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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, ∆eQ and work, ∆eW,
∆H = ∆eQ + ∆eW [Eq. 1]
Whereas ∆H describes the energy change (enthalpy) of the system, heat and work are external changes (subscript e). 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.
An equally famous energy balance equation considers energy changes of the system only:
∆H = ∆G + T∙∆S = ∆G + ∆B [Eq. 2]
The total energy (enthalpy, ∆H) change of a system (at constant pressure and temperature) is the sum of free energy change (Gibbs energy, ∆G) and bound energy change (bound energy, ∆B = T∙∆S). 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,
∆H = ∆iH + ∆eH [Eq. 3a]
∆G = ∆iG + ∆eG [Eq. 3b]
The energy conservation law of thermodynamics (First Law) can be formulated as ∆iH = 0 (at constant gas pressure), whereas the generally netative sign of the dissipated energy, ∆iG ≡ ∆iD ≤ 0, is a formulation of the Second Law of Thermodynamics. Insertion into Eq. 3 for closed systems yields,
∆H = ∆eH [Eq. 4a]
∆G = ∆iD + ∆eW [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
MitoPedia concepts: Ergodynamics
