Exothermic reaction - wikipedia Exothermic reaction This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. ( June 2013 ) 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 energy by light or heat. It is the opposite of an endothermic reaction. Expressed in a chemical equation : reactants → products + energy. Exothermic Reaction means `` exo '' meaning releases and `` thermic '' means heat. So the reaction in which there is release of heat with or without light is called exothermic reaction. Contents 1 Overview 2 Examples of exothermic reactions 3 Other points to think about 4 Measurement 5 See also 6 References 7 External links Overview An exothermic reaction is a chemical or physical 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. When the medium in which the reaction is taking place gains 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 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 : 2H ( g ) + O ( g ) → 2H O ( g ) ΔH = − 483.6 kJ / mol of O In an adiabatic system, the temperature raise due to enthalpy change can be expressed as - ΔH = ∫ C dT+ ∫ ( C - C ) dT where ΔH is the standard enthalpy of reaction at 298K, T and T are the initial and final temperature of the system, respectively, and C and C are the heat capacities of the product and reactant, respectively. Assuming the heat capacity of the system remains as a constant value C = C = C, the change of temperature ΔT = T - T can be expressed as - ΔH = ∫ C dT = ΔTC The most commonly available hand warmers make use of the oxidation of iron to achieve an exothermic reaction : 4Fe ( s ) + 3O ( g ) → 2Fe O ( s ). Examples of exothermic reactions Play media Video of an exothermic reaction. Ethanol vapor is ignited inside a bottle, causing combustion. Combustion reactions of fuels or a substance Neutralization Deposition of dry ice ( carbon dioxide ) from the gaseous state Adding water to anhydrous copper ( II ) sulfate The thermite reaction Reactions taking place in a self - heating can based on lime aluminium Many corrosion reactions such as oxidation of metals Most polymerization reactions The Haber process of ammonia production Respiration Decomposition of vegetable matter into compost Solution of sulfuric acid into water Dehydration of sugars upon contact with sulfuric acid Detonation of nitroglycerin Nuclear fission of uranium - 235 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. Measurement 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 Chemical thermodynamics Differential scanning calorimetry Endergonic Exergonic Endergonic reaction Exergonic reaction Exothermic process Endothermic reaction Endotherm References Jump up ^ 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. Jump up ^ `` Endothermic and Exothermic Reactions ''. About Chemistry. 3 February 2013. Archived from the original on 18 March 2016. Retrieved 5 April 2016. Jump up ^ `` Archived copy ''. Archived from the original on 2013 - 07 - 08. Retrieved 2013 - 07 - 20. ^ Jump up to : 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. External links Chemistry portal Retrieved from `` https://en.wikipedia.org/w/index.php?title=Exothermic_reaction&oldid=813927475 '' Categories : Thermochemistry Hidden categories : Articles needing additional references from June 2013 All articles needing additional references Talk Contents About Wikipedia Български Català Čeština Dansk Deutsch Eesti Ελληνικά Español Esperanto فارسی Français Galego 한국어 हिन्दी Ido Italiano עברית Kreyòl ayisyen Lietuvių Magyar Македонски Nederlands 日本 語 Norsk Norsk nynorsk ਪੰਜਾਬੀ Polski Português Română Русский Seeltersk Simple English Slovenčina Slovenščina Suomi Svenska தமிழ் Українська اردو 中文 Edit links This page was last edited on 5 December 2017, at 23 : 58. Text is available under the Creative Commons Attribution - ShareAlike License ; additional terms may apply. By using this site, you agree to the Terms of Use and Privacy Policy. Wikipedia ® is a registered trademark of the Wikimedia Foundation, Inc., a non-profit organization. About Wikipedia

Question: when is heat released in a chemical reaction
Answer: exothermic reaction