Thermodynamics
Understanding the principles of thermodynamics is of interest for multiple scientific disciplines. Thermodynamic analysis is the bread and butter of many physicists and engineers. For them, it is usually unproblematic to understand the basics of thermodynamics because of their strong background in mathematics. On the other end of the spectrum, chemists also frequently have to analyze thermodynamics in order to predict and optimize chemical reactions, and biologists also come into contact with the topic in the context of toxycology, enzymology or metabolic flux analysis. For this audience it is often not easy to thoroughly understand the concepts of thermodynamics. To make things worse, in chemistry text books like “Physical Chemistry” (Atkins 2006), the considered thermodynamic systems are not always defined rigorously enough. The following unreviewed article is a summary of the authors understanding of the topic as a graduate student in biochemistry. This text is provided without guarantees of completeness or accuracy.
Thermodynamics in reactive open systems
A scientific law is a statement that describes what phenomena happen (observed or predicted). Overlap between scientific theory and scientific laws allows for predictions. By repeated successful predictions, the appropriate field of application for the law can be extended. If we consider a system which together with its surroundings constitutes a larger, isolated system (the universe), one important law is the law of conversion of energy which states that the energy of the universe is constant. Thus
where
If the kinetic energy of the system as a whole and potential Energy of the system as a whole (e.g. due to interaction with an external gravitational field) is not considered, i.e. for
The zeroth law of thermodynamics states that if two thermodynamic systems are each in thermal equilibrium with a third one, then they are in thermal equilibrium with each other. Maxwell expressed it in the words “All heat is of the same kind” (Maxwell, Clerk 1871). It can also expressed by saying that there is precisely one kind of non-mechanical, non-matter-transferring contact equilibrium between thermodynamic systems.
The most general case that can be considered is a reactive, open system that can exchange energy and matter with its surroundings. For an open, reaktive system where elementary chemical reactions k take place the first principle of thermodynamics can be expressed as
with
The second principle of thermodynamics can then be expressed as (Petrucci et al. 2016, p.600):
For reversible processes,
For irreversible processes,
According to the second principle of thermodynamics (De Decker 2015),
with
Thus
If the heat
Here,
Inserting
Different approaches towards a thermodynamic description of systems far from equilibrium (Jazinsky 2011; Esposito and Van den Broeck 2011; Prokovskii 2013) and description of chemically reactive systems were presented in recent literature (De Decker 2015; Rao and Esposito 2016).
Prigogine introduced the concept of internal variables
with
Prigogine showed that for chemically reactive systems, the extend of reaction
The rate of reaction is (Davis and Davis 2003)
The vector
Unreactive open systems
For an unreactive open system that is connected to other systems via different walls that are only permeable to either heat (diathermic wall), or work (piston for adiabatic work) or matter of a specific compound i (semipermeable membrane that is permeable only by compound i) the internal energy of the open system is (Knuiman et al. 2012):
with
According to the first law of thermodynamics,
The mass exchange is quantized, therefore the last equation becomes
with
Equation
Quasistatic processes and reversible processes
If the process of mass exchange is quasistatic or even reversible (quasistatic, no dissipation), the expression for the work is
Since for the given process
When the mass exchange is a quasistatic (or even reversible) transfer, the second law of thermodynamics holds:
The mass exchange is quantized, hence
with
Inserting
In equation
That seperation vanishes when the equation is simplified to
with
and
Notably, in equation
For any system and any general process the Gibbs equation is:
Equation
Fundamental equations of thermodynamics
For any system and any general process the Gibbs equation is given by
Insertion of
Insertion of
Insertion of
From
For homogenous macroscopic systems, it follows from equation
Insertion of
Insertion of
Spontaneous irreversible processes and reversible processes
From the second law of thermodynamics
For reversible processes,
Irreversible processes are spontaneous if
Reorganizing
with
For an isolated system (
For closed systems with varying constant parameters, the inequality for spontaneous change is given below. Indices indicate constant parameters.
The changes in the potentials of thermodynamics above are equal to
Confusingly, in the equations above, the symbol
Generalized work
In any case, the relevant generalized work terms relevant for the system and process under consideration are required:
where
The most relevant intrinsic and extrinsic variables are given in the table below. For details, see (Balmer 2011).
Type of work | Symbol | Generalized force | extrinsic parameter |
---|---|---|---|
volume-pressure | pressure ( |
volume ( |
|
electrochemical | electromotive force ( |
charge ( |
In the context of electrochemical cells, the electromotive force is called source voltage. It is equal to the electric potential difference for zero current through the cell IUPAC 2019.
Closed systems and unreactive systems
For closed systems
Thus, from
For an isochoric system (
For an isobaric system (
For an isobaric system, if temperature is held constant (
Confusingly, the symbol
Inserting
With
The chemical potential of substance i can be calculated from two seperate terms:
with
Inserting
with
For a system at thermodynamic equilibrium,
with
Electrochemical thermodynamics - Nernst equation
Equation
For an isochoric system (
For an isobaric system (
When the reaction proceeds for
Inserting
with
Inserting
and
For reversible processes,
Reorganizing yields:
From
Thus
See Balmer 2011.
The cell potential can be calculated from the electrode potentials of the two balanced half-reactions (
For a electrochemical cell with the cell formula
Anode (oxidation)
See here
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