General Chemistry 2: Gibbs Free Energy and Reaction Direction

Second Law of Thermodynamics

• States that entropy of the universe increases in spontaneous processes and remains unchanged in equilibrium processes
• Entropy represents disorder or randomness in a system
• In energy transfer or transformation, some energy becomes lost as heat
• Heat always flows from a hot object to a cold object
• Irreversibility of natural processes is a principle of increasing entropy

Entropy

• Measure of disorder or randomness in a system
• Increases over time in a closed system

Predicting Spontaneity

• Gibbs free energy (∆G) is used to predict spontaneity of a process
• ∆G < 0 indicates a spontaneous process at constant temperature and pressure
• ∆G = 0 indicates equilibrium
• K > 1 indicates a spontaneous process

Types of Processes

• Rusting of iron is a spontaneous process
• Melting of ice cubes at -50°C and 1 atm is non-spontaneous
• Arranging books on a shelf by height does not show increasing entropy
• Endothermic process absorbs heat from the surroundings
• Ice formation is an example of a non-spontaneous process

Gibbs Free Energy

• ∆G = ∆H – T∆S is the equation correctly representing the relationship between Gibbs free energy, enthalpy, and entropy
• ∆G is typically measured in Joules (J)
• Negative ∆G indicates a spontaneous reaction
• Factors affecting spontaneity of a reaction: temperature (T), enthalpy change (∆H), and entropy change (∆S)