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}{ ext{rxn}} = ext{Sm}S{ ext{products}} - ext{Sn}S_{ ext{reactants}}

• Example Calculation: For the reaction 2 ext{SO}2 (g) + ext{O}2 (g)
  ightarrow 2 ext{SO}_3 (g):

  • ext{ΔS}{ ext{rxn}} = 2 imes S{ ext{SO}3} - (S{ ext{SO}2} + S{ 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}{ ext{sys}} = ext{ΔH}{ 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.