### Topic Page: Gibbs, Josiah Willard (1839 - 1903)

**Gibbs, Josiah Willard**

*The Hutchinson Unabridged Encyclopedia with Atlas and Weather Guide*

US theoretical physicist and chemist who developed a mathematical approach to thermodynamics and established vector methods in physics. He devised the phase rule and formulated the Gibbs adsorption isotherm.

Gibbs showed how many thermodynamic laws could be interpreted in terms of the results of the movements of enormous numbers of bodies such as molecules. His ensemble method equated the behaviour of a large number of systems at once to that of a single system over a period of time.

Gibbs was born in New Haven, Connecticut, and studied at Yale and in Europe1866–69. From 1871 he was professor of mathematical physics at Yale.

The phase rule, published 1876–78, may be stated as:

f = n + 2 – r

where f is the number of degrees of freedom, n the number of chemical components, and r the number of phases – solid, liquid or gas; degrees of freedom are quantities such as temperature and pressure that may be altered without changing the number of phases. He also described his concept of free energy, which can be used as a measure of the feasibility of a given chemical reaction. It is defined in terms of the enthalpy, or heat content, and entropy, a measure of the disorder of a chemical system. From this Gibbs developed the notion of chemical potential, which is a measure of how the free energy of a particular phase depends on changes in composition.

The Gibbs adsorption isotherm showed that changes in the concentration of a component of a solution in contact with a surface occur if there is an alteration in the surface tension.

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