Open main menu
Home
Random
Recent changes
Special pages
Community portal
Preferences
About Wikipedia
Disclaimers
Incubator escapee wiki
Search
User menu
Talk
Dark mode
Contributions
Create account
Log in
Editing
Standard enthalpy of formation
(section)
Warning:
You are not logged in. Your IP address will be publicly visible if you make any edits. If you
log in
or
create an account
, your edits will be attributed to your username, along with other benefits.
Anti-spam check. Do
not
fill this in!
{{Short description|Change of enthalpy during the formation of a compound from its elements}} In [[chemistry]] and [[thermodynamics]], the '''standard enthalpy of formation''' or '''standard heat of formation''' of a [[Chemical compound|compound]] is the change of [[enthalpy]] during the formation of 1 [[mole (unit)|mole]] of the substance from its constituent [[Chemical element|elements in their reference state]], with all substances in their [[standard state]]s. The standard [[pressure]] value {{nowrap|1={{math|''p''{{sup|⦡}}}} = 10{{sup|5}} [[Pascal (unit)|Pa]]}} {{nowrap|1=(= 100 kPa = 1 [[bar (unit)|bar]])}} is recommended by [[IUPAC]], although prior to 1982 the value 1.00 [[atmosphere (unit)|atm]] (101.325 kPa) was used.<ref>{{GoldBookRef| file=S05921 | title = standard pressure}}</ref> There is no standard temperature. Its symbol is Ξ<sub>f</sub>''H''<sup>⦡</sup>. The superscript [[Standard state#Typesetting|Plimsoll]] on this symbol indicates that the process has occurred under standard conditions at the specified temperature (usually 25 Β°C or 298.15 K). Standard states are defined for various types of substances. For a gas, it is the hypothetical state the gas would assume if it obeyed the [[ideal gas equation]] at a pressure of 1 bar. For a gaseous or solid [[solute]] present in a diluted [[ideal solution]], the standard state is the hypothetical state of concentration of the solute of exactly one mole per liter (1 [[molar concentration|M]]) at a pressure of 1 bar extrapolated from infinite dilution. For a pure substance or a [[solvent]] in a condensed state (a liquid or a solid) the standard state is the pure liquid or solid under a pressure of 1 bar. For elements that have multiple [[Allotropy|allotropes]], the reference state usually is chosen to be the form in which the element is most stable under 1 bar of pressure. One exception is [[phosphorus]], for which the most stable form at 1 bar is [[black phosphorus]], but white phosphorus is chosen as the standard reference state for zero enthalpy of formation.<ref>{{cite book |last1=Oxtoby |first1=David W |url=https://books.google.com/books?id=fJWpg4ZJ2esC&q=standard+exception+white+phosphorus+enthalpy&pg=PA547 |title=Principles of Modern Chemistry |last2=Pat Gillis |first2=H |last3=Campion |first3=Alan |date=2011 |isbn=978-0-8400-4931-5 |page=547|publisher=Cengage Learning }}</ref> For example, the standard enthalpy of formation of [[carbon dioxide]] is the enthalpy of the following reaction under the above conditions: :<chem>C(s, graphite) + O2(g) -> CO2(g)</chem> All elements are written in their standard states, and one mole of product is formed. This is true for all enthalpies of formation. The standard enthalpy of formation is measured in units of energy per amount of substance, usually stated in [[Joule per mole|kilojoule per mole]] (kJ mol<sup>β1</sup>), but also in [[kilocalorie per mole]], [[joule]] per mole or kilocalorie per [[gram (unit)|gram]] (any combination of these units conforming to the energy per mass or amount guideline). All elements in their reference states ([[oxygen]] gas, solid [[carbon]] in the form of [[graphite]], etc.) have a standard enthalpy of formation of zero, as there is no change involved in their formation. The formation reaction is a constant pressure and constant temperature process. Since the pressure of the standard formation reaction is fixed at 1 bar, the standard formation enthalpy or reaction heat is a function of temperature. For tabulation purposes, standard formation enthalpies are all given at a single temperature: 298 K, represented by the symbol {{math|Ξ{{sub|f}}''H''{{su|b=298 K|p=⦡}}}}.
Edit summary
(Briefly describe your changes)
By publishing changes, you agree to the
Terms of Use
, and you irrevocably agree to release your contribution under the
CC BY-SA 4.0 License
and the
GFDL
. You agree that a hyperlink or URL is sufficient attribution under the Creative Commons license.
Cancel
Editing help
(opens in new window)