Everything about Thermodynamic Free Energy totally explained
In
thermodynamics, the term
thermodynamic free energy is the amount
of
mechanical (or other)
work that can be extracted from a
system, and is helpful in
engineering applications. It is a subtraction of the
entropy of a system ("useless energy") from the total energy, yielding a
thermodynamic state function which represents the "
useful energy".
Overview
In short, free energy is that portion of any
First-Law energy that's available for doing thermodynamic
work;
for example, work mediated by thermal energy. Since free energy is subject to
irreversible loss in the course of such work and First-Law energy is always conserved, it's evident that free energy is an expendable,
Second-Law kind of energy that can make things happen within finite amounts of time.
In solution
chemistry and
biochemistry, the
Gibbs free energy change (denoted by Δ
G) is commonly used merely as a surrogate for (−
T times) the entropy
produced by spontaneous
chemical reactions in situations where there's no work done; or at least no "useful" work; for example, other than
pd
V. As such, it serves as a particularization of the
second law of thermodynamics, giving it the
physical dimensions of energy, even though the inherent meaning in terms of entropy would be more to the point.
The free energy
functions are
Legendre transforms of the
internal energy. For
processes involving a system at
constant pressure p and
temperature T, the
Gibbs free energy is the most useful because, in addition to subsuming any entropy change due merely to heat
flux, it does the same for the
pd
V work needed to "make space for additional molecules" produced by various processes. (Hence its utility to
solution-
phase chemists, including biochemists.) The
Helmholtz free energy has a special
theoretical importance since it's proportional to the
logarithm of the
partition function for the
canonical ensemble in
statistical mechanics. (Hence its utility to
physicists; and to
gas-phase chemists and engineers, who don't want to ignore
pd
V work.)
The (historically earlier)
Helmholtz free energy is defined as
A =
U −
TS, where
U is the internal energy,
T is the
absolute temperature, and
S is the
entropy. Its change is equal to the amount of
reversible work done on, or obtainable from, a system at constant
T. Thus its appellation "
work content", and the designation
A from
arbeit, the German word for work. Since it makes no reference to any quantities involved in work (such as
p and
V), the Helmholtz function is completely general: its decrease is the maximum amount of work which can be done
by a system, and it can increase at most by the amount of work done
on a system.
The
Gibbs free energy G =
H −
TS, where
H is the
enthalpy. (
H =
U +
pV, where
p is the pressure and
V is the volume.)
There has been historical controversy:
Since both fields use both functions, a
compromise has been suggested, using
A to denote the Helmholtz function, with
G for the Gibbs function. While
A is preferred by
IUPAC,
F is sometimes still in use, and the correct free energy function is often implicit in manuscripts and presentations.
Application
The
experimental usefulness of these functions is restricted to conditions where certain variables (
T, and
V or
external p) are held constant, although they also have theoretical importance in deriving
Maxwell relations. Work other than
pd
V may be added, for example, for
electrochemical cells, or
f ˑd
x work in
elastic materials and in
muscle contraction. Other forms of work which must sometimes be considered are
stress-
strain,
magnetic, as in
adiabatic de
magnetization used in the approach to
absolute zero, and work due to electric
polarization. These are described by
tensors.
In most cases of interest there are internal
degrees of freedom and processes, such as
chemical reactions and
phase transitions, which create entropy. Even for homogeneous "bulk" materials, the free energy functions depend on the (often suppressed)
composition, as do all proper
thermodynamic potentials (
extensive functions), including the internal energy.
| Name |
Definition |
Natural variables |
| Helmholtz free energy |
|
Any decrease in the Gibbs function of a system is the upper limit for any isothermal, isobaric work that can be captured in the surroundings, or it may simply be dissipated, appearing as T times a corresponding increase in the entropy of the system and/or its surrounding.
Further Information
Get more info on 'Thermodynamic Free Energy'.
|
External Link Exchanges
Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:
<a href="http://thermodynamic_free_energy.totallyexplained.com">Thermodynamic free energy Totally Explained</a>
Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned. |
|
