standard enthalpy of formation

Standard enthalpy of formationFrom Wikipedia, the free encyclopedia

The standard enthalpy of formation or standard heat of formation of a compound is the change of enthalpyfrom the formation of 1 mole of the compound from its constituent elements, with all substances in theirstandard states at 101.3 kPa and 298 K. Its symbol is ΔHf

O or ΔfHO. The superscript theta (zero) on this symbol

indicates that the process has been carried out under standard conditions. Standard States are as follows:

For a gas: standard state is a pressure of exactly 1 atmosphere1.For a substance present in a solution: a concentration of exactly 1 M at a pressure of 1 atm2.For a pure substance in a condensed state (a liquid or a solid): the pure liquid or solid under a pressure of1 atm

3.

For an element: the form in which the element is most stable under 1 atm of pressure and the specifiedtemperature. (Usually 25 degrees Celsius or 298.15 K) One exception is phosphorus: most stable under 1atm is black phosphorus, but white phosphorus is used as the reference for zero enthalpy of formation[1]

4.

For example, the standard enthalpy of formation of carbon dioxide would be the enthalpy of the followingreaction under the conditions above:

C(s,graphite) + O2(g) → CO2(g)

Note that all elements are written in their standard states, and one mole of product is formed. This is true for allenthalpies of formation.

The standard enthalpy of formation is measured in units of energy per amount of substance. Most are defined inkilojoules per mole (kJ mol−1), but can also be measured in calories per mole, joules per mole or kilocaloriesper gram (any combination of these units conforming to the energy per mass or amount guideline). In physicsthe energy per particle is often expressed in electronvolts which corresponds to about 100 kJ mol−1.

All elements in their standard states (oxygen gas, solid carbon in the form of graphite, etc.) have a standardenthalpy of formation of zero, as there is no change involved in their formation.

Contents

1 Mechanics2 Standard enthalpy of reaction3 Key concepts for doing enthalpy calculations4 Subcategories5 Examples: Inorganic compounds (at 25°C, 298 K)6 See also7 External links8 References

Mechanics

Standard enthalpy of formation - Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Standard_enthalpy_of_formation

1 of 5 26-Nov-13 3:23 PM

Standard enthalpy change of formationBorn-Haber diagram for lithium fluoride.

The standard enthalpy of formation is equivalent to the sum of many separate processes included in theBorn-Haber cycle of synthesis reactions. For example, to calculate the standard enthalpy of formation of sodiumchloride, we use the following reaction:

Na(s) + (1/2)Cl2(g) → NaCl(s)

This process is made of many separate sub-processes, each with their own enthalpies. Therefore, we must takeinto account:

The standard enthalpy of atomization of solid sodium1.The first ionization energy of gaseous sodium2.The standard enthalpy of atomization of chlorine gas3.The electron affinity of chlorine atoms4.The lattice enthalpy of sodium chloride5.

The sum of all these values will give the standard enthalpy offormation of sodium chloride.

Additionally, applying Hess's Law shows that the sum of theindividual reactions corresponding to the enthalpy change offormation for each substance in the reaction is equal to theenthalpy change of the overall reaction, regardless of thenumber of steps or intermediate reactions involved. This isbecause enthalpy is a state function. In the example above thestandard enthalpy change of formation for sodium chloride isequal to the sum of the standard enthalpy change of formation for each of the steps involved in the process. Thisis especially useful for very long reactions with many intermediate steps and compounds.

Chemists may use standard enthalpies of formation for a reaction that is hypothetical. For instance carbon andhydrogen will not directly react to form methane, yet the standard enthalpy of formation for methane isdetermined to be -74.8 kJ mol−1 from using other known standard enthalpies of reaction with Hess's law. That itis negative shows that the reaction, if it were to proceed, would be exothermic; that is, it is enthalpically morestable than hydrogen gas and carbon.

It is possible to predict heat of formations for simple unstrained organic compounds with the Heat of formationgroup additivity method.

Standard enthalpy of reaction

The standard enthalpy of formation is used in thermochemistry to find the standard enthalpy change of reaction.This is done by subtracting the sum of the standard enthalpies of formation of the reactants (each beingmultiplied by its respective stoichiometric coefficient, ν) from the sum of the standard enthalpies of formationof the products (each also multiplied by its respective stoichiometric coefficient), as shown in the equationbelow:

ΔH° = Σ(ν × ΔHf°) (products) - Σ(ν × ΔHf°) (reactants)

For example, for the reaction CH4 + 2 O2 → CO2 + 2 H2O:


2 of 5 26-Nov-13 3:23 PM

ΔHr° = [(1 × ΔHf°(CO2)) + (2 × ΔHf°(H2O))] (products) - [(1 × ΔHf°(CH4)) + (2 × ΔHf°(O2))] (reactants)

If the standard enthalpy of the products is less than the standard enthalpy of the reactants, the standard enthalpyof reaction will be negative. This implies that the reaction is exothermic. The converse is also true; the standardenthalpy of reaction will be positive for an endothermic reaction.

Key concepts for doing enthalpy calculations

When a reaction is reversed, the magnitude of ΔH stays the same, but the sign changes.1.When the balanced equation for a reaction is multiplied by an integer, the corresponding value of ΔHmust be multiplied by that integer as well.

2.

The change in enthalpy for a reaction can be calculated from the enthalpies of formation of the reactantsand the products

3.

Elements in their standard states are not included in the enthalpy calculations for the reaction since theenthalpy of an element in its standard state is zero.

4.

Subcategories

Standard enthalpy of neutralization is the change in enthalpy that occurs when an acid and base undergo aneutralization reaction to form one mole of water under standard conditions, as previously defined.Standard enthalpy of sublimation, or heat of sublimation, is defined as the enthalpy required to sublimeone mole of the substance under standard conditions, as previously defined.Standard enthalpy of solution (or enthalpy change of dissolution or heat of solution) is the enthalpychange associated with the dissolution of a substance in a solvent at constant pressure under standardconditions, as previously defined.Standard enthalpy of hydrogenation is defined as the enthalpy change observed when one mole of anunsaturated compound reacts with an excess of hydrogen to become fully saturated under standardconditions, as previously defined.

Examples: Inorganic compounds (at 25°C, 298 K)

Chemical Compound Phase(matter)

Chemicalformula

Δ Hf0 in

kJ/molAmmonia (AmmoniumHydroxide) aq NH3 (NH4OH) -80.8

Ammonia g NH3 -46.1

Copper (II) sulfate aq CuSO4 -769.98

Sodium carbonate s Na2CO3 -1131

Sodium chloride (table salt) aq NaCl -407

Sodium chloride (table salt) s NaCl -411.12

Sodium chloride (table salt) l NaCl -385.92

Sodium chloride (table salt) g NaCl -181.42

Sodium hydroxide aq NaOH -470.1


3 of 5 26-Nov-13 3:23 PM

Sodium hydroxide s NaOH -426.7

Sodium nitrate aq NaNO3 -446.2

Sodium nitrate s NaNO3 -424.8

Sulfur dioxide g SO2 -297

Sulfuric acid l H2SO4 -814

Silica s SiO2 -911

Nitrogen dioxide g NO2 +33.2

Nitrogen monoxide g NO +91.3

Water l H2O -285.8

Water g H2O -241.82

Carbon dioxide(CO2) g CO2 -393.5

Hydrogen g H2 0

Fluorine g F2 0

Chlorine g Cl2 0

Bromine l Br2 0

Bromine g Br2 +31

Iodine s I2 0

Iodine g I2 +62

Zinc sulfate s ZnSO4 -980.14

(State: g = gaseous; l = liquid; s = solid; aq = aqueous)

See also

ThermochemistryEnthalpyCalorimetryStandard enthalpy change of formation (data table)

External links

NIST Chemistry WebBook (http://webbook.nist.gov/chemistry/)

References[2]

^ Principles of Modern Chemistry 547p (http://books.google.co.kr/books?id=fJWpg4ZJ2esC&pg=PA547&1.


4 of 5 26-Nov-13 3:23 PM

lpg=PA547&dq=standard+exception+white+phosphorus+enthalpy&source=bl&ots=W76OtR_Hr3&sig=GFbrpTTGMookqRUlf2lxWsdajCQ&hl=ko&sa=X&ei=KJcnUILTN4OkiQfbkoGgBA&ved=0CGUQ6AEwCQ#v=onepage&q=standard%20exception%20white%20phosphorus%20enthalpy&f=false)^ Zumdahl, Steven (2009). Chemical Principles, Ed. 6 p. 384-387. Houghton Mifflin, Boston. New York. ISBN978-0-547-19626-8.

2.

Retrieved from "http://en.wikipedia.org/w/index.php?title=Standard_enthalpy_of_formation&oldid=580981391"Categories: Enthalpy Thermochemistry

This page was last modified on 10 November 2013 at 01:46.Text is available under the Creative Commons Attribution-ShareAlike License; additional terms mayapply. 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.


5 of 5 26-Nov-13 3:23 PM

standard enthalpy of formation

Documents