General Chemistry 2 Study Guide: Gibbs Free Energy and Reaction Direction

The Second Law of Thermodynamics

• The second law of thermodynamics states that the entropy of the universe increases in a spontaneous process and remains unchanged in an equilibrium process.
• Entropy is a measure of the disorder or randomness in a system.
• Heat always flows from a hot object to a cold object, which is consistent with the second law of thermodynamics.

Entropy and Its Implications

• In a closed system, entropy increases over time.
• Some energy transformations are irreversible, which is an implication of the second law of thermodynamics.
• Entropy is often referred to as the measure of disorder.

Predicting Spontaneity of a Process

• The thermodynamic quantity used to predict the spontaneity of a process is Gibbs free energy (∆G).
• At equilibrium, the value of ∆G for a process is zero.
• ∆G < 0 shows a spontaneous process at a constant temperature and pressure.

Gibbs Free Energy and Its Relationship

• The equation ∆G = ∆H – T∆S correctly represents the relationship between Gibbs free energy, enthalpy, and entropy.
• Joules (J) is the unit typically used for Gibbs free energy.
• A spontaneous reaction is indicated by a negative ∆G.
• Factors that affect the spontaneity of a reaction according to Gibbs free energy include:
  • Temperature (T)
  • Enthalpy change (∆H)
  • Entropy change (∆S)

Gibbs Free Energy Equations

• The equation ∆G = RT in JK relates to the spontaneity of a process.
• The equation ∆G = ∆H – T∆S correctly represents Gibbs free energy (∆G).

Examples of Spontaneous and Non-Spontaneous Processes

• Rusting of iron is an example of a spontaneous process.
• Melting of ice cubes at -50C and 1atm is an example of a non-spontaneous process.
• Ice formation is another example of a non-spontaneous process.
• Arranging books in a shelf by height does not show an increasing entropy.
• A type of process that absorbs heat from the surroundings is endothermic.