World Library  
Flag as Inappropriate
Email this Article

Exothermic process

Article Id: WHEBN0000010201
Reproduction Date:

Title: Exothermic process  
Author: World Heritage Encyclopedia
Language: English
Subject: Thermodynamic processes, Chemical thermodynamics
Collection: Chemical Thermodynamics, Thermodynamic Processes
Publisher: World Heritage Encyclopedia
Publication
Date:
 

Exothermic process

Explosions are some of the most violent exothermic reactions.

In thermodynamics, the term exothermic process (exo- : "outside") describes a process or reaction that releases energy from the system, 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").[1] 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) are terms usually used to describe two types of chemical reactions or systems found in nature:

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).

Whereas, in an endothermic reaction, or system, energy is taken from the surroundings in the course of the reaction. An example of an endothermic reaction: a first aid cold pack, in which the reaction of two chemicals, or dissolving of one in another, requires calories from the surroundings, as 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.

Contents

  • Overview 1
  • Examples 2
  • Implications for chemical reactions 3
  • Contrast between thermodynamic and biological terminology 4
  • See also 5
  • References 6
  • External links 7

Overview

Exothermic refers to a transformation in which a system releases energy (heat) to the surroundings:

Q < 0

When the transformation occurs at constant pressure:

∆H < 0

and constant volume:

∆U < 0

In an adiabatic system (e.g. a system that does not give off heat to the surroundings), an exothermic process results in an increase in temperature.[2]

In chemical reactions, the heat that is absorbed is in the form of electromagnetic energy. The loss of kinetic energy via reacting electrons causes light to be released. This light is equivalent in energy to the stabilization energy of the energy for the chemical reaction, i.e. the bond energy. This light that is released can be absorbed by other molecules in solution to give rise to molecular vibrations or rotations, which gives rise to the classical understanding of heat. In contrast, when endothermic reactions occur, energy is absorbed to place an electron in a higher energy state, such that the electron can associate with another atom to form another chemical complex. The loss of energy within solution is absorbed by the endothermic reaction and therefore is a loss of heat. This is the physical understanding of exothermic and endothermic reactions within solution. Therefore in an exothermic reaction the energy needed for the reaction to occur is less than the total energy released.

Examples

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

Some examples of exothermic processes are:[3]

Implications for chemical reactions

Chemical exothermic reactions are generally more spontaneous than their counterparts, endothermic reactions. In a thermochemical reaction that is exothermic, the heat may be listed among the products of the reaction.

Contrast between thermodynamic and biological terminology

Note that because of historical accident, students encounter a source of possible confusion between the terminology of physics and biology. Whereas the thermodynamic terms "exothermic" and "endothermic" respectively refer to processes that give out heat energy and processes that absorb heat energy, in biology the sense is effectively inverted. The metabolic terms "ectothermic" and "endothermic" respectively refer to organisms that rely largely on external heat to achieve a full working temperature, and to organisms that produce heat from within as a major factor in controlling their bodily temperature.

See also

References

Notes

  1. ^ "Gate for the Greek language" on-line dictionary
  2. ^ Perrot, Pierre (1998). A to Z of Thermodynamics. Oxford University Press.  
  3. ^ Exothermic - Endothermic examples

External links

  • http://chemistry.about.com/b/a/184556.htm Observe exothermic reactions in a simple experiment
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
 
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
 
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.
 


Copyright © World Library Foundation. All rights reserved. eBooks from Project Gutenberg are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.