Exothermic reaction

An exothermic reaction is a chemical reaction that releases energy through light or heat. It is the opposite of an endothermic reaction.[1]

Expressed in a chemical equation: reactants → products + energy. Exothermic Reaction means "exo" (derived from the greek word: "έξω", literally translated to "out") meaning releases and "thermic" means heat. So the reaction in which there is release of heat with or without light is called exothermic reaction.

An exothermic thermite reaction using iron(III) oxide. The sparks flying outwards are globules of molten iron trailing smoke in their wake.


An exothermic reaction is a chemical reaction that releases heat. It gives net energy to its surroundings. That is, the energy needed to initiate the reaction is less than the energy released.[2]

When the medium in which the reaction is taking place collects heat, the reaction is exothermic. When using a calorimeter, the total amount of heat that flows into (or through) the calorimeter is the negative of the net change in energy of the system.

The absolute amount of energy in a chemical system is difficult to measure or calculate. The enthalpy change, ΔH, of a chemical reaction is much easier to work with. The enthalpy change equals the change in internal energy of the system plus the work needed to change the volume of the system against constant ambient pressure. A bomb calorimeter is very suitable for measuring the energy change, ΔH, of a combustion reaction. Measured and calculated ΔH values are related to bond energies by:

ΔH = (energy used in forming product bonds) − (energy released in breaking reactant bonds)
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An energy profile of an exothermic reaction

In an exothermic reaction, by definition, the enthalpy change has a negative value:

ΔH < 0

since a larger value (the energy released in the reaction) is subtracted from a smaller value (the energy used for the reaction). For example, when hydrogen burns:

2H2 (g) + O2 (g) → 2H2O (g)
ΔH = −483.6 kJ/mol of O2 [3]

In an adiabatic system, the temperature raise due to enthalpy change can be expressed as

−ΔH298.15 K = T1
Cp, pdT + T0
298 K
(Cp, pCp, r)dT

where ΔH298.15 K is the standard enthalpy of reaction at 298 K, T0 and T1 are the initial and final temperature of the system, respectively, and Cp,p and Cp,r are the heat capacities of the product and reactant, respectively.

Assuming the heat capacity of the system remains as a constant value Cp,p=Cp,r=Cp, the change of temperature ΔT=T1T0 can be expressed as

−ΔH298.15 K = T0T
Cp, pdT = ΔTCp, p

The most commonly available hand warmers make use of the oxidation of iron to achieve an exothermic reaction:

4Fe (s) + 3O2 (g) → 2Fe2O3 (s).

Examples of exothermic reactions

Video of an exothermic reaction. Ethanol vapor is ignited inside a bottle, causing combustion.

Other points to think about

  • The concept and its opposite number endothermic relate to the enthalpy change in any process, not just chemical reactions.
  • In endergonic reactions and exergonic reactions it is the sign of the Gibbs free energy that determines the equilibrium point, and not enthalpy. The related concepts endergonic and exergonic apply to all physical processes.
  • The conceptually related endotherm and ectotherm (or sometimes exotherm) are concepts in animal physiology.
  • In quantum numbers, when any excited energy level goes down to its original level for example: when n=4 fall to n=2, energy is released so, it is exothermic.
  • Where an exothermic reaction causes heating of the reaction vessel which is not controlled, the rate of reaction can increase, in turn causing heat to be evolved even more quickly. This positive feedback situation is known as thermal runaway. An explosion can also result from the problem.


Heat production or absorption in either a physical process or chemical reaction is measured using calorimetry. One common laboratory instrument is the reaction calorimeter, where the heat flow into or from the reaction vessel is monitored. The technique can be used to follow chemical reactions as well as physical processes such as crystallization and dissolution.

Energy released is measured in Joule per mole. The reaction has a negative ΔH(heat change) value due to heat loss. e.g.: -123 J/mol

See also


  1. ^ Article written by Anne Marie Helmenstine, Ph.D on exothermic and endothermic reactions "Archived copy". Archived from the original on 2016-03-18. Retrieved 2016-04-05.CS1 maint: Archived copy as title (link)
  2. ^ "Endothermic and Exothermic Reactions". About Chemistry. 3 February 2013. Archived from the original on 18 March 2016. Retrieved 5 April 2016.
  3. ^ "Archived copy". Archived from the original on 2013-07-08. Retrieved 2013-07-20.CS1 maint: Archived copy as title (link)
  4. ^ a b Yin, Xi; Wu, Jianbo; Li, Panpan; Shi, Miao; Yang, Hong (January 2016). "Self-Heating Approach to the Fast Production of Uniform Metal Nanostructures". ChemNanoMat. 2 (1): 37–41. doi:10.1002/cnma.201500123.

A bridgewire, sometimes spelled as bridge wire, also known as a hot bridge wire (HBW) is a relatively thin resistance wire used to set off a pyrotechnic composition serving as pyrotechnic initiator. By passing of electric current it is heated to a high temperature that starts the exothermic chemical reaction of the attached composition. After successful firing, the bridgewire melts, resulting in an open circuit.

Usually a thin nichrome wire is used. Some applications also use platinum-silver alloy; other bridgewire materials in use are platinum, gold, silver, tungsten, etc. Care has to be taken when selecting the material as it is in direct contact with the pyrotechnic composition and should not undergo corrosion in such conditions. Another material, able to actively release chemical energy, is Pyrofuze, aluminium wire clad with palladium; when being heated it undergoes strongly exothermic reaction as the molten metals form an alloy. A variant with the same function consists of laminated thin alternate layers of aluminium and nickel. Carbon bridge is a thin spot of colloidal graphite used as the bridgewire. Some variants use a conductive pyrotechnic composition as the resistive material. In amateur rocketry, grossly overloaded low wattage metal film resistors and 0805 Surface-mount technology resistors are also used.A shallow notch cut into the center of the bridgewire promotes gross localized overheating instead of homogeneous heating of the entire bridgewire. This may improve the bridgewire performance in some applications.

Bridgewires are used in diverse applications; to trigger detonators, electric matches, squibs, electric blasting caps, pyrotechnic fasteners, and more. Bridgewires dipped in a suitable pyrotechnic composition (pyrogen) are known as electric matches. Pyrogens with content of magnesium allow reaching very high combustion temperatures.

Devices using bridgewires, whether for initiating an explosion ("electroexplosive") or for nonexplosive purposes, are called bridge wire actuated devices (BWAD).Bridgewires, especially connected to longer cables, may be susceptible to initiation by currents induced by external electromagnetic fields.By passing an extremely high amount of electric current through the bridgewire, it gets rapidly vaporized, causing a small explosion. This is exploited in exploding-bridgewire detonators (EBWs), used for very safe and highly precise initiation of explosives, e.g. in nuclear weapons.

Calcium oxide

Calcium oxide (CaO), commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline, crystalline solid at room temperature. The broadly used term lime connotes calcium-containing inorganic materials, in which carbonates, oxides and hydroxides of calcium, silicon, magnesium, aluminium, and iron predominate. By contrast, quicklime specifically applies to the single chemical compound calcium oxide. Calcium oxide that survives processing without reacting in building products such as cement is called free lime.Quicklime is relatively inexpensive. Both it and a chemical derivative (calcium hydroxide, of which quicklime is the base anhydride) are important commodity chemicals.

Condensed aerosol fire suppression

Condensed aerosol fire suppression is a particle-based form of fire extinction. It is similar to gaseous fire suppression (or dry chemical fire extinction). It employs a fire extinguishing agent consisting of: very fine solid particles as well as gaseous matter. The condensed aerosol microparticles and effluent gases are generated by the exothermic reaction; the particles remain in vapor state until the process of being discharged from the device. Then, it is "condensed" and cooled within the device and discharged as solid particles.

Compared to gaseous suppressants (which emit only gas) and dry chemical suppression agents (which are powder-like particles of a large size - 25–150 micrometres), condensed aerosols are defined as those which release finely-divided solids of less than 10 micrometres in diameter (by the National Fire Protection Association).

The solid particulates have a considerably smaller mass median aerodynamic diameter (MMAD) than those of dry chemical suppression agents. The particulates also remain airborne significantly longer and leave much less residue within the protected area.

Condensed aerosols are flooding agents and therefore effective regardless of the location and height of the fire. This can be contrasted with dry chemical systems, which must be directly aimed at the flame.

The condensed aerosol agent can be delivered by means of mechanical operation, electric operation, or combined electro-mechanical operation.

Wet chemical systems such as the kind generally found in foam extinguishers, must, similarly to dry chemical systems, be sprayed directionally onto the fire.

Electron acceptor

An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process. Electron acceptors are sometimes mistakenly called electron receptors.

Typical oxidizing agents undergo permanent chemical alteration through covalent or ionic reaction chemistry, resulting in the complete and irreversible transfer of one or more electrons. In many chemical circumstances, however, the transfer of electronic charge from an electron donor may be only fractional, meaning an electron is not completely transferred, but results in an electron resonance between the donor and acceptor. This leads to the formation of charge transfer complexes in which the components largely retain their chemical identities.

The electron accepting power of an acceptor molecule is measured by its electron affinity which is the energy released when filling the lowest unoccupied molecular orbital (LUMO).

The energy required to remove one electron from the electron donor is its ionization energy (I). The energy liberated by attachment of an electron to the electron acceptor is the negative of its electron affinity (A). The overall system energy change (ΔE) for the charge transfer is then . For an exothermic reaction, the energy liberated is of interest and is equal to .

In chemistry, a class of electron acceptors that acquire not just one, but a set of two paired electrons that form a covalent bond with an electron donor molecule, is known as a Lewis acid. This phenomenon gives rise to the wide field of Lewis acid-base chemistry. The driving forces for electron donor and acceptor behavior in chemistry is based on the concepts of electropositivity (for donors) and electronegativity (for acceptors) of atomic or molecular entities.

Exergonic process

An exergonic process is one in 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" (from the prefix exo-, derived for the Greek word ἔξω exō, "outside" and the suffix -ergonic, derived from the Greek word ἔργον ergon, "work") means "releasing energy in the form of work". In thermodynamics, work is defined as the energy moving from the system (the internal region) to the surroundings (the external region) during a given process.

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 also exergonic when spontaneous. Thus in this type of reactions the Gibbs free energy decreases. The entropy is included in any change of the Gibbs free energy. This differs from a exothermic reaction or a endothermic reaction where the entropy is not included. The Gibbs free energy is calculated with the Gibbs–Helmholtz equation:


T = temperature in kelvins (K)
ΔG = change in the Gibbs free energy
ΔS = change in entropy (at 298 K) as ΔS = Σ{S(Product)} − Σ{S(Reagent)}
ΔH = change in enthalpy (at 298 K) as ΔH = Σ{H(Product)} − Σ{H(Reagent)}

A chemical reaction progresses only spontaneously when the Gibbs free energy decreases, in that case the ΔG is negative. In exergonic reactions the ΔG is negative and in endergonic reactions the ΔG is positive:



equals the change in the Gibbs free energy after completion of a chemical reaction.
Exothermic process

In thermodynamics, the term exothermic process (exo- : "outside") describes a 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 (e.g. a spark, flame, or flash), electricity (e.g. a battery), or sound (e.g. explosion heard when burning hydrogen). Its etymology stems from the Greek prefix έξω (exō, which means "outwards") and the Greek word θερμικός (thermikόs, which means "thermal"). The term exothermic was first coined by Marcellin Berthelot. The opposite of an exothermic process is an endothermic process, one that absorbs energy in the form of heat.

The concept is frequently applied in the physical sciences to chemical reactions, where as in chemical bond energy that will be converted to thermal energy (heat).

Exothermic (and endothermic) describe two types of chemical reactions or systems found in nature, as follows.

Simply stated, after an exothermic reaction, more energy has been released to the surroundings than was absorbed to initiate and maintain the reaction. An example would be the burning of a candle, wherein the sum of calories produced by combustion (found by looking at radiant heating of the surroundings and visible light produced, including increase in temperature of the fuel (wax) itself, which with oxygen, have become hot CO2 and water vapor,) exceeds the number of calories absorbed initially in lighting the flame and in the flame maintaining itself. (i.e. some energy produced by combustion is reabsorbed and used in melting, then vaporizing the wax, etc. but is (far) outstripped by the energy produced in breaking carbon-hydrogen bonds and combination of oxygen with the resulting carbon and hydrogen).

On the other hand, in an endothermic reaction or system, energy is taken from the surroundings in the course of the reaction. An example of an endothermic reaction is a first aid cold pack, in which the reaction of two chemicals, or dissolving of one in another, requires calories from the surroundings, and the reaction cools the pouch and surroundings by absorbing heat from them. An endothermic system is seen in the production of wood: trees absorb radiant energy, from the sun, use it in endothermic reactions such as taking apart CO2 and H2O and combining the carbon and hydrogen generated to produce cellulose and other organic chemicals. These products, in the form of wood, say, may later be burned in a fireplace, exothermically, producing CO2 and water, and releasing energy in the form of heat and light to their surroundings, e.g., to a home's interior and chimney gasses.


A flame (from Latin flamma) is the visible, gaseous part of a fire. It is caused by a highly exothermic reaction taking place in a thin zone. Very hot flames are hot enough to have ionized gaseous components of sufficient density to be considered plasma.

Gravity dam

A gravity dam is a dam constructed from concrete or stone masonry and designed to hold back water by primarily using the weight of the material alone to resist the horizontal pressure of water pushing against it. Gravity dams are designed so that each section of the dam is stable, independent of any other dam section.Gravity dams generally require stiff rock foundations of high bearing strength (slightly weathered to fresh); although they have been built on soil foundations in rare cases. The bearing strength of the foundation limits the allowable position of the resultant which influences the overall stability. Also, the stiff nature of the gravity dam structure is unforgiving to differential foundation settlement; which can induce cracking of the dam structure.

Gravity dams provide some advantages over embankment dams. The main advantage being that they can tolerate minor over-topping flows as the concrete is resistant to scouring. Large over-topping flows are still a problem, as they can scour the foundations if not accounted for in the design. A disadvantage of gravity dams is that due to their large footprint, they are susceptible to uplift pressures which act as a de-stabilising force. Uplift pressures (buoyancy) can be reduced by internal and foundation drainage systems which reduces the pressures.

During construction, the setting concrete produces a exothermic reaction. This heat expands the plastic concrete and can take up to several decades to cool. When cooling, the concrete is in a stiff state and is susceptible to cracking. It is the designer's task to ensure this does not occur.

Iodine pentafluoride

Iodine pentafluoride is an interhalogen compound with chemical formula IF5. It is a fluoride of iodine. It is a colorless or yellow liquid with a density of 3.250 g cm−3. It was first synthesized by Henri Moissan in 1891 by burning solid iodine in fluorine gas. This exothermic reaction is still used to produce iodine pentafluoride, although the reaction conditions have been improved.

I2 + 5 F2 → 2 IF5

Isodesmic reaction

An isodesmic reaction is a chemical reaction in which the type of chemical bonds broken in the reactant are the same as the type of bonds formed in the reaction product. This type of reaction is often used as a hypothetical reaction in thermochemistry.

An example of an isodesmic reaction is

CH3− + CH3X → CH4 + CH2X− (1)X = F, Cl, Br, IEquation 1 describes the deprotonation of a methyl halide by a methyl anion. The energy change associated with this exothermic reaction which can be calculated in silico increases going from fluorine to chlorine to bromine and iodine making the CH2I− anion the most stable and least basic of all the halides. Although this reaction is isodesmic the energy change in this example also depends on the difference in bond energy of the C-X bond in the base and conjugate acid. In other cases, the difference may be due to steric strain. This difference is small in fluorine but large in iodine (in favor of the anion) and therefore the energy trend is as described despite the fact that C-F bonds are stronger than C-I bonds.The related term homodesmotic reaction also takes into account orbital hybridization and in addition there is no change in the number of carbon to hydrogen bonds.

M-SG reducing agent

In M-SG an alkali metal is absorbed into silica gel at elevated temperatures. The resulting black powder material is an effective reducing agent and safe to handle as opposed to the pure metal. The material can also be used as a desiccant and as a hydrogen source.The metal is either sodium or a sodium - potassium alloy (Na2K). The molten metal is mixed with silica gel under constant agitation at room temperature. This phase 0 material must be handled in an inert atmosphere. Heating phase 0 at 150 °C (302 °F) takes it to phase I. When this material is exposed to dry oxygen the reducing power is not affected. At further heating to 400 °C (752 °F) phase II can be handled safely in an ambient environment.

The metal reacts with the silica gel in an exothermic reaction in which Na4Si4 nanoparticles are formed. The powder reacts with water to form hydrogen.

Compounds such as biphenyl and naphthalene are reduced by the powder and form highly coloured radical anions. The powder can also be introduced in a column chromatography setup and eluted with organic reactants in order to probe the reducing power. The powder is mixed with additional (wet) silica gel which provides additional hydrogen. A Birch reduction of naphthalene takes 5 minutes elution time. The column converts benzyl chloride to bibenzyl in a Wurtz coupling and in a similar fashion dibenzothiophene is reduced to biphenyl.


N-Methylhydroxylamine or methylhydroxylamine is a hydroxylamine derivative with a methyl group replacing one of the hydrogens of the amino group. It is an isomer of methoxyamine and aminomethanol. It decomposes in an exothermic reaction (-63 kJ/mol) into methane and azanone unless stored as a hydrochloride salt.

The compound is commercially available as its hydrochloride salt. This can be produced by electrochemical reduction of nitromethane in hydrochloric acid using a copper anode and a graphite cathode.

Plasma recombination

Plasma recombination is a process by which positive ions of a plasma capture a free (energetic) electron and combine with electrons or negative ions to form new neutral atoms (gas). Recombination is an exothermic reaction, meaning heat releasing.

Recombination usually takes place in the whole volume of a plasma (volume recombination), although in some cases it is confined to some special region of it. Each kind of reaction is called a recombining mode and their individual rates are strongly affected by the properties of the plasma such as its energy (heat), density of each species, pressure and temperature of the surrounding environment. An everyday example of rapid plasma recombination occurs when a fluorescent lamp is switched off. The low-density plasma in the lamp (which generates the light by bombardment of the fluorescent coating on the inside of the glass wall) recombines in a fraction of a second after the plasma-generating electric field is removed by switching off the electric power source.

Hydrogen recombination modes are of vital importance in the development of divertor regions for tokamak reactors. In fact they will provide a good way for extracting the energy produced in the core of the plasma. At the present time, it is believed that the most likely plasma losses observed in the recombining region are due to two different modes: electron ion recombination (EIR) and molecular activated recombination (MAR).

Potassium hexafluorophosphate

Potassium hexafluorophosphate is the chemical compound with the formula KPF6. This colourless salt consists of potassium cations and hexafluorophosphate anions. It is prepared by the reaction:

PCl5 + KCl + 6 HF → KPF6 + 6 HClThis exothermic reaction is conducted in liquid hydrogen fluoride. The salt is stable in hot alkaline aqueous solution, from which it can be recrystallized. The sodium and ammonium salts are more soluble in water whereas the rubidium and caesium salts are less so.

KPF6 is a common laboratory source of the hexafluorophosphate anion, a non-coordinating anion that confers lipophilicity to its salts. These salts are often less soluble than the closely related tetrafluoroborates.

Pouillet effect

In physics, the term Pouillet effect refers to an exothermic reaction that takes place when a liquid is added to a powder. It was first observed by Leslie in 1802 when dry sawdust was wetted with water. Claude Pouillet later described this phenomenon in 1822 when it came to be known as the Pouillet effect in France.


Pyrometallurgy is a branch of extractive metallurgy. It consists of the thermal treatment of minerals and metallurgical ores and concentrates to bring about physical and chemical transformations in the materials to enable recovery of valuable metals. Pyrometallurgical treatment may produce products able to be sold such as pure metals, or intermediate compounds or alloys, suitable as feed for further processing. Examples of elements extracted by pyrometallurgical processes include the oxides of less reactive elements like iron, copper, zinc, chromium, tin, and manganese.Pyrometallurgical processes are generally grouped into one or more of the following categories:




refining.Most pyrometallurgical processes require energy input to sustain the temperature at which the process takes place. The energy is usually provided in the form of combustion or from electrical heat. When sufficient material is present in the feed to sustain the process temperature solely by exothermic reaction (i.e. without the addition of fuel or electrical heat), the process is said to be "autogenous". Processing of some sulfide ores exploit the exothermicity of their combustion

Self-heating can

A self-heating can is an enhancement of the common food can. Self-heating cans have dual chambers, one surrounding the other, making a self-heating food package.

In one version, the inner chamber holds the food or drink, and the outer chamber houses chemicals which undergo an exothermic reaction when combined. When the user wants to heat the contents of the can, a ring on the can - when pulled - breaks the barrier which keeps the chemicals in the outer chamber apart from the water. In another type, the chemicals are in the inner chamber and the beverage surrounds it in the outer chamber. To heat the contents of the can, the user pushes on the bottom of the can to break the barrier separating the chemical from the water. This design has the advantages of being more efficient (less heat is lost to the surrounding air) as well as reducing excessive heating of the product's exterior, causing possible discomfort to the user. In either case, after the heat from the reaction has been absorbed by the food, the user can enjoy a hot meal or drink.

Self-heating cans offer benefits to campers and people without access to oven, stove or camp-fire, but their use is not widespread. This is because self-heating cans are considerably more expensive than the conventional type, take more space, and have problems with uneven heating of their contents.

Sodium silicide

Sodium silicide (NaSi, Na4Si4) is a binary inorganic compound consisting of sodium and silicon. It is a solid black or grey crystalline material.Sodium silicide reacts readily with water yielding gaseous hydrogen and aqueous sodium silicate in an exothermic reaction (~175 kJ·mol−1):

2 NaSi + 5 H2O → 5 H2 + Na2Si2O5This is used in hydrogen technologies to generate hydrogen as a fuel.

Urea nitrate

Urea nitrate is a fertilizer-based high explosive that has been used in improvised explosive devices in Afghanistan, Pakistan, Iraq, and various other terrorist acts elsewhere in the world, like the 1993 World Trade Center bombings. It has a destructive power similar to better-known ammonium nitrate explosives, with a velocity of detonation between 11,155 ft/s (3,400 m/s) and 15,420 ft/s (4,700 m/s).Urea nitrate is produced in one step by reaction of urea with nitric acid. This is an exothermic reaction, so steps must be taken to control the temperature.

Urea nitrate explosions may be initiated using a blasting cap.

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