Free energy is the portion of a system’s total energy that is available to do useful work. The concept comes from thermodynamics, where it helps predict whether a chemical reaction will happen on its own or needs an energy push. The term “free” doesn’t mean energy from nothing. It refers to energy that is “freed up,” available to power something, after accounting for energy lost to disorder in the system.
The Core Idea Behind Free Energy
Every physical or chemical process involves two competing factors: the total energy contained in the system and the natural tendency of that system toward disorder. Free energy captures both in a single number. When a log burns in a fireplace, some of the energy stored in the wood’s chemical bonds escapes as heat that warms the room, while some goes toward increasing the randomness of the ash, smoke, and gases produced. Free energy is the piece you can actually harness.
The formal version most people encounter is Gibbs free energy, named after the 19th-century physicist Josiah Willard Gibbs. It applies to the conditions we deal with most often in chemistry and biology: constant pressure and temperature. The formula is straightforward:
G = H − TS
G is free energy, H is enthalpy (the total heat content of the system), T is temperature in Kelvin, and S is entropy (a measure of disorder). The key insight is that temperature amplifies disorder. At higher temperatures, the entropy term (T × S) grows larger, meaning more energy gets “locked up” in disorder and less remains free to do work.
Why the Change in Free Energy Matters Most
Scientists rarely care about the absolute free energy of a system. What matters is the change in free energy, written as ΔG (delta G), during a reaction or process. This single value tells you three things:
- ΔG is negative: The reaction releases usable energy and will proceed on its own. These are called exergonic reactions. Burning fuel, rusting iron, and digesting food all have negative ΔG values.
- ΔG is positive: The reaction requires an input of energy to move forward. These endergonic reactions won’t happen spontaneously. Photosynthesis, for instance, needs sunlight to drive it.
- ΔG equals zero: The system is at equilibrium. Products and reactants are being formed at equal rates, and no net change occurs.
“Spontaneous” in this context doesn’t mean instant. It means the reaction is energetically favorable. Iron rusting is spontaneous but takes years. The ΔG value tells you about direction, not speed.
A Familiar Example: Your Body Running on ATP
Your cells store and release energy using a molecule called ATP. When ATP breaks apart (a process called hydrolysis), it releases roughly 28 to 34 kilojoules per mole of usable energy, depending on conditions inside the cell. That negative ΔG is what powers muscle contraction, nerve signaling, and virtually every active process in your body. Cells then use energy from food to rebuild ATP, completing the cycle.
This is free energy at work in biology: the energy released by breaking ATP’s chemical bonds is “free” to drive other reactions that wouldn’t happen on their own. Your cells constantly couple exergonic reactions (ones that release free energy) to endergonic ones (ones that need it), using ATP as the go-between.
Standard Conditions and Real Conditions
When you see ΔG° (with a small circle), that refers to the free energy change measured under standard conditions: gases at 1 bar of pressure, solutions at 1 molar concentration, and typically 25°C. These standardized values let scientists compare reactions on equal footing.
Real-world conditions differ. Inside a living cell, concentrations of molecules vary constantly, temperatures fluctuate, and the actual free energy change can be quite different from the standard value. That’s why biochemists often work with adjusted calculations that account for the true concentrations present in the system.
Gibbs vs. Helmholtz: Two Versions of Free Energy
Gibbs free energy applies when pressure stays constant, which covers most chemical reactions happening in open containers or inside living organisms. There’s a second version called Helmholtz free energy, used when volume stays constant instead of pressure. Its formula is similar: A = U − TS, where U is the internal energy of the system. You’d use Helmholtz free energy for reactions happening inside sealed, rigid containers, like certain industrial processes or theoretical problems in physics. For everyday chemistry and biology, Gibbs free energy is the relevant one.
How Free Energy Connects to Entropy
The second law of thermodynamics states that the total entropy of the universe always increases during any real process. Free energy provides a convenient way to track this without having to measure the entropy of the entire universe. When a system’s free energy decreases (negative ΔG), the entropy of the universe increases by a corresponding amount. The two concepts are mirror images: a spontaneous drop in free energy within a system always means rising disorder somewhere in the bigger picture.
This is why no process can convert 100% of energy into work. Some energy inevitably goes toward increasing entropy. Free energy represents the theoretical maximum work you could extract, and in practice, you always get less due to friction, heat loss, and other inefficiencies.
Free Energy in Neuroscience
The term “free energy” has also been borrowed by brain science, though with a different meaning. The Free Energy Principle, proposed by neuroscientist Karl Friston, suggests that the brain constantly works to minimize “surprise,” meaning the mismatch between what it predicts and what it actually senses. In this framework, variational free energy is a mathematical quantity that puts an upper bound on surprise. The brain reduces it in two ways: updating its internal predictions (perception) or acting on the world to make sensory input match expectations (action).
This isn’t thermodynamic free energy. It borrows the mathematical structure from physics but applies it to how the brain processes information. Under this theory, perception, learning, memory, and attention all serve the same goal of minimizing prediction errors, providing a single unifying principle for how brains organize themselves.
What Free Energy Is Not
The phrase “free energy” has been widely misused in popular culture to describe perpetual motion machines or devices that supposedly generate unlimited energy from nothing. These claims violate the first and second laws of thermodynamics. Energy cannot be created or destroyed, and every real process loses some energy to entropy. No device can produce more usable energy than it consumes. When physicists and chemists use the term “free energy,” they’re describing the fraction of existing energy that’s available for work, not energy that appears from nowhere.