temperature affect entropy?

Increasing temperature leads to an increase in the number of microstates and therefore an increase in entropy.

Q: What is standard molar entropy?

Standard molar entropy is the absolute entropy of a substance at one atmosphere pressure and a specified temperature, compared to a hypothetical perfect crystal at zero Kelvin.

Q: Why do gases generally have higher standard molar entropies than solids?

Gases have more freedom of movement and a greater number of possible arrangements of particles, leading to an increase in entropy compared to solids.

Summary & Key Takeaways

• Entropy can be measured on an absolute scale, with a point of zero entropy reached at absolute zero temperature for pure crystalline substances.

• The number of microstates, or possible arrangements of particles, determines entropy according to Boltzmann's equation.

• The standard molar entropy compares the absolute entropy of a substance at one atmosphere pressure and a specified temperature to a hypothetical perfect crystal at zero Kelvin.