|
Search over 100 encyclopedias and dictionaries: |
Research categories | Follow us on Twitter |
Research categories
View all topics in the newsView all reference sources at Encyclopedia.com |
|||
enthalpy
enthalpy , measure of the heat content of a chemical or physical system; it is a quantity derived from the heat and work relations studied in thermodynamics . As a system changes from one state to another the enthalpy change, Δ H, is equal to the enthalpy of the products minus the enthalpy of the reactants. If heat is given off during a transformation from one state to another, then the final state will have a lower heat content than the initial state, the enthalpy change Δ H will be negative, and the process is said to be exothermic. If heat is absorbed during the transformation, then the final state will have a higher heat content, Δ H will be positive, and the process is said to be endothermic. The enthalpy change accompanying a chemical reaction is called the heat of the reaction. For a reaction in which a compound is formed from its composite elements, the enthalpy increase or decrease is called the heat of formation of the compound. Changes of state, or phase, of matter are also accompanied by enthalpy changes; the change associated with the solid-liquid transition is called the heat of fusion and the change associated with the liquid-gas transition is called the heat of vaporization (see latent heat ). The enthalpy change for a given reaction often may be used to tell how favorable the reaction is; an exothermic reaction involves a loss of heat and a consequent lower final energy and thus tends to be favorable, while an endothermic reaction tends to be unfavorable because it involves an increase in energy. However, there are other factors, such as entropy changes, which must also be taken into account in determining whether or not a given process can occur. |
|
|
Cite this article
"enthalpy." The Columbia Encyclopedia, 6th ed.. 2008. Encyclopedia.com. 11 Feb. 2012 <http://www.encyclopedia.com>. "enthalpy." The Columbia Encyclopedia, 6th ed.. 2008. Encyclopedia.com. (February 11, 2012). http://www.encyclopedia.com/doc/1E1-enthalpy.html "enthalpy." The Columbia Encyclopedia, 6th ed.. 2008. Retrieved February 11, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1E1-enthalpy.html |
|
enthalpy
enthalpy Symbol H. A thermodynamic property of a system defined by H = U + pV, where H is the enthalpy, U is the internal energy of the system, p its pressure, and V its volume. In a chemical reaction carried out in the atmosphere the pressure remains constant and the enthalpy of reaction, ΔH, is equal to ΔU + pΔV. For an exothermic reaction ΔH is taken to be negative.
|
|
|
Cite this article
"enthalpy." A Dictionary of Biology. 2004. Encyclopedia.com. 11 Feb. 2012 <http://www.encyclopedia.com>. "enthalpy." A Dictionary of Biology. 2004. Encyclopedia.com. (February 11, 2012). http://www.encyclopedia.com/doc/1O6-enthalpy.html "enthalpy." A Dictionary of Biology. 2004. Retrieved February 11, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O6-enthalpy.html |
|
enthalpy
enthalpy (symbol H) In thermodynamics, amount of heat energy possessed by a substance. The enthalpy of a system equals the sum of its internal energy and the product of pressure and volume. In an endothermic reaction, there is an increase in enthalpy. The reverse occurs in an exothermic reaction.
|
|
|
Cite this article
"enthalpy." World Encyclopedia. 2005. Encyclopedia.com. 11 Feb. 2012 <http://www.encyclopedia.com>. "enthalpy." World Encyclopedia. 2005. Encyclopedia.com. (February 11, 2012). http://www.encyclopedia.com/doc/1O142-enthalpy.html "enthalpy." World Encyclopedia. 2005. Retrieved February 11, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O142-enthalpy.html |
|
enthalpy
enthalpy (H) Heat content per unit mass of a substance measured as the internal energy plus the product of its volume and pressure.
|
|
|
Cite this article
AILSA ALLABY and MICHAEL ALLABY. "enthalpy." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. 11 Feb. 2012 <http://www.encyclopedia.com>. AILSA ALLABY and MICHAEL ALLABY. "enthalpy." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. (February 11, 2012). http://www.encyclopedia.com/doc/1O13-enthalpy.html AILSA ALLABY and MICHAEL ALLABY. "enthalpy." A Dictionary of Earth Sciences. 1999. Retrieved February 11, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O13-enthalpy.html |
|
|
Search over 100 encyclopedias and dictionaries:
HighBeam™ Research, Inc. © Copyright 2012. All rights reserved.
The Encyclopedia.com advertising network includes:
