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Standard enthalpy of formation
From Wikipedia, the free encyclopedia
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 of a substance in its standard state from its constituent
elements in their standard states (the most stable form of the element at 1 bar of pressure and the specified
temperature, usually 298.15 K or 25 degrees Celsius). Its symbol is ΔHfO or ΔfHO.
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) + O2(g) → CO2(g)
The standard enthalpy of formation is measured in units of energy per amount of substance. Most are defined
in kilojoules per mole (kJ mol−1), but can also be measured in calories per mole, joules per mole or kilocalories
per 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 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 standard
enthalpy of formation of zero, as there is no change involved in their formation.
Contents
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
1 Mechanics

2 Standard enthalpy of reaction

3 Subcategories

4 Examples: Inorganic compounds (at 25 °C)
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5 See also
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6 External links
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7 References
Mechanics
The standard enthalpy of formation is equivalent to the sum of many separate processes included in the BornHaber cycle of synthesis reactions. For example, to calculate the standard enthalpy of formation of sodium
chloride, we use the following reaction:
Na(s) + (1/2)Cl2(g) → NaCl(s)
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This process is made of many separate sub-processes, each with their own enthalpies. Therefore, we must
take into account:
Standard enthalpy change of formation Born-Haber diagram for lithium fluoride.
1.
The standard enthalpy of atomization of solid sodium
2.
The first ionization energy of gaseous sodium
3.
The standard enthalpy of atomization of chlorine gas
4.
The electron affinity of chlorine atoms
5.
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−1 from using other known standard enthalpies of reaction with Hess's law. That it
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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.
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
being multiplied by its respective stoichiometric coefficient, ν) from the sum of the standard enthalpies of
formation of the products (each also multiplied by its respective stoichiometric coefficient), as shown in the
equation below:
ΔH° = Σ(ν × ΔHf°) (products) - Σ(ν × ΔHf°) (reactants)
For example, for the reaction CH4 + 2 O2 → CO2 + 2 H2O:
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
enthalpy of reaction will be negative. This implies that the reaction is exothermic. The converse is also true; the
standard enthalpy of reaction will be positive for an endothermic reaction.
Subcategories

Standard enthalpy of neutralization is the change in enthalpy that occurs when an acid
and base undergo a neutralization 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 sublime one 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 enthalpy change associated with the dissolution of a substance in a solvent at
constant pressure under standard conditions, as previously defined.

Standard enthalpy of hydrogenation is defined as the enthalpy change observed when
one mole of an unsaturated compound reacts with an excess of hydrogen to become
fully saturated under standard conditions, as previously defined.
Examples: Inorganic compounds (at 25 °C)
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Phase (matter) Chemical formula Δ Hf0 in kJ/mol
Chemical Compound
Ammonia (Ammonium Hydroxide) 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
-469.6
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
4
Silica
s
SiO2
-911
Nitrogen dioxide
g
NO2
+33
Nitrogen monoxide
g
NO
+90
Water
l
H2O
-285.8
Water
g
H2O
-241.82
Carbon dioxide
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
aq
ZnSO4
-980.14
(State: g = gaseous; l = liquid; s = solid; aq = aqueous)
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See also

Thermochemistry

Enthalpy

Calorimetry

Standard enthalpy change of formation (data table)

NIST Chemistry WebBook
External links
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