Standard enthalpy change of formation
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The '''standard enthalpy of formation''' or "standard heat of formation" of a compound is the change of [[enthalpy]] that accompanies the formation of 1 [[mole (unit)|mole]] of a substance in its [[standard state]] from its constituent elements in their [[standard state]]s (the most stable form of the element at 101.325 kPa of pressure and the specified temperature, usually 298 K or 25 degrees Celsius). Its symbol is Δ''H''<sub>f</sub><sup><s>O</s></sup>.
A similar type of enthalpy change, known as the '''standard enthalpy change of hydrogenation''' is defined as the enthalpy change observed when 1 [[mole (unit)|mol]] of an [[unsaturated]] compound reacts with an excess of hydrogen to become fully saturated, all elements within the reaction being within their [[standard state]]s.
For example, the standard enthalpy of formation of [[carbon dioxide]] would be the enthalpy of the following reaction under the conditions above:
:C(s,graphite) + O<sub>2</sub>(g) → CO<sub>2</sub>(g)
The standard enthalpy change of formation is measured in units of energy per amount of substance. Most are defined in kilojoules per [[mole (unit)|mole]], or kJ mol<sup>-1</sup>, but can also be measured in [[calories]] per mole, [[joules]] per [[gram (unit)|mole]] or kilocalories per [[gram (unit)|gram]] (any combination of these units conforming to the energy per mass or amount guideline).
In [[physics]] the energy per particle is often expressed in [[electronvolt]]s which corresponds to about 100 kJ mol<sup>-1</sup>.
All elements in their standard 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 standard enthalpy change 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 from the sum of the standard enthalpies of formation of the products, as shown in the equation below.
Δ''H''<sub>reaction</sub><sup><s>O</s></sup> = ΣΔ''H''<sub>f</sub><sup><s>O</s></sup> (Products) - ΣΔ''H''<sub>f</sub><sup><s>O</s></sup> (Reactants)
The standard enthalpy of formation is equivalent to the sum of many separate processes included in the [[Born-Haber cycle]] of synthesis reactions. For example, to calculate the standard enthalpy of formation of [[sodium chloride]],
we use the following reaction:
:Na<sub>(s)</sub> + (1/2)Cl<sub>2(g)</sub> → NaCl<sub>(s)</sub>
This process is made of many separate sub-processes, each with their own enthalpies. Therefore, we must take into account:
[[Image:BornHaberLiF.PNG|280px|thumb|right|Standard enthalpy change of formation [[Born-Haber cycle|Born-Haber]] diagram for lithium fluoride.]]
#The [[standard enthalpy of atomization]] of solid sodium
#The [[first ionization energy]] of gaseous sodium
#The standard enthalpy of atomization of chlorine gas
#The electron affinity of chlorine atoms
#The lattice enthalpy of sodium chloride
The sum of all these values will give the standard enthalpy of formation of sodium chloride.
Additionally, applying [[Hess's Law]] shows that the ''sum'' of the individual reactions corresponding to the enthalpy change of formation for each substance in the reaction '''is equal to''' the enthalpy change of the overall reaction, regardless of the number of steps or intermediate reactions involved. In the example above the standard enthalpy change of formation for sodium chloride is equal to the sum of the standard enthalpy change of formation for each of the steps involved in the process. This is 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 and hydrogen will not directly react to form methane, yet the standard enthalpy of formation for methane is determined to be -74.8 kJ mol<sup>-1</sup> from using other known standard enthalpies of reaction with [[Hess's law]]. That it is negative shows that the reaction, if it were to proceed, would be [[exothermic]]; that is, it is enthalpically more stable than hydrogen gas and carbon.
It is possible to predict heat of formations for simple unstrained organic compounds with the [[Heat of formation group additivity]] method.
==== Examples: Inorganic compounds (at 25 °C) ====
{| border="2" cellspacing="0" width="500px" cellpadding="4" rules="all" style="border-collapse:collapse; empty-cells:show; margin: 2ex 2em; border: solid 1px #aaaaa; font-size: 95%; text-align: middle"
|- style="text-align: center" {{highlight1}}
!| Chemical Compound || [[Phase (matter)]]
|| [[Chemical formula]] ||Δ ''H''<sub>''f''</sub><sup>0</sup> in kJ/mol
|-
| style="text-align: left" | [[Ammonia]] (Ammonium Hydroxide) || aq
|| NH<sub>3</sub> (NH<sub>4</sub>OH) || -80.8
|-
| style="text-align: left" | Ammonia || g
|| NH<sub>3</sub> || -46.1
|-
| style="text-align: left" | Copper (II) sulfate || aq
|| CuSO<sub>4</sub> || -769.98
|-
| style="text-align: left" | [[Sodium carbonate]] || s
|| Na<sub>2</sub>CO<sub>3</sub> || -1131
|-
| style="text-align: left" | [[Sodium chloride]] (table salt) || aq
|| NaCl || -407
|-
| style="text-align: left" | Sodium chloride (table salt) || s
|| NaCl || -411.12
|-
| style="text-align: leftt" | Sodium chloride (table salt) || l
|| NaCl || -385.92
|-
| style="text-align: left" | Sodium chloride (table salt) || g
|| NaCl || -181.42
|-
| style="text-align: left" | [[Sodium hydroxide]] || aq
|| NaOH || -469.6
|-
| style="text-align: left" | Sodium hydroxide || s
|| NaOH || -426.7
|-
| style="text-align: left" | [[Sodium nitrate]] || aq
|| NaNO<sub>3</sub> || -446.2
|-
| style="text-align: left" | Sodium nitrate || s
|| NaNO<sub>3</sub> || -424.8
|-
| style="text-align: left" | [[Sulfur dioxide]] || g
|| SO<sub>2</sub> || -297
|-
| style="text-align: left" | [[Sulfuric acid]] || l
|| H<sub>2</sub>SO<sub>4</sub> || -814
|-
| style="text-align: left" | [[Silica]] || s
|| SiO<sub>2</sub> || -911
|-
| style="text-align: left" | [[Nitrogen dioxide]] || g
|| NO<sub>2</sub> || +33
|-
| style="text-align: left" | [[Nitrogen monoxide]] || g
|| NO || +90
|-
| style="text-align: left" | [[Water (data page)|Water]] || l
|| H<sub>2</sub>O || -286
|-
| style="text-align: left" | Water || g
|| H<sub>2</sub>O || -241.8
|-
| style="text-align: left" | [[Carbon dioxide]] || g
|| CO<sub>2</sub> || -393.5
|-
| style="text-align: left" | [[Hydrogen]] || g
|| H<sub>2</sub> || 0
|-
| style="text-align: left" | [[Fluorine]] || g
|| F<sub>2</sub> || 0
|-
| style="text-align: left" | [[Chlorine]] || g
|| Cl<sub>2</sub> || 0
|-
| style="text-align: left" | [[Bromine]] || l
|| Br<sub>2</sub> || 0
|-
| style="text-align: left" | Bromine || g
|| Br<sub>2</sub> || +31
|-
| style="text-align: left" | [[Iodine]]|| s
|| I<sub>2</sub> || 0
|-
| style="text-align: left" | Iodine|| g
|| I<sub>2</sub> || +62
|-
| style="text-align: left" | Zinc sulfate|| aq
|| ZnSO<sub>4</sub> || -980.14
|-
|}
:(State: g = gaseous; l = liquid; s = solid; aq = aqueous)
==See also==
*[[Thermochemistry]]
*[[Enthalpy]]
*[[Calorimetry]]
*[[Standard enthalpy change of formation (data table)]]
==External links==
*[http://webbook.nist.gov/chemistry/ NIST Chemistry WebBook]
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