An
exothermic reaction is a
chemical reactionA chemical reaction is a process that leads to the transformation of one set of chemical substances to another. They are studied by chemists under a field of science called chemistry. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, often coming about...
that releases energy in the form of
heatIn physics and thermodynamics, heat is the process of energy transfer from one body or system due to thermal contact, which in turn is defined as an energy transfer to a body in any other way than due to work performed on the body....
. It is the opposite of an endothermic reaction. Expressed in a
chemical equationA chemical equation may be described as a chemical reaction or a means of writing out and describing such a phenomenon. The coefficients next to the symbols and formulae of entities are the absolute values of the stoichiometric numbers...
:
- reactants → products + energy
An exothermic reaction is a chemical reaction that is accompanied by the release of heat. In other words, the energy needed for the reaction to occur is less than the total energy released.
An
exothermic reaction is a
chemical reactionA chemical reaction is a process that leads to the transformation of one set of chemical substances to another. They are studied by chemists under a field of science called chemistry. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, often coming about...
that releases energy in the form of
heatIn physics and thermodynamics, heat is the process of energy transfer from one body or system due to thermal contact, which in turn is defined as an energy transfer to a body in any other way than due to work performed on the body....
. It is the opposite of an endothermic reaction. Expressed in a
chemical equationA chemical equation may be described as a chemical reaction or a means of writing out and describing such a phenomenon. The coefficients next to the symbols and formulae of entities are the absolute values of the stoichiometric numbers...
:
- reactants → products + energy
Overview
An exothermic reaction is a chemical reaction that is accompanied by the release of heat. In other words, the energy needed for the reaction to occur is less than the total energy released. As a result of this, the extra energy is released, usually in the form of heat.
When using a
calorimeterA calorimeter is a device used for calorimetry, the science of measuring the heat of chemical reactions or physical changes as well as heat capacity. The word calorimeter is derived from the Latin word calor, meaning heat. Differential scanning calorimeters, isothermal microcalorimeters, titration...
, the change in heat of the calorimeter is equal to the opposite of the change in heat of the system. This means that when the medium in which the reaction is taking place gains heat, the reaction is exothermic.
The absolute amount of energy in a chemical system is extremely difficult to measure or calculate. The
enthalpyIn thermodynamics and molecular chemistry, the enthalpy is a thermodynamic property of a thermodynamic system. It can be used to calculate the heat transfer during a quasistatic process taking place in a closed thermodynamic system under constant pressure...
change, ΔH, of a chemical reaction is much easier to measure and calculate. A bomb calorimeter is very suitable for measuring the energy change, ΔH, of a
combustionCombustion or burning is a complex sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat or both heat and light in the form of either a glow or flames, appearance of light flickering.Direct combustion by atmospheric oxygen is a reaction...
reaction. Measured and calculated ΔH values are related to bond energies by:
- ΔH = energy used in bond breaking reactions − energy released in bond making products
by definition the enthalpy change has a negative value:
- ΔH < 0
For an exothermic reaction, this gives a negative value for ΔH, 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 + O2 → 2H2O
- ΔH = −483.6 kJ/mol of O2
Examples of exothermic reactions
- Combustion
Combustion or burning is a complex sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat or both heat and light in the form of either a glow or flames, appearance of light flickering.Direct combustion by atmospheric oxygen is a reaction...
reactions of fuelFuel is any material that is burned or altered to obtain energy and to heat or to move object. Fuel releases its energy either through a chemical reaction means, such as combustion, or nuclear means, such as nuclear fission or nuclear fusion...
s
- Neutralization reactions such as direct reaction of acid and base
- Adding concentrated acid to water
- Adding water to anhydrous
As a general term, a substance is said to be anhydrous if it contains no water. The way of achieving the anhydrous form differs from one substance to another.-Solvents:...
copper(II) sulfateCopper sulfate is the chemical compound with the formula CuSO
4. This salt exists as a series of compounds that differ in their degree of hydration. The anhydrous form is a pale green or gray-white powder, whereas the pentahydrate, the most commonly encountered salt, is bright blue. The...
- The thermite
Thermite is a pyrotechnic composition of a metal powder and a metal oxide, which produces an aluminothermic reaction known as a thermite reaction. It is not explosive, but can create short bursts of extremely high temperatures focused on a very small area for a short period of time.Thermites can...
reaction
- Reactions taking place in a 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. The inner chamber holds the food or drink, and the outer chamber houses chemicals that undergo an exothermic reaction when combined...
based on limeLime is a general term for calcium-containing inorganic materials, in which carbonates, oxides and hydroxides predominate. Strictly speaking, lime is calcium oxide or calcium hydroxide. It is also the name for a single mineral of the CaO composition, occurring very rarely...
and aluminum
- The setting of cement
In the most general sense of the word, a cement is a binder, a substance which sets and hardens independently, and can bind other materials together. The word "cement" traces to the Romans, who used the term "opus caementicium" to describe masonry which resembled concrete and was made from crushed...
and concreteConcrete is a construction material composed of cement as well as other cementitious materials such as fly ash and slag cement, aggregate , water, and chemical admixtures...
- Many corrosion
Corrosion is the disintegration of a material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means a loss of electrons of metals reacting with water and oxygen. Weakening of iron due to oxidation of the iron atoms is a...
reactions such as oxidation of metals
- Most polymerisation reactions
- The Haber-Bosch process
The Haber process, also called the Haber–Bosch process, is the nitrogen fixation reaction of nitrogen gas and hydrogen gas, over an enriched iron catalyst, to produce ammonia. The Haber process is important because ammonia is difficult to produce on an industrial scale, and the fertilizer...
of ammonia productionBecause of its many uses, ammonia is one of the most highly-produced inorganic chemicals. There are numerous large-scale ammonia production plants worldwide, producing a total of 109,000,000 metric tons of ammonia in 2004. China produced 28.4% of the worldwide production, followed by India with...
Key points
- The concept and its opposite number endothermic
In thermodynamics, the word endothermic "within-heating" describes a process or reaction that absorbs energy in the form of heat. Its etymology stems from the Greek prefix endo-, meaning “inside” and the Greek suffix –thermic, meaning “to heat”. The opposite of an endothermic process is an...
relate to the enthalpy change in any process, not just chemical reactions.
- In endergonic reaction
In chemical thermodynamics, an endergonic reaction is a chemical reaction in which the standard change in free energy is positive, and energy is absorbed...
s and exergonic reactionAn exergonic reaction is a chemical reaction where the change in the Gibbs free energy is negative, indicating a spontaneous reaction. Symbolically, the release of Gibbs free energy, G, in an exergonic reaction is denoted as...
s it is the sign of the Gibbs free energyIn thermodynamics, the Gibbs free energy is a thermodynamic potential that measures the "useful" or process-initiating work obtainable from an isothermal, isobaric thermodynamic system...
that determines the equilibrium point, and not enthalpyIn thermodynamics and molecular chemistry, the enthalpy is a thermodynamic property of a thermodynamic system. It can be used to calculate the heat transfer during a quasistatic process taking place in a closed thermodynamic system under constant pressure...
. The related concepts endergonicEndergonic means absorbing energy in the form of work. Its etymology stems from the suffix -ergonic, as derived from the Greek root ergon, meaning work, combined with the prefix end-, as derived from the Greek root en, meaning put into. Endergonic reactions are not spontaneous...
and exergonicExergonic means to release energy in the form of work. Its etymology stems from the suffix -ergonic, as derived from the Greek root ergon, meaning work, combined with the Greek prefix ex-, meaning out of. By thermodynamic standards, work, a form of energy, is defined as moving from the system to...
apply to all physical processes.
- the conceptually related endotherm
In biology, a warm-blooded animal species is one whose members maintain thermal homeostasis; that is, they keep their body temperature at a roughly constant level, regardless of the ambient temperature. This involves the ability to cool down or produce more body heat. Warm-blooded animals mainly...
and exothermIn biology, a warm-blooded animal species is one whose members maintain thermal homeostasis; that is, they keep their body temperature at a roughly constant level, regardless of the ambient temperature. This involves the ability to cool down or produce more body heat. Warm-blooded animals mainly...
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
Thermal runaway refers to a situation where an increase in temperature changes the conditions in a way that causes a further increase in temperature leading to a destructive result. It is a kind of positive feedback.-Chemical engineering:...
. An ExplosionAn explosion is a rapid increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases. An explosion creates a shock wave. If the shock wave is a supersonic detonation, then the source of the blast is called a "high explosive"...
can also result from the problem.
Measurement
Heat production or absorption in either a physical process or chemical reaction is measured using
calorimetryCalorimetry is the science of measuring the heat of chemical...
. One common laboratory instrument is the Differential scanning calorimeter or DSC, where a small sample is slowly heated in a controlled way and the heat flow into or from the sample chamber is monitored. The technique can be used to follow chemical reactions as well as physical processes, such as melting and boiling (both of which are
endothermicIn thermodynamics, the word endothermic "within-heating" describes a process or reaction that absorbs energy in the form of heat. Its etymology stems from the Greek prefix endo-, meaning “inside” and the Greek suffix –thermic, meaning “to heat”. The opposite of an endothermic process is an...
). Crystallization and condensation, the reverse processes, are both exothermic.
See also
- 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...
- Differential scanning calorimetry
Differential scanning calorimetry or DSC is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference are measured as a function of temperature. Both the sample and reference are maintained at nearly the same...
- Endergonic
Endergonic means absorbing energy in the form of work. Its etymology stems from the suffix -ergonic, as derived from the Greek root ergon, meaning work, combined with the prefix end-, as derived from the Greek root en, meaning put into. Endergonic reactions are not spontaneous...
- Exergonic
Exergonic means to release energy in the form of work. Its etymology stems from the suffix -ergonic, as derived from the Greek root ergon, meaning work, combined with the Greek prefix ex-, meaning out of. By thermodynamic standards, work, a form of energy, is defined as moving from the system to...
- Endergonic reaction
In chemical thermodynamics, an endergonic reaction is a chemical reaction in which the standard change in free energy is positive, and energy is absorbed...
- Exergonic reaction
An exergonic reaction is a chemical reaction where the change in the Gibbs free energy is negative, indicating a spontaneous reaction. Symbolically, the release of Gibbs free energy, G, in an exergonic reaction is denoted as...
- Exothermic
In thermodynamics, the term exothermic describes a process or reaction that releases energy usually in the form of heat, but also in the form of light , electricity , or sound. Its etymology stems from the Greek prefix ex- and the Greek word thermein...
- Endothermic
In thermodynamics, the word endothermic "within-heating" describes a process or reaction that absorbs energy in the form of heat. Its etymology stems from the Greek prefix endo-, meaning “inside” and the Greek suffix –thermic, meaning “to heat”. The opposite of an endothermic process is an...
reaction
- Endotherm
In biology, a warm-blooded animal species is one whose members maintain thermal homeostasis; that is, they keep their body temperature at a roughly constant level, regardless of the ambient temperature. This involves the ability to cool down or produce more body heat. Warm-blooded animals mainly...
- Exotherm
Ectothermic, from the Greek ectos, "outside" and therme, "heat," refers to organisms that control body temperature through external means. As a result, organisms are dependent on environmental heat sources and have relatively low metabolic rates. For example, many reptiles regulate their body...