In
chemical thermodynamics
Chemical thermodynamics is the study of the interrelation of heat and work with chemical reactions or with physical changes of state within the confines of the laws of thermodynamics. Chemical thermodynamics involves not only laboratory measurem ...
, an endergonic reaction (; also called a heat absorbing nonspontaneous reaction or an unfavorable reaction) is a
chemical reaction
A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...
in which the standard change in
free energy is positive, and an
additional driving force is needed to perform this reaction. In layman's terms, the total amount of useful energy is negative (it takes more energy to start the reaction than what is received out of it) so the total energy is a net negative result. For an overall gain in the net result, see
exergonic reaction. Another way to phrase this is that useful energy must be absorbed from the
surroundings
Surroundings are the area around a given physical or geographical point or place. The exact definition depends on the field. Surroundings can also be used in geography (when it is more precisely known as vicinity, or vicinage) and mathematics, ...
into the workable system for the reaction to happen.
Under constant temperature and constant pressure conditions, this means that the change in the standard
Gibbs free energy
In thermodynamics, the Gibbs free energy (or Gibbs energy; symbol G) is a thermodynamic potential that can be used to calculate the maximum amount of work that may be performed by a thermodynamically closed system at constant temperature and ...
would be positive,
:
for the reaction at
standard state (i.e. at standard pressure (1
bar), and standard concentrations (1
molar) of all the reagents).
In
metabolism
Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run c ...
, an endergonic process is
anabolic, meaning that energy is stored; in many such anabolic processes energy is supplied by coupling the reaction to
adenosine triphosphate (ATP) and consequently resulting in a high energy, negatively charged organic phosphate and positive
adenosine diphosphate.
Equilibrium constant
The
equilibrium constant for the reaction is related to Δ''G''° by the relation:
:
where ''T'' is the
absolute temperature and ''R'' is the
gas constant
The molar gas constant (also known as the gas constant, universal gas constant, or ideal gas constant) is denoted by the symbol or . It is the molar equivalent to the Boltzmann constant, expressed in units of energy per temperature increment per ...
. A positive value of Δ''G''° therefore implies
:
so that starting from molar stoichiometric quantities such a reaction would move backwards toward equilibrium, not forwards.
Nevertheless, endergonic reactions are quite common in nature, especially in
biochemistry
Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology and ...
and
physiology. Examples of endergonic reactions in cells include
protein synthesis, and the
Na+/K+ pump which drives
nerve conduction and
muscle contraction.
Gibbs free energy for endergonic reactions
All physical and chemical systems in the universe follow the
second law of thermodynamics and proceed in a downhill, i.e., ''exergonic'', direction. Thus, left to itself, any physical or chemical system will proceed, according to the second law of thermodynamics, in a direction that tends to lower the
free energy of the system, and thus to expend energy in the form of work. These reactions occur spontaneously.
A chemical reaction is endergonic when non spontaneous. Thus in this type of reaction the
Gibbs free energy
In thermodynamics, the Gibbs free energy (or Gibbs energy; symbol G) is a thermodynamic potential that can be used to calculate the maximum amount of work that may be performed by a thermodynamically closed system at constant temperature and ...
increases. The
entropy is included in any change of the Gibbs free energy. This differs from an
endothermic reaction where the entropy is not included. The Gibbs free energy is calculated with the
Gibbs–Helmholtz equation:
:
where:
: = temperature in
kelvins
The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and phy ...
(K)
: = change in the Gibbs free energy
: = change in entropy (at 298 K) as
: = change in enthalpy (at 298 K) as
A chemical reaction progresses non spontaneously when the Gibbs free energy increases, in that case the is positive. In
exergonic reactions the is negative and in endergonic reactions the is positive:
:
exergon
:
endergon
where
equals the change in the Gibbs free energy after completion of a chemical reaction.
Making endergonic reactions happen
Endergonic reactions can be achieved if they are either ''pulled'' or ''pushed'' by an
exergonic (stability increasing, negative change in
free energy) process. Of course, in all cases the net reaction of the ''total'' system (the reaction under study plus the puller or pusher reaction) is exergonic.
Pull
Reagents can be ''pulled'' through an endergonic reaction, if the reaction products are cleared rapidly by a subsequent exergonic reaction. The concentration of the products of the endergonic reaction thus always remains low, so the reaction can proceed.
A classic example of this might be the first stage of a reaction which proceeds via a
transition state. The process of getting to the top of the
activation energy barrier to the transition state is endergonic. However, the reaction can proceed because having reached the transition state, it rapidly evolves via an exergonic process to the more stable final products.
Push
Endergonic reactions can be ''pushed'' by coupling them to another reaction which is strongly exergonic, through a shared intermediate.
This is often how biological reactions proceed. For example, on its own the reaction
:
may be too endergonic to occur. However it may be possible to make it occur by coupling it to a strongly exergonic reaction – such as, very often, the decomposition of
ATP into
ADP and inorganic phosphate ions, ATP → ADP + P
i, so that
:
:
This kind of reaction, with the ATP decomposition supplying the free energy needed to make an endergonic reaction occur, is so common in cell biochemistry that ATP is often called the "universal energy currency" of all living organisms.
Examples
Endergonic reactions include any processes that do not happen by themselves, and they could be said to "not want" to happen. A very simple and easily visualizable example could be the massive pile of paperwork on the corner of your desk that you absolutely do not want to complete, or, a more scientific example, a car moving forward while on a completely flat surface. Any chemical processes that would naturally not be completed without external input embody endergonic reactions. This external input doesn't always have to be physical or visible, as a chemical catalyst counts as one, and if a reaction ''needs'' a
catalyst
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
to happen, then it is endergonic.
Here are a few scientific examples of endergonic processes:
# Water freezing at room temperature (must be below room temperature)
# Water decomposing into gaseous hydrogen and oxygen (
electrolysis) (needs an added electrical current)
#
Photosynthesis (needs sunlight energy)
# Formation of ozone (O
3) from oxygen (needs ultraviolet light from the sun)
See also
*
Exergonic
*
Exergonic reaction
*
Exothermic
*
Endothermic
*
Exothermic reaction
In thermochemistry, an exothermic reaction is a "reaction for which the overall standard enthalpy change Δ''H''⚬ is negative." Exothermic reactions usually release heat. The term is often confused with exergonic reaction, which IUPAC defines ...
*
Endothermic reaction
*
Endotherm
*
Exotherm
References
{{Portal bar, Chemistry
Thermochemistry
Thermodynamic processes