Apa itu Entropi? | Dr. Grandprix Thomryes Marth Kadja, M.Si.

Summary

In this lecture on thermodynamics, the focus is on the second law, which emphasizes that entropy in a closed system increases over time. The speaker defines entropy as a measure of randomness or disorder, using water in different phases (solid, liquid, gas) to explain the varying levels of entropy. Key concepts are illustrated with examples, such as the transformation of calcium carbonate to calcium oxide and CO2 gas, where entropy increases, and the condensation of water vapor, where entropy decreases. The speaker guides on how to predict entropy changes based on the phases and number of molecules in a reaction.

Takeaways

• 😀 Thermodynamics is an extension of the first topic studied in Basic Chemistry 1, which is thermochemistry, focusing on the second and third laws of thermodynamics.
• 😀 The second law of thermodynamics states that the entropy of a system in a closed environment will increase over time.
• 😀 Entropy (S) represents the randomness or disorder of a system, with higher entropy indicating greater disorder.
• 😀 The most disordered state for water is when it is in the gas phase (steam), which has the highest entropy compared to solid (ice) and liquid (water).
• 😀 Entropy reflects the number of possible energy transfer configurations or degrees of freedom a molecule or substance has.
• 😀 Entropy is a state function, similar to enthalpy (H), and it can change during a chemical reaction.
• 😀 A positive change in entropy (ΔS > 0) indicates that the disorder or randomness has increased, such as when solid calcium carbonate (CaCO3) decomposes into calcium oxide (CaO) and carbon dioxide (CO2) gas.
• 😀 A negative change in entropy (ΔS < 0) indicates a decrease in randomness or more order, such as when water vapor (gas) condenses into liquid water.
• 😀 The entropy of gases is much greater than that of liquids or solids due to the greater freedom of motion and energy transfer in the gaseous state.
• 😀 To predict changes in entropy (ΔS), focus on the phases of the substances involved in a reaction, particularly gas molecules. If gases are formed or consumed, entropy will likely increase or decrease accordingly.

Q & A

What is the second law of thermodynamics?

The second law of thermodynamics states that the entropy of an isolated system will always increase over time, meaning systems naturally evolve towards more disordered states.

How is entropy defined in the context of thermodynamics?

Entropy is a measure of the disorder or randomness of a system. The higher the randomness in molecular movement, the higher the entropy.

What is the relationship between entropy and phases of matter?

In thermodynamics, gases have the highest entropy because their molecules move more freely compared to solids and liquids. Solids have the lowest entropy due to their more ordered molecular structure.

Why does a gas phase have the highest entropy?

A gas phase has the highest entropy because its molecules have more degrees of freedom, allowing for greater randomness in their movement compared to solids or liquids.

What happens to entropy when a solid turns into a gas?

When a solid turns into a gas, entropy increases because the molecular arrangement becomes more random and the molecules have more freedom to move.

What does a positive ΔS (change in entropy) indicate?

A positive ΔS indicates that the entropy of the system has increased, meaning the system has become more disordered or random.

Give an example of a reaction where ΔS is positive.

An example is the reaction where calcium carbonate (CaCO3) solid breaks down into calcium oxide (CaO) solid and carbon dioxide (CO2) gas. The entropy increases because the product side contains more disordered gas molecules.

What does a negative ΔS (change in entropy) indicate?

A negative ΔS indicates that the entropy of the system has decreased, meaning the system has become more ordered or less random.

Give an example of a reaction where ΔS is negative.

An example is the condensation of water vapor (H2O gas) to liquid water (H2O liquid). The system becomes more ordered, resulting in a decrease in entropy.

How do you predict the change in entropy (ΔS) for reactions involving gas molecules?

To predict ΔS for reactions with gases, compare the number of gas molecules on both sides of the equation. The side with more gas molecules will generally have higher entropy.