Thermodynamics Concepts

Chapter 17: Thermodynamics Overview

• Key Topics Covered:
  • Spontaneous Processes
  • Entropy and the Laws of Thermodynamics
  • Free Energy
  • Temperature and Spontaneity
  • Equilibrium and Thermodynamics

17.1 Spontaneous Processes

• System and Surroundings:

  • System: Part of the universe being studied.
  • Surroundings: Everything else in the universe.

• Spontaneous Change: A transformation that occurs without continuous energy input.

  • Examples: Freezing of ice, burning of carbon.

• Nonspontaneous Change: Requires energy input from surroundings to occur.

• Importance of Entropy:

  • Entropy (S) measures the disorder or randomness in a system.
  •  Changes in entropy can help predict spontaneity.

• Energy Changes in Reactions:

  • 1st Law of Thermodynamics: Energy cannot be created or destroyed; it’s conserved.
  • When $ext{ΔH}$ is exothermic, spontaneous processes are usually favored.
  • For endothermic $ext{ΔH}$, entropy can be the determining factor for spontaneity.

17.2 Entropy and The 2nd Law of Thermodynamics

• Second Law of Thermodynamics: Entropy of the universe increases in a spontaneous process ( ext{DS}_{ ext{universe}} > 0).
• Standard Entropy (S°):
  • Defined at 25 °C, under specific standard conditions (1 atm for gases, 1 M for solutions).
• Third Law of Thermodynamics: The entropy of a perfect crystalline substance at absolute zero is zero (0 K).

17.3 Absolute Entropy and Molecular Structure

• Absolute Entropy Values (S°): Used to calculate changes in entropy for reactions and processes.
• Factors affecting entropy:
  • Phase changes: Solid < Liquid < Gas (entropy increases with phase change).
  • Moles of substances: More moles of gaseous products increase entropy.
• Example States:
  • For CO2 (s) to CO2 (g), ext{ΔS}_{ ext{sys}} > 0 due to increased disorder.

17.4 Calculating Entropy Changes

• Entropy Change Formula:

  • $ext{ΔS}<em>{ ext{rxn}} = ext{Sm}S</em>{ ext{products}} - ext{Sn}S_{ ext{reactants}}$

• Example Calculation: For the reaction $2 ext{SO}<em>2 (g) + ext{O}</em>2 (g) <br /> ightarrow 2 ext{SO}_3 (g)$:

  • $ext{ΔS}<em>{ ext{rxn}} = 2 imes S</em>{ ext{SO}<em>3} - (S</em>{ ext{SO}<em>2} + S</em>{ ext{O}_2})$

• Surroundings and Entropy:

  • Entropy of surroundings is linked to $- ext{ΔH}_{ ext{sys}}/T$.

17.5 Free Energy

• Gibbs Free Energy (G): Helps determine spontaneity of a reaction without additional energy input.
  • $ext{ΔG}<em>{ ext{sys}} = ext{ΔH}</em>{ ext{sys}} - T ext{ΔS}_{ ext{sys}}$
• Spontaneity Rules:
  • ext{ΔG} < 0: Spontaneous process
  • ext{ΔG} > 0: Nonspontaneous process
  • $ext{ΔG} = 0$: System at equilibrium.

17.6 Temperature and Spontaneity

• Temperature affects spontaneity (crossover temperature determines when a reaction becomes spontaneous).
• Crossover temperature calculation:
  • At the crossover temperature, $ext{ΔG} = 0$ and $ext{ΔH} = T ext{ΔS}$

17.7 Equilibrium & Thermodynamics

• Equilibrium Defined: No net change in the concentration of products and reactants over time.
• Gibbs Free Energy and reaction quotient (Q) is used to determine the direction of the reaction towards equilibrium:
  • If Q < K: Reaction proceeds forward.
  • If Q > K: Reaction proceeds in reverse.
• Relationship: $ext{ΔG} = ext{ΔG}° + RT ext{ln}Q$

Sample Problems and Practice Questions

• Practice predicting signs of entropy change and spontaneity for various reactions.
• Calculate Gibbs Free Energy changes using standard formation values.
• Calculate equilibrium constants from Gibbs Free Energy values.