Principle of maximum work
In the
Overview
Berthelot independently enunciated a generalization (commonly known as Berthelot's Third Principle, or Principle of Maximum Work), which may be briefly stated as: every pure chemical reaction is accompanied by evolution of heat. Whilst this principle is undoubtedly applicable to the great majority of chemical actions under ordinary conditions, it is subject to numerous exceptions, and cannot therefore be taken (as its authors originally intended) as a secure basis for theoretical reasoning on the connection between thermal effect and chemical affinity. The existence of reactions which are reversible on slight alteration of conditions at once invalidates the principle, for if the action proceeding in one direction evolves heat, it must absorb heat when proceeding in the reverse direction. As the principle was abandoned even by its authors, it is now only of historical importance, although for many years it exerted considerable influence on thermochemical research.[1]
Thus, to summarize, in 1875 by the French chemist
In 1876, however, through the works of
For all thermodynamic processes between the same initial and final state, the delivery of work is a maximum for a reversible process.
The principle of work was a precursor to the development of the thermodynamic concept of free energy.
Thermochemistry
In thermodynamics, the Gibbs free energy or Helmholtz free energy is essentially the energy of a chemical reaction "free" or available to do external work. Historically, the "free energy" is a more advanced and accurate replacement for the thermochemistry term “affinity” used by chemists of olden days to describe the “force” that caused chemical reactions. The term dates back to at least the time of Albertus Magnus in 1250.
According to Nobelist and chemical engineering professor Ilya Prigogine: “as motion was explained by the Newtonian concept of force, chemists wanted a similar concept of ‘driving force’ for chemical change? Why do chemical reactions occur, and why do they stop at certain points? Chemists called the ‘force’ that caused chemical reactions affinity, but it lacked a clear definition.[2]
During the entire 18th century, the dominant view in regard to heat and light was that put forward by Isaac Newton, called the “Newtonian hypothesis”, which stated that light and heat are forms of matter attracted or repelled by other forms of matter, with forces analogous to gravitation or to chemical affinity.
In the 19th century, the French chemist
Thermodynamics
With the development of the first two
Define:
The loss of internal energy by the primary system The gain in entropy of the primary system The gain in internal energy of the reversible work source The gain in entropy of the reversible work source The gain in internal energy of the reversible heat source The gain in entropy of the reversible heat source The temperature of the reversible heat source
We may now make the following statements
(First law of thermodynamics) (Second law of thermodynamics) (Reversible work source) (Reversible heat source)
Eliminating , , and gives the following equation:
When the primary system is reversible, the equality will hold and the amount of work delivered will be a maximum. Note that this will hold for any reversible system which has the same values of dU and dS .
See also
- Chemical thermodynamics
- Thomsen-Berthelot principle
- Thermochemistry
References
- ^ Encyclopædia Britannica 1911
- ^ Source: Ilya Prigogine's 1998 textbook Modern Thermodynamics