What is the primary difference between a spontaneous process and a system at equilibrium?

A spontaneous process is one that occurs without external intervention, driven by a decrease in Gibbs free energy ($Delta G 0$). It represents the system moving *towards* a more stable state. Equilibrium, on the other hand, is the *state* where the system has reached its most stable condition under the given constraints, and there is no longer any net driving force for change. At equilibrium, $Delta G = 0$ (at constant T, P) or $Delta S_{universe} = 0$. A spontaneous process ceases to be spontaneous once equilibrium is attained.

Why is Gibbs free energy ($Delta G$) the preferred criterion for equilibrium in most chemical reactions?

Most chemical reactions and biological processes occur under conditions of constant temperature and pressure, which are typical laboratory and atmospheric conditions. Under these specific conditions, Gibbs free energy provides a direct and convenient measure of spontaneity and equilibrium. Its change, $Delta G$, directly indicates whether a process is spontaneous ($Delta G 0$), or at equilibrium ($Delta G = 0$). Using $Delta S_{universe}$ would require calculating changes in both the system and surroundings, which is often more complex than directly evaluating $Delta G$ for the system.

Does $Delta G = 0$ mean that no reaction is occurring at all?

No, absolutely not. $Delta G = 0$ signifies a state of dynamic equilibrium. This means that the forward reaction is occurring at the exact same rate as the reverse reaction. Individual molecules are still reacting and transforming, but the net concentrations of reactants and products remain constant over time. It's a continuous, balanced process, not a static halt. For example, in a saturated salt solution, solid salt is continuously dissolving, and dissolved ions are continuously precipitating, but the overall concentration of dissolved salt remains constant.

How does temperature affect the equilibrium criterion $Delta G = 0$?

Temperature plays a critical role in the $Delta G = Delta H - TDelta S$ equation. At equilibrium, $Delta G = 0$, which implies $Delta H = TDelta S$. This means the equilibrium temperature ($T_{eq}$) can be calculated as $T_{eq} = rac{Delta H}{Delta S}$. For processes like phase transitions, this equation directly gives the transition temperature (e.g., melting point, boiling point). For chemical reactions, changing temperature can shift the equilibrium position by altering the relative contributions of the enthalpy and entropy terms to $Delta G$, thus changing the point at which $Delta G$ becomes zero.

What is the criterion for equilibrium in an isolated system?

For an isolated system, which cannot exchange either energy or matter with its surroundings, the criterion for equilibrium is that the entropy of the system ($Delta S_{system}$) reaches its maximum possible value. At this maximum entropy, any infinitesimal change within the system would result in $Delta S_{system} = 0$. This aligns with the Second Law of Thermodynamics, which states that the entropy of an isolated system tends to increase for spontaneous processes until it reaches a maximum at equilibrium.

Can a non-spontaneous reaction ever reach equilibrium?

Yes, but not on its own. A 'non-spontaneous' reaction ($Delta G > 0$) means that the reverse reaction is spontaneous. For the non-spontaneous forward reaction to proceed and reach equilibrium, external energy input is required. This is what happens in electrolytic cells or when coupling a non-spontaneous reaction with a highly spontaneous one (e.g., ATP hydrolysis in biological systems). Once the external energy drives the reaction sufficiently, it will eventually reach an equilibrium state where $Delta G = 0$ under the new conditions, or if the external energy input is removed, it will revert to the original equilibrium state.

Criteria for Equilibrium

Chemistry

NEET UG

Version 1Updated 22 Mar 2026

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In thermodynamics, a system is said to be in equilibrium when its macroscopic properties, such as temperature, pressure, and concentration, do not change with time. This state is characterized by a balance of opposing forces or processes, resulting in a net zero change. For a system at constant temperature and pressure, the criterion for equilibrium is that the Gibbs free energy change ( $Delta G$ )…

Quick Summary

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.

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Key Concepts

Gibbs Free Energy and Equilibrium (

Delta G = 0

)

For most chemical reactions and phase changes occurring under constant temperature and pressure conditions,…

Entropy of the Universe and Equilibrium (

Delta S_{universe} = 0

)

The Second Law of Thermodynamics states that for any spontaneous process, the total entropy of the universe…

Relationship between

Delta G^circ

and Equilibrium Constant (K)

The standard Gibbs free energy change ( $Delta G^circ$ ) relates to the equilibrium constant ( $K$ ) of a…

Equilibrium (Constant T, P): —

Delta G = 0

Equilibrium (Constant T, V): —

Delta A = 0

Equilibrium (Isolated System): —

Delta S_{system} = 0

(at maximum entropy)

Spontaneity (Constant T, P): —

Delta G < 0

Spontaneity (Isolated System): —

Delta S_{system} > 0

Gibbs Free Energy Equation: —

Delta G = Delta H - TDelta S

Equilibrium Temperature: —

T_{eq} = \frac{Delta H}{Delta S}

(when

Delta G = 0

)

Relationship with K: —

Delta G^circ = -RT ln K

GET A S.I.T.E. for Equilibrium!

Gibbs = 0 for Equilibrium at constant Temperature, Pressure. A = 0 for constant Temperature, Volume. System Isolated: $Delta S_{system}$ is Top (maximum), so $Delta S_{system}$ Equals 0.