1876

Gibbs Free Energy

Josiah Willard Gibbs (1839–1903)

If you want to look under the hood of chemistry and see what really makes things work, study thermodynamics. It measures changes in energy, the driving force for all chemical processes. Credit here goes to American scientist Josiah Willard Gibbs, whose theoretical insights and great mathematical ability turned thermodynamics into a precise scientific tool with applications in every possible area of chemistry, physics, and biology.

In 1876, he published his work on chemical systems and the “free energy” of reactions (now called Gibbs free energy, or G, in his honor). When a system changes from one state into another (chemically, as in a reaction, or physically, as in melting or boiling), the change in G (called ∆G or delta-G) is the work exchanged by the system with its surroundings (for example, the heat that is given off). Chemical reactions that can spontaneously give off energy show a negative ∆G. A fire is a perfect example. Reactions that have even larger negative ∆G values (such as the thermite reaction or the decomposition of nitroglycerine) can be dangerously energetic. In contrast, reactions with a positive ∆G—photosynthesis in plants, for example—require the addition of external energy, such as sunlight.

The other key thing to know about ∆G is that it’s made up of two parts: enthalpy and entropy. Enthalpy (designated by the letter H) can be thought of as a pure measure of heat and energy, while entropy (S) is related to disorder and the reactants’ “degrees of freedom” (i.e., how many different ways that they can move and vibrate). Chemists think in these terms constantly, gaining great insight into reactions by keeping all these factors in mind.

Some chemical reactions are spontaneous even though they actually get cold and soak up heat from their surroundings, like an instant “cold pack.” This can happen because the entropy of the final state is so much higher (∆S) than that of the starting materials, canceling out an unfavorable enthalpy change (∆H), and giving an overall favorable ∆G. If both ∆H and ∆S are large and negative, though, you have an explosion in the making!

Josiah Willard Gibbs, 1903.

This explosive thermite reaction has a large negative ∆G value.