Entropy Notes

Entropy

• Entropy is often misunderstood, unlike enthalpy which is more intuitive, especially in dramatic exothermic reactions like combustion.
• Entropy may seem less intuitive initially, but it describes everyday phenomena.

Examples of Entropy in Everyday Life

• Hot tea cooling down.
• Frozen drinks melting.
• Iron rusting.
• Buildings crumbling.
• Balloons deflating.
• Living things dying and decaying.

Common Denominator

• Energy goes from being localized or concentrated to being spread out or dispersed in all examples.

Specific Examples

• Hot tea: Thermal energy disperses into cooler air.
• Frozen drink: Thermal energy disperses into the drink.
• Iron rusting: Chemical energy in iron and oxygen bonds disperses as iron oxide (rust) forms.
• Building crumbling: Potential energy disperses as light, sound, and heat.
• Balloon deflating: Pressurized air energy disperses into the atmosphere.
• Death and decay: Chemical energy in organic molecules disperses into the environment.

Second Law of Thermodynamics

• Energy spontaneously disperses from being localized to becoming spread out if it is not hindered from doing so.

Entropy vs. Disorder

• Do not literally equate entropy with disorder.
• The analogy of a messy room is deficient and can cause confusion.

Definition of Entropy

• Entropy is the measure of the spontaneous dispersal of energy at a specific temperature.
• It quantifies how much or how widely energy spreads out in a process.

Equation for Calculating Change in Entropy

• $\triangle S = \frac{Q_{REV}}{T}$
  • $\triangle S$: Change in entropy.
  • $Q_{REV}$: Heat gained or lost in a reversible process.
  • $T$: Temperature in Kelvin.
• Units of entropy: J/(mol⋅K).

Entropy Change and Energy Distribution

• When energy is distributed into a system at a given temperature, entropy increases.
• When energy is distributed out of a system at a given temperature, entropy decreases.

Spontaneity and Energy Concentration

• Energy will spontaneously disperse.
• Concentration of energy will rarely happen spontaneously in a closed system.
• Work must be done to concentrate energy.

Refrigerators and Entropy

• Refrigerators counteract the spontaneous flow of heat, concentrating energy outside the system.
• Refrigerators consume energy to move heat against the temperature gradient.

Second Law as Time's Arrow

• The second law imposes a unidirectional limitation on energy movement, indicating the direction of time.
• Example: Recognizing whether a video of an explosion is running forward or backward.

Entropy in a Closed System

• Energy in a closed system will spontaneously spread out, and entropy will increase if not hindered.
• A system can be defined to include the entire universe.

Entropy of the Universe

• The second law claims that the entropy of the universe is increasing.
• \triangle S{universe} = \triangle S{system} + \triangle S_{surroundings} > 0

Entropy as a State Function

• Change in entropy from one equilibrium state to another is pathway independent.
• It only depends upon the difference in entropies of the final and initial states.

Standard Entropy Change for a Reaction

• The standard entropy change for a reaction ($\triangle S_{RXN}^\circ$) can be calculated using the standard entropies of reactants and products.
• $\triangle S<em>{RXN}^\circ = \sum \triangle S</em>F^\circ \text{ products } - \sum \triangle S_F^\circ \text{ reactants }$