If ΔG is negative, does that mean the reaction happens instantly?

No. A negative ΔG indicates the reaction is thermodynamically *favored* or spontaneous, meaning it can proceed without an ongoing input of energy. However, it says nothing about the *speed* or *rate* of the reaction. That is governed by kinetics and activation energy. A reaction with a negative ΔG could still be extremely slow, like the rusting of iron, without a suitable catalyst or initiation.

Can a reaction be spontaneous if it's endothermic (ΔH > 0)?

Yes. If the reaction results in a large enough increase in the disorder of the system (a positive ΔS), the –TΔS term in the Gibbs equation can outweigh the positive ΔH, making ΔG negative. A common example is the melting of ice at temperatures above 0°C; it absorbs heat (endothermic) but the increased molecular disorder drives the process.

Why does the simulator only show the sign of ΔG and not calculate an equilibrium constant?

This simulator is designed to teach the foundational relationship ΔG = ΔH – TΔS and how its sign predicts spontaneity under standard-state conditions. The connection to the equilibrium constant K, where ΔG° = –RT ln K, is a crucial next step but involves additional complexity (the reaction quotient Q). This model simplifies the concept to build an intuitive understanding first.

What's the difference between 'spontaneous' and 'exothermic'?

Exothermic refers specifically to a process that releases heat (ΔH < 0). Spontaneous refers to a process that can proceed on its own without continuous external intervention, as determined by a negative ΔG. While many spontaneous reactions are exothermic, they are not the same. An endothermic process can be spontaneous if driven by a large entropy increase, as shown in the simulator.

Gibbs Free Energy

ΔG = ΔH − TΔS; sign vs spontaneity at constant p,T (no Q or K).

ΔG < 0 — forward direction favored (model)

Endothermic (ΔH > 0) can still be spontaneous if TΔS is large enough; exothermic with large negative ΔS can become non-spontaneous at low T.

About this model

The Gibbs Free Energy simulator visualizes the central thermodynamic equation ΔG = ΔH – TΔS, which determines the spontaneity of a process at constant temperature and pressure. It models how the interplay between enthalpy change (ΔH), entropy change (ΔS), and absolute temperature (T) dictates the sign of the Gibbs free energy change (ΔG). A negative ΔG indicates a spontaneous (thermodynamically favorable) process, a positive ΔG indicates a non-spontaneous one, and ΔG = 0 signifies equilibrium. The core learning is that spontaneity is not governed by enthalpy or entropy alone, but by their combined effect as moderated by temperature. For example, an endothermic reaction (ΔH > 0) can still be spontaneous if it involves a large increase in entropy (ΔS > 0) and the temperature is sufficiently high. This simulator simplifies the real world by focusing on the *sign* of ΔG for a single, defined process, not its exact numerical value. It does not incorporate reaction quotients (Q), equilibrium constants (K), or the kinetics of how fast a reaction proceeds. By adjusting sliders for ΔH, ΔS, and T, students directly observe the transition between spontaneous and non-spontaneous regimes, reinforcing the predictive power of the Gibbs equation and the conditions under which entropy or enthalpy dominates the driving force of a reaction.

Who it's for: High school and introductory undergraduate chemistry students learning chemical thermodynamics, particularly the concepts of spontaneity, Gibbs free energy, and the second law.

Key terms

Gibbs Free Energy
Enthalpy
Entropy
Spontaneity
Thermodynamics
Second Law of Thermodynamics
Endothermic
Exothermic

How it works

At fixed temperature and pressure, the sign of ΔG (for a reaction written as products − reactants) tells you whether the process tends to run forward (ΔG < 0) in the thermodynamic limit. ΔG = ΔH − TΔS: enthalpy and entropy oppose or reinforce depending on T. This is a state-function sketch — no activities, non-standard ΔG vs Q, or equilibrium K here.

Key equations

ΔG = ΔH − TΔS · (constant p, T; sign → spontaneity forward)

Frequently asked questions

If ΔG is negative, does that mean the reaction happens instantly?: No. A negative ΔG indicates the reaction is thermodynamically *favored* or spontaneous, meaning it can proceed without an ongoing input of energy. However, it says nothing about the *speed* or *rate* of the reaction. That is governed by kinetics and activation energy. A reaction with a negative ΔG could still be extremely slow, like the rusting of iron, without a suitable catalyst or initiation.
Can a reaction be spontaneous if it's endothermic (ΔH > 0)?: Yes. If the reaction results in a large enough increase in the disorder of the system (a positive ΔS), the –TΔS term in the Gibbs equation can outweigh the positive ΔH, making ΔG negative. A common example is the melting of ice at temperatures above 0°C; it absorbs heat (endothermic) but the increased molecular disorder drives the process.
Why does the simulator only show the sign of ΔG and not calculate an equilibrium constant?: This simulator is designed to teach the foundational relationship ΔG = ΔH – TΔS and how its sign predicts spontaneity under standard-state conditions. The connection to the equilibrium constant K, where ΔG° = –RT ln K, is a crucial next step but involves additional complexity (the reaction quotient Q). This model simplifies the concept to build an intuitive understanding first.
What's the difference between 'spontaneous' and 'exothermic'?: Exothermic refers specifically to a process that releases heat (ΔH < 0). Spontaneous refers to a process that can proceed on its own without continuous external intervention, as determined by a negative ΔG. While many spontaneous reactions are exothermic, they are not the same. An endothermic process can be spontaneous if driven by a large entropy increase, as shown in the simulator.

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Gibbs Free Energy

About this model

How it works

Key equations

Frequently asked questions

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About this model

How it works

Key equations

Frequently asked questions