Enthalpy, Entropy, and Gibbs Free Energy - SS2 Chemistry Lesson Note

Enthalpy, entropy, and Gibbs free energy are fundamental concepts in thermodynamics that describe the energy and spontaneity of processes. These thermodynamic properties play a crucial role in understanding and predicting the behaviour of chemical reactions and physical processes.

Enthalpy (H):

Enthalpy is a thermodynamic property that represents the total heat content of a system. It encompasses the internal energy (U) of the system and the work (PΔV) done on or by the system at constant pressure. Enthalpy is denoted by the symbol "H."

Mathematically, enthalpy can be expressed as:

H = U + PΔV

Key Points about Enthalpy:

●     Heat Transfer: Enthalpy accounts for the heat transfer that occurs during a process at constant pressure.

●     Exothermic and Endothermic Processes: In exothermic processes, where heat is released, the change in enthalpy (ΔH) is negative. In endothermic processes, where heat is absorbed, ΔH is positive.

●     Standard Enthalpy Change: The standard enthalpy change (ΔH°) is the enthalpy change that occurs when all reactants and products are in their standard states at a specified temperature and pressure.

Entropy (S):

Entropy is a thermodynamic property that measures the degree of randomness or disorder in a system. It represents the distribution of energy among the possible microstates of a system. Entropy is denoted by the symbol "S."

Key Points about Entropy:

●     Microscopic Disorder: Entropy is related to the number of ways energy can be distributed in a system, reflecting its microscopic disorder.

●     Change in Entropy (ΔS): ΔS is positive when there is an increase in disorder or randomness in a process and negative when there is a decrease.

●     Boltzmann's Formula: The entropy of a system can be calculated using Boltzmann's formula, which relates entropy to the number of microstates (W) available to the system: S = k ln(W), where k is Boltzmann's constant.

Gibbs Free Energy (G):

Gibbs free energy is a thermodynamic potential that combines the effects of both enthalpy and entropy. It determines whether a process is spontaneous or nonspontaneous and provides insight into the maximum work obtainable from a system. Gibbs free energy is denoted by the symbol "G."

Mathematically, Gibbs free energy can be expressed as:

ΔG = ΔH - TΔS

Key Points about Gibbs Free Energy:

●     Spontaneity of a Process: A process is spontaneous if ΔG is negative, indicating that the process can occur without external intervention. Non-spontaneous processes have a positive ΔG.

●     Equilibrium: At equilibrium, ΔG is zero, indicating that the system is in a state of maximum stability.

●     Free Energy Change (ΔG°): The standard free energy change represents the change in Gibbs free energy under standard conditions (1 atm pressure, 298 K temperature).

●     Relationship with Equilibrium Constant: For a chemical reaction at equilibrium, ΔG° is related to the equilibrium constant (K) through the equation: ΔG° = -RT ln(K), where R is the gas constant and T is the temperature.

Applications of Enthalpy, Entropy, and Gibbs Free Energy:

●     Reaction Spontaneity: Enthalpy, entropy, and Gibbs free energy help determine the spontaneity of chemical reactions and the direction they will proceed.

●     Phase Transitions: These thermodynamic properties explain phase transitions, such as melting, boiling, and sublimation, by considering the changes in enthalpy and entropy.

●     Equilibrium and Reaction Quotient: Gibbs free energy is used to calculate the equilibrium constant and reaction quotient, providing insights into the direction and extent of a chemical reaction.

●     Energy Conversion: Enthalpy and Gibbs free energy provide a basis for understanding and optimising energy conversion processes, such as fuel combustion and power generation.

Enthalpy, entropy, and Gibbs free energy are key thermodynamic properties that characterise the energy and spontaneity of processes. Enthalpy accounts for the heat transfer during a process, while entropy measures the degree of disorder. Gibbs free energy combines both enthalpy and entropy, providing insight into the spontaneity of a process and the maximum work obtainable. Understanding and manipulating these properties are essential for studying and predicting the behaviour of chemical reactions, phase transitions, and energy conversion processes.