Criteria for Equilibrium — Core Principles

Core Principles

Equilibrium in chemistry refers to a dynamic state where the rates of forward and reverse processes are equal, leading to no net change in macroscopic properties. It's not a static halt but a continuous balance.

The thermodynamic criteria for equilibrium depend on the specific conditions of the system. For systems at constant temperature and pressure, which is common for most chemical reactions, the Gibbs free energy change ($Delta G$) must be zero.

This signifies that the system has reached its minimum Gibbs free energy. If $Delta G < 0$, the process is spontaneous; if $Delta G > 0$, it's non-spontaneous. For systems at constant temperature and volume, the Helmholtz free energy change ($Delta A$) must be zero.

Lastly, for an isolated system, equilibrium is achieved when the entropy of the system ($Delta S_{system}$) reaches its maximum value, meaning $Delta S_{system} = 0$ for any infinitesimal change. Understanding these criteria is fundamental for predicting reaction feasibility and the extent of chemical transformations.

Important Differences

vs Spontaneity

Aspect	This Topic	Spontaneity
Definition	The inherent tendency of a process to occur without continuous external intervention.	A dynamic state where opposing processes occur at equal rates, resulting in no net change in macroscopic properties.
Thermodynamic Criterion (Constant T, P)	$Delta G < 0$ (Gibbs free energy decreases).	$Delta G = 0$ (Gibbs free energy is at a minimum).
Thermodynamic Criterion (Isolated System)	$Delta S_{system} > 0$ (Entropy of the system increases).	$Delta S_{system} = 0$ (Entropy of the system is at a maximum).
Direction of Change	Indicates the direction in which a process will proceed.	Indicates the state where there is no net direction of change.
Relationship	A spontaneous process drives the system *towards* equilibrium.	Equilibrium is the *final state* achieved by a spontaneous process.

Spontaneity describes the natural tendency of a process to occur, driven by a reduction in Gibbs free energy or an increase in total entropy. It dictates the direction of change. Equilibrium, conversely, is the stable state where no further net change occurs because the forward and reverse rates are balanced. A spontaneous process will continue until it reaches equilibrium, at which point its spontaneity ceases. Thus, spontaneity is the journey, and equilibrium is the destination, with distinct thermodynamic criteria defining each state.