Exergonic reaction

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An exergonic reaction (such as cellular respiration) is a reaction that releases free energy in the process of the reaction. The progress of the reaction is shown by the line. Activation energy (1) slows down the reaction. The change of Gibbs free energy (DG) in an exergonic reaction (that takes place at constant pressure and temperature) is negative because energy is lost (2). Exergonic Reaction.svg
An exergonic reaction (such as cellular respiration) is a reaction that releases free energy in the process of the reaction. The progress of the reaction is shown by the line. Activation energy (1) slows down the reaction. The change of Gibbs free energy (ΔG) in an exergonic reaction (that takes place at constant pressure and temperature) is negative because energy is lost (2).

In chemical thermodynamics, an exergonic reaction is a chemical reaction where the change in the free energy is negative (there is a net release of free energy). [1] This indicates a spontaneous reaction if the system is closed and initial and final temperatures are the same. For processes that take place in a closed system at constant pressure and temperature, the Gibbs free energy is used, whereas the Helmholtz energy is relevant for processes that take place at constant volume and temperature. Any reaction occurring at constant temperature without input of electrical or photon energy is exergonic, according to the second law of thermodynamics. An example is cellular respiration.

Symbolically, the release of free energy, , in an exergonic reaction (at constant pressure and temperature) is denoted as

Although exergonic reactions are said to occur spontaneously, this does not imply that the reaction will take place at an observable rate. For instance, the disproportionation of hydrogen peroxide releases free energy but is very slow in the absence of a suitable catalyst. It has been suggested that eager would be a more intuitive term in this context. [2]

More generally, the terms exergonic and endergonic relate to the free energy change in any process, not just chemical reactions. By contrast, the terms exothermic and endothermic relate to an enthalpy change in a closed system during a process, usually associated with the exchange of heat.

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<span class="mw-page-title-main">Exothermic process</span> Thermodynamic process that releases energy to its surroundings

In thermodynamics, an exothermic process is a thermodynamic process or reaction that releases energy from the system to its surroundings, usually in the form of heat, but also in a form of light, electricity, or sound. The term exothermic was first coined by 19th-century French chemist Marcellin Berthelot.

<span class="mw-page-title-main">Enthalpy</span> Measure of energy in a thermodynamic system

Enthalpy is the sum of a thermodynamic system's internal energy and the product of its pressure and volume. It is a state function in thermodynamics used in many measurements in chemical, biological, and physical systems at a constant external pressure, which is conveniently provided by the large ambient atmosphere. The pressure–volume term expresses the work that was done against constant external pressure to establish the system's physical dimensions from to some final volume , i.e. to make room for it by displacing its surroundings. The pressure-volume term is very small for solids and liquids at common conditions, and fairly small for gases. Therefore, enthalpy is a stand-in for energy in chemical systems; bond, lattice, solvation, and other chemical "energies" are actually enthalpy differences. As a state function, enthalpy depends only on the final configuration of internal energy, pressure, and volume, not on the path taken to achieve it.

An endothermic process is a chemical or physical process that absorbs heat from its surroundings. In terms of thermodynamics and thermochemistry, it is a thermodynamic process with an increase in the enthalpy H of the system. In an endothermic process, the heat that a system absorbs is thermal energy transfer into the system. Thus, an endothermic reaction generally leads to an increase in the temperature of the system and a decrease in that of the surroundings.

<span class="mw-page-title-main">Thermodynamics</span> Physics of heat, work, and temperature

Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of thermodynamics, which convey a quantitative description using measurable macroscopic physical quantities, but may be explained in terms of microscopic constituents by statistical mechanics. Thermodynamics applies to a wide variety of topics in science and engineering, especially physical chemistry, biochemistry, chemical engineering and mechanical engineering, but also in other complex fields such as meteorology.

<span class="mw-page-title-main">Thermochemistry</span> Study of the heat energy associated with chemical reactions and/or physical transformations

Thermochemistry is the study of the heat energy which is associated with chemical reactions and/or phase changes such as melting and boiling. A reaction may release or absorb energy, and a phase change may do the same. Thermochemistry focuses on the energy exchange between a system and its surroundings in the form of heat. Thermochemistry is useful in predicting reactant and product quantities throughout the course of a given reaction. In combination with entropy determinations, it is also used to predict whether a reaction is spontaneous or non-spontaneous, favorable or unfavorable.

<span class="mw-page-title-main">Thermodynamic free energy</span> State function whose change relates to the systems maximal work output

In thermodynamics, the thermodynamic free energy is one of the state functions of a thermodynamic system. The change in the free energy is the maximum amount of work that the system can perform in a process at constant temperature, and its sign indicates whether the process is thermodynamically favorable or forbidden. Since free energy usually contains potential energy, it is not absolute but depends on the choice of a zero point. Therefore, only relative free energy values, or changes in free energy, are physically meaningful.

In chemistry, the standard molar entropy is the entropy content of one mole of pure substance at a standard state of pressure and any temperature of interest. These are often chosen to be the standard temperature and pressure.

In thermodynamics, the chemical potential of a species is the energy that can be absorbed or released due to a change of the particle number of the given species, e.g. in a chemical reaction or phase transition. The chemical potential of a species in a mixture is defined as the rate of change of free energy of a thermodynamic system with respect to the change in the number of atoms or molecules of the species that are added to the system. Thus, it is the partial derivative of the free energy with respect to the amount of the species, all other species' concentrations in the mixture remaining constant. When both temperature and pressure are held constant, and the number of particles is expressed in moles, the chemical potential is the partial molar Gibbs free energy. At chemical equilibrium or in phase equilibrium, the total sum of the product of chemical potentials and stoichiometric coefficients is zero, as the free energy is at a minimum. In a system in diffusion equilibrium, the chemical potential of any chemical species is uniformly the same everywhere throughout the system.

<span class="mw-page-title-main">Gibbs free energy</span> Type of thermodynamic potential

In thermodynamics, the Gibbs free energy is a thermodynamic potential that can be used to calculate the maximum amount of work, other than pressure-volume work, that may be performed by a thermodynamically closed system at constant temperature and pressure. It also provides a necessary condition for processes such as chemical reactions that may occur under these conditions. The Gibbs free energy is expressed asWhere:

<span class="mw-page-title-main">Hess's law</span> Observation that total enthalpy change of a chemical reaction is independent of the steps taken

Hess’ law of constant heat summation, also known simply as Hess' law, is a relationship in physical chemistry named after Germain Hess, a Swiss-born Russian chemist and physician who published it in 1840. The law states that the total enthalpy change during the complete course of a chemical reaction is independent of the sequence of steps taken.

<span class="mw-page-title-main">Thermodynamic potential</span> Scalar physical quantities representing system states

A thermodynamic potential is a scalar quantity used to represent the thermodynamic state of a system. Just as in mechanics, where potential energy is defined as capacity to do work, similarly different potentials have different meanings. The concept of thermodynamic potentials was introduced by Pierre Duhem in 1886. Josiah Willard Gibbs in his papers used the term fundamental functions. While thermodynamic potentials cannot be measured directly, they can be predicted using computational chemistry.

In thermodynamics, a spontaneous process is a process which occurs without any external input to the system. A more technical definition is the time-evolution of a system in which it releases free energy and it moves to a lower, more thermodynamically stable energy state. The sign convention for free energy change follows the general convention for thermodynamic measurements, in which a release of free energy from the system corresponds to a negative change in the free energy of the system and a positive change in the free energy of the surroundings.

<span class="mw-page-title-main">Internal energy</span> Energy contained within a system

The internal energy of a thermodynamic system is the energy of the system as a state function, measured as the quantity of energy necessary to bring the system from its standard internal state to its present internal state of interest, accounting for the gains and losses of energy due to changes in its internal state, including such quantities as magnetization. It excludes the kinetic energy of motion of the system as a whole and the potential energy of position of the system as a whole, with respect to its surroundings and external force fields. It includes the thermal energy, i.e., the constituent particles' kinetic energies of motion relative to the motion of the system as a whole. The internal energy of an isolated system cannot change, as expressed in the law of conservation of energy, a foundation of the first law of thermodynamics. The notion has been introduced to describe the systems characterized by temperature variations, temperature being added to the set of state parameters, the position variables known in mechanics, in a similar way to potential energy of the conservative fields of force, gravitational and electrostatic. Internal energy changes equal the algebraic sum of the heat transferred and the work done. In systems without temperature changes, potential energy changes equal the work done by/on the system.

<span class="mw-page-title-main">Isothermal process</span> Thermodynamic process in which temperature remains constant

An isothermal process is a type of thermodynamic process in which the temperature T of a system remains constant: ΔT = 0. This typically occurs when a system is in contact with an outside thermal reservoir, and a change in the system occurs slowly enough to allow the system to be continuously adjusted to the temperature of the reservoir through heat exchange (see quasi-equilibrium). In contrast, an adiabatic process is where a system exchanges no heat with its surroundings (Q = 0).

Chemical energy is the energy of chemical substances that is released when the substances undergo a chemical reaction and transform into other substances. Some examples of storage media of chemical energy include batteries, food, and gasoline. Breaking and re-making chemical bonds involves energy, which may be either absorbed by or evolved from a chemical system. If reactants with relatively weak electron-pair bonds convert to more strongly bonded products, energy is released. Therefore, relatively weakly bonded and unstable molecules store chemical energy.

<span class="mw-page-title-main">Thermodynamic system</span> Body of matter in a state of internal equilibrium

A thermodynamic system is a body of matter and/or radiation separate from its surroundings that can be studied using the laws of thermodynamics.

<span class="mw-page-title-main">Endergonic reaction</span> Chemical reaction which requires more energy to initiate than it produces

In chemical thermodynamics, an endergonic reaction is a chemical reaction 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 so the total energy is a net negative result, as opposed to a net positive result in an exergonic reaction. Another way to phrase this is that useful energy must be absorbed from the surroundings into the workable system for the reaction to happen.

<span class="mw-page-title-main">Exothermic reaction</span> Chemical reaction that releases energy as light or heat

In thermochemistry, an exothermic reaction is a "reaction for which the overall standard enthalpy change ΔH⚬ is negative." Exothermic reactions usually release heat. The term is often confused with exergonic reaction, which IUPAC defines as "... a reaction for which the overall standard Gibbs energy change ΔG⚬ is negative." A strongly exothermic reaction will usually also be exergonic because ΔH⚬ makes a major contribution to ΔG. Most of the spectacular chemical reactions that are demonstrated in classrooms are exothermic and exergonic. The opposite is an endothermic reaction, which usually takes up heat and is driven by an entropy increase in the system.

An exergonic process is one which there is a positive flow of energy from the system to the surroundings. This is in contrast with an endergonic process. Constant pressure, constant temperature reactions are exergonic if and only if the Gibbs free energy change is negative (∆G < 0). "Exergonic" means "releasing energy in the form of work". In thermodynamics, work is defined as the energy moving from the system to the surroundings during a given process.

References

  1. IUPAC Gold Book definition: exergonic reaction (exoergic reaction)
  2. Hamori, Eugene; James E. Muldrey (1984). "Use of the word "eager" instead of "spontaneous" for the description of exergonic reactions". Journal of Chemical Education. 61 (8): 710. Bibcode:1984JChEd..61..710H. doi:10.1021/ed061p710.