Energy

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Energy

Description

Energy [J] is a fundamental term that is used in physics and physical chemistry with various meanings. These meanings become explicit in the following equations relating to systems at constant temperature and pressure. Energy is exchanged between a system and the environment across the system boundaries in the form of heat, d_eQ, work, d_eW, and matter, d_matH,

dH = (deQ + deW) + dmatH [Eq. 1]

Whereas dH describes the energy change (enthalpy) of the system, heat and work are external energy changes (subscript e), and d_matH is the exchange of matter expressed in energy (enthaply) equivalents. In closed systems, d_matH = 0. The energy balance equation [Eq. 1] is a form of the First Law of Thermodynamics, which is the law of energy conservation stating that energy cannot be generated or destroyed: energy can only be transformed into different forms of work and heat, and transferred in the form of matter.

An equally famous energy balance equation considers energy changes of the system only:

dH = dG + T∙dS = dG + dB [Eq. 2]

The total energy (enthalpy, dH) change of a system (at constant pressure and temperature) is the sum of free energy change (Gibbs energy, dG) and bound energy change (bound energy, dB = T∙dS). The bound energy is that part of the total energy change that is always bound to an exchange of heat (at constant temperature).

A third energy balance equation accounts for changes of the system in terms of irreversible internal processes (i) occuring within the system boundaries, and reversible external processes (e) of transfer across the system boundaries,

 dH = diH + deH [Eq. 3a]

 dG = diG + deG [Eq. 3b]

The energy conservation law of thermodynamics (First Law) can be formulated as d_iH = 0 (at constant gas pressure), whereas the generally negative sign of the dissipated energy, d_iG ≡ d_iD ≤ 0, is a formulation of the Second Law of Thermodynamics. Insertion into Eq. 3 yields,

 dH = deH [Eq. 4a]

 dG = diD + deW + dmatG [Eq. 4b]

When talking about energy transformations, the term energy is used in a general sense without specification of these various forms of energy.

Abbreviation: E; various [J]

Reference: Gnaiger 1993 Pure Appl Chem

Communicated by Gnaiger E 2018-12-29; last update: 2018-12-31

MitoPedia concepts: Ergodynamics 

References

1. Gnaiger 1993 Pure Appl Chem
2. Coopersmith 2010 Oxford Univ Press