Standard enthalpy of formation

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The standard enthalpy of formation or standard heat of formation of a compound is the change of enthalpy during the formation of 1 mole of the compound from its constituent elements, with all substances in their standard states at 1 atmosphere (1 atm or 101.3 kPa). Its symbol is ΔHo
f
or ΔfHo. The superscript theta (zero) on this symbol indicates that the process has occurred under standard conditions at the specified temperature (usually 25 °C or 298.15 K). Standard states are as follows:

  1. For a gas: the standard state is a pressure of exactly 1 atm
  2. For a solute present in an ideal solution: a concentration of exactly one mole per liter (1 M) at a pressure of 1 atm
  3. 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 atm
  4. For an element: the form in which the element is most stable under 1 atm of pressure. One exception is phosphorus, for which the most stable form at 1 atm is black phosphorus, but white phosphorus is chosen as the standard reference state for zero enthalpy of formation.[1]

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)

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 kilojoule per mole (kJ mol−1), but also in calorie per mole, joule per mole or kilocalorie 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.

The formation reaction is a constant pressure and constant temperature process. Since the pressure of the standard formation reaction is fixed at 1 atm, 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 ΔHo
f298
.

Calculation[edit]

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(s) + 12 Cl2(g) → NaCl(s)

This process is made of many separate sub-processes, each with its own enthalpy. Therefore, we must take into account:

Standard enthalpy change of formation in 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. This is because enthalpy is a state function. 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 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[edit]

Standard enthalpies of formation are used in thermochemistry to find the standard enthalpy change of any 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:

ΔHo = Σ(νΔHo
f
) (products) − Σ(νΔHo
f
) (reactants)[2]

This calculation has a tacit assumption of ideal solution between reactants and products where the enthalpy of mixing is zero.

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

ΔHo
r
= [(ΔHo
f
(CO2)) + (2ΔHo
f
(H2O))] (products) – [(ΔHo
f
(CH4)) + (2ΔHo
f
(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.

Key concepts for doing enthalpy calculations[edit]

  1. When a reaction is reversed, the magnitude of ΔH stays the same, but the sign changes.
  2. When the balanced equation for a reaction is multiplied by an integer, the corresponding value of ΔH must be multiplied by that integer as well.
  3. The change in enthalpy for a reaction can be calculated from the enthalpies of formation of the reactants and the products
  4. Elements in their standard states make no contribution to the enthalpy calculations for the reaction since the enthalpy of an element in its standard state is zero. Allotropes of an element other than the standard state generally have non-zero standard enthalpies of formation.

Subcategories[edit]

  • 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.

Hydrocarbons[edit]

Formula     Name                                                       SMILES ΔHo
f
(kcal/mol)
CAS registry number ΔHo
f
(kJ/mol)
           
H2 Hydrogen [H][H] 0.0 001333-74-0 0.0
CH4 Methane C −17.9 000074-82-8 −74.9
C2H6 Ethane CC −20.0 000074-84-0 −83.7
C2H2 Acetylene CC 54.2 000074-86-2 226.8
C3H8 n-Propane CCC −25.0 000074-98-6 −104.6
C4H10 n-Butane CCCC −30.0 000106-97-8 −125.5
C5H12 n-Pentane CCCCC −35.1 000109-66-0 −146.9
C6H14 n-Hexane CCCCCC −40.0 000110-54-3 −167.4
C7H16 n-Heptane CCCCCCC −44.9 000142-82-5 −187.9
C8H18 n-Octane CCCCCCCC −49.8 000111-65-9 −208.4
C9H20 n-Nonane CCCCCCCCC −54.8 000111-84-2 −229.3
C10H22 n-Decane CCCCCCCCCC −59.6 000124-18-5 −249.4
           
C4H10 Isobutane (methylpropane) CC(C)C −32.1 000075-28-5 −134.3
           
C5H12 Neopentane (dimethylpropane) CC(C)(C)C −40.1 000463-82-1 −167.8
C5H12 Isopentane (methylbutane) CCC(C)C −36.9 000078-78-4 −154.4
           
C6H14 2,2-Dimethylbutane CCC(C)(C)C −44.5 000075-83-2 −186.2
C6H14 2,3-Dimethylbutane CC(C)C(C)C −42.5 000079-29-8 −177.8
C6H14 2-Methylpentane (isohexane) CCCC(C)C −41.8 000107-83-5 −174.9
C6H14 3-Methylpentane CCC(C)CC −41.1 000096-14-0 −172.0
           
C7H16 2,2-Dimethylpentane CCCC(C)(C)C −49.2 000590-35-2 −205.9
C7H16 2,2,3-Trimethylbutane CC(C)C(C)(C)C −49.0 000464-06-2 −205.0
C7H16 3,3-Dimethylpentane CCC(C)(C)CC −48.1 000562-49-2 −201.3
C7H16 2,3-Dimethylpentane CCC(C)C(C)C −47.3 000565-59-3 −197.9
C7H16 2,4-Dimethylpentane CC(C)CC(C)C −48.2 000108-08-7 −201.7
C7H16 2-Methylhexane CCCCC(C)C −46.5 000591-76-4 −194.6
C7H16 3-Methylhexane CCCC(C)CC −45.7 000589-34-4 −191.2
C7H16 3-ethylpentane CCC(CC)CC −45.3 000617-78-7 −189.5
           
C8H18 2,3-Dimethylhexane CCCC(C)C(C)C −55.1 000584-94-1 −230.5
C8H18 2,2,3,3-Tetramethylbutane CC(C)(C)C(C)(C)C −53.9 000594-82-1 −225.5
C8H18 2,2-Dimethylhexane CCCCC(C)(C)C −53.7 000590-73-8 −224.7
C8H18 2,2,4-Trimethylpentane (isooctane) CC(C)CC(C)(C)C −53.5 000540-84-1 −223.8
C8H18 2,5-Dimethylhexane CC(C)CCC(C)C −53.2 000592-13-2 −222.6
C8H18 2,2,3-Trimethylpentane CCC(C)C(C)(C)C −52.6 000564-02-3 −220.1
C8H18 3,3-Dimethylhexane CCCC(C)(C)CC −52.6 000563-16-6 −220.1
C8H18 2,4-Dimethylhexane CCC(C)CC(C)C −52.4 000589-43-5 −219.2
C8H18 2,3,4-Trimethylpentane CC(C)C(C)C(C)C −51.9 000565-75-3 −217.1
C8H18 2,3,3-Trimethylpentane CCC(C)(C)C(C)C −51.7 000560-21-4 −216.3
C8H18 2-Methylheptane CCCCCC(C)C −51.5 000592-27-8 −215.5
C8H18 3-Ethyl-3-Methylpentane CCC(C)(CC)CC −51.4 001067-08-9 −215.1
C8H18 3,4-Dimethylhexane CCC(C)C(C)CC −50.9 000583-48-2 −213.0
C8H18 3-Ethyl-2-Methylpentane CCC(CC)C(C)C −50.4 000609-26-7 −210.9
           
C9H20 2,2,4,4-Tetramethylpentane CC(C)(C)CC(C)(C)C −57.8 001070-87-7 −241.8
C9H20 2,2,3,3-Tetramethylpentane CCC(C)(C)C(C)(C)C −56.7 007154-79-2 −237.2
C9H20 2,2,3,4-Tetramethylpentane CC(C)C(C)C(C)(C)C −56.6 001186-53-4 −236.8
C9H20 2,3,3,4-Tetramethylpentane CC(C)C(C)(C)C(C)C −56.4 016747-38-9 −236.0
C9H20 3,3-Diethylpentane CCC(CC)(CC)CC −55.7 001067-20-5 −233.0

Miscellaneous compounds[edit]

Thermochemical properties of selected substances at 298 K and 1 atm

Species Phase Chemical formula ΔHo
f
(kJ/mol)
Aluminium
Aluminium Solid Al 0
Aluminium chloride Solid AlCl3 −705.63
Aluminium oxide Solid Al2O3 −1669.8
Aluminium hydroxide Solid Al(OH)3 −1277
Aluminium sulphate Solid Al2(SO4)3 −3440
Barium
Barium chloride Solid BaCl2 −858.6
Barium carbonate Solid BaCO3 −1213
Barium hydroxide Solid Ba(OH)2 −944.7
Barium oxide Solid BaO −548.1
Barium sulfate Solid BaSO4 −1473.2
Beryllium
Beryllium Solid Be 0
Beryllium hydroxide Solid Be(OH)2 −902.9999
Beryllium oxide Solid BeO −609.4(25)
Boron
Boron trichloride Solid BCl3 −402.96
Bromine
Bromine Liquid Br2 0
Bromine Liquid Br −121
Bromine Gas Br 111.884
Bromine Gas Br2 30.91
Bromine trifluoride Gas BrF3 −255.60
Hydrobromic acid Gas HBr −36.29
Cadmium
Cadmium Solid Cd 0
Cadmium oxide Solid CdO −258
Cadmium hydroxide Solid Cd(OH)2 −561
Cadmium sulfide Solid CdS −162
Cadmium sulfate Solid CdSO4 −935
Calcium
Calcium Solid Ca 0
Calcium Gas Ca 178.2
Calcium(II) ion Gas Ca2+ 1925.90
Calcium carbide Solid CaC2 −59.8
Calcium carbonate (Calcite) Solid CaCO3 −1206.9
Calcium chloride Solid CaCl2 −795.8
Calcium chloride Aqueous CaCl2 −877.3
Calcium phosphate Solid Ca3(PO4)2 −4132
Calcium fluoride Solid CaF2 −1219.6
Calcium hydride Solid CaH2 −186.2
Calcium hydroxide Solid Ca(OH)2 −986.09
Calcium hydroxide Aqueous Ca(OH)2 −1002.82
Calcium oxide Solid CaO −635.09
Calcium sulfate Solid CaSO4 −1434.52
Calcium sulfide Solid CaS −482.4
Wollastonite Solid CaSiO3 −1630
Caesium
Caesium Solid Cs 0
Caesium Gas Cs 76.50
Caesium Liquid Cs 2.09
Caesium(I) ion Gas Cs+ 457.964
Caesium chloride Solid CsCl −443.04
Carbon Alkanes Above
Benzene Liquid C6H6 48.95
Benzoic acid Solid C7H6O2 −385.2
Glucose Solid C6H12O6 −1271
Carbon (Graphite) Solid C 0
Carbon (Diamond) Solid C 1.9
Carbon Gas C 716.67
Carbon dioxide Gas CO2 −393.509
Carbon disulfide Liquid CS2 89.41
Carbon disulfide Gas CS2 116.7
Carbon monoxide Gas CO −110.525
Carbon tetrachloride Liquid CCl4 −135.4
Carbon tetrachloride Gas CCl4 −95.98
Carbonyl chloride (Phosgene) Gas COCl2 −218.8
Ethane Gas C2H6 −83.85
Ethanol Liquid C2H5OH −277.0
Ethanol Gas C2H5OH −235.3
Ethene Gas C2H4 52.3
Vinyl chloride Solid C2H3Cl −94.12
Ethyne Gas C2H2 226.73
Methane Gas CH4 −74.87
Methanol (methyl alcohol) Liquid CH3OH −238.4
Methanol (methyl alcohol) Gas CH3OH −201.0
Methyl linoleate (Biodiesel) Gas C19H34O2 −356.3
Methyl trichloride (Chloroform) Liquid CHCl3 −134.47
Methyl trichloride (Chloroform) Gas CHCl3 −103.18
Propane Gas C3H8 −104.7
CO2 (aqueous, unionized) Aqueous CO2(aq) −419.26
HCO3 HCO3 −689.93
Carbon trioxide CO3 −675.23
Chlorine
Monatomic chlorine Gas Cl 121.7
Chloride ion Aqueous Cl −167.2
Chlorine Gas Cl2 0
Chromium
Chromium Solid Cr 0
Copper
Copper Solid Cu 0
Copper(II) oxide Solid CuO −155.2
Fluorine
Fluorine Gas F2 0
Hydrogen
Monatomic hydrogen Gas H 218
Hydrogen Gas H2 0
Water Gas H2O −241.818
Water Liquid H2O −285.8
Hydrogen ion Aqueous H+ 0
Hydroxide ion Aqueous OH −230
Hydrogen peroxide Liquid H2O2 −187.8
Phosphoric acid Liquid H3PO4 −1288
Hydrogen cyanide Gas HCN 130.5
Hydrogen bromide Liquid HBr −36.3
Hydrogen chloride Gas HCl −92.30
Hydrogen chloride Aqueous HCl −167.2
Hydrogen fluoride Gas HF −273.3
Hydrogen iodide Gas HI 26.5
Iodine
Iodine Solid I2 0
Iodine Gas I2 62.438
Iodine Aqueous I2 23
Iodide ion Aqueous I −55
Iron
Iron Solid Fe 0
Iron carbide (Cementite) Solid Fe3C 5.4
Iron(II) carbonate (Siderite) Solid FeCO3 -750.6
Iron(III) chloride Solid FeCl3 −399.4
Iron(II) oxide (Wüstite) Solid FeO −272
Iron(II,III) oxide (Magnetite) Solid Fe3O4 −1118
Iron(III) oxide (Hematite) Solid Fe2O3 −824.2
Iron(II) sulfate Solid FeSO4 −929
Iron(III) sulfate Solid Fe2(SO4)3 −2583
Iron(II) sulfide Solid FeS −102
Pyrite Solid FeS2 −178
Lead
Lead Solid Pb 0
Lead dioxide Solid PbO2 −277
Lead sulfide Solid PbS −100
Lead sulfate Solid PbSO4 −920
Lead(II) nitrate Solid Pb(NO3)2 −452
Lead(II) sulfate Solid PbSO4 −920
Lithium
Magnesium
Magnesium Solid Mg 0
Magnesium ion Aqueous Mg2+ −466.85
Magnesium carbonate Solid MgCO3 −1095.797
Magnesium chloride Solid MgCl2 −641.8
Magnesium hydroxide Solid Mg(OH)2 −924.54
Magnesium hydroxide Aqueous Mg(OH)2 −926.8
Magnesium oxide Solid MgO −601.6
Magnesium sulfate Solid MgSO4 −1278.2
Manganese
Manganese Solid Mn 0
Manganese(II) oxide Solid MnO −384.9
Manganese(IV) oxide Solid MnO2 −519.7
Manganese(III) oxide Solid Mn2O3 −971
Manganese(II,III) oxide Solid Mn3O4 −1387
Permanganate Aqueous MnO
4
−543
Mercury
Mercury(II) oxide (red) Solid HgO −90.83
Mercury sulfide (red, cinnabar) Solid HgS −58.2
Nickel
Nitrogen
Ammonia Aqueous NH3 −80.8
Ammonia Gas NH3 −45.90
Ammonium chloride Solid NH4Cl −314.55
Nitrogen dioxide Gas NO2 33.2
Nitrous oxide Gas N2O 82.05
Nitric oxide Gas NO 90.29
Dinitrogen tetroxide Gas N2O4 9.16
Dinitrogen pentoxide Solid N2O5 −43.1
Dinitrogen pentoxide Gas N2O5 11.3
Oxygen
Monatomic oxygen Gas O 249
Oxygen Gas O2 0
Ozone Gas O3 143
Phosphorus
Phosphorus trichloride Liquid PCl3 −319.7
Phosphorus trichloride Gas PCl3 −278
Phosphorus pentachloride Solid PCl5 −440
Potassium
Potassium bromide Solid KBr −392.2
Potassium carbonate Solid K2CO3 −1150
Potassium chlorate Solid KClO3 −391.4
Potassium chloride Solid KCl −436.68
Potassium fluoride Solid KF −562.6
Potassium oxide Solid K2O −363
Potassium perchlorate Solid KClO4 −430.12
Silicon
Silicon Gas Si 368.2
Silicon carbide Solid SiC −73.22
Silicon tetrachloride Liquid SiCl4 −640.1
Silica (Quartz) Solid SiO2 −910.86
Silver
Silver bromide Solid AgBr −99.5
Silver chloride Solid AgCl −127.01
Silver iodide Solid AgI −62.4
Silver oxide Solid Ag2O −31.1
Silver sulfide Solid Ag2S −31.8
Sodium
Sodium carbonate Solid Na2CO3 −1130.77
Sodium chloride Aqueous NaCl −407.27
Sodium chloride Solid NaCl −411.12
Sodium chloride Liquid NaCl −385.92
Sodium chloride Gas NaCl −181.42
Sodium fluoride Solid NaF −569.0
Sodium hydroxide Aqueous NaOH −469.15
Sodium hydroxide Solid NaOH −425.93
Sodium nitrate Aqueous NaNO3 −446.2
Sodium nitrate Solid NaNO3 −424.8
Sodium oxide Solid Na2O −414.2
Sulfur
Hydrogen sulfide Gas H2S −20.63
Sulfur dioxide Gas SO2 −296.84
Sulfur trioxide Gas SO3 −395.7
Sulfuric acid Liquid H2SO4 −814
Tin
Titanium
Titanium Gas Ti 468
Titanium tetrachloride Gas TiCl4 −763.2
Titanium tetrachloride Liquid TiCl4 −804.2
Titanium dioxide Solid TiO2 −944.7
Zinc
Zinc Gas Zn 130.7
Zinc chloride Solid ZnCl2 −415.1
Zinc oxide Solid ZnO −348.0

Examples: standard enthalpies of formation (at 25 °C, 298 K)[edit]

Chemical compound Phase Chemical formula ΔHo
f
(kJ/mol)
Acetone l C3H6O −248.4
Acetylene g C2H2 +227.4
Ammonia (ammonium hydroxide) aq NH3 (NH4OH) −80.8
Ammonia g NH3 −46.1
Ammonium nitrate s NH4NO3 −365.6
Benzene l C6H6 +49.1
Bromine l Br2 0
Bromine g Br2 +31
Bromine g Br +111.9
Hydrogen bromide g HBr −36.3
Calcium s Ca 0
Calcium oxide s CaO −634.9
Calcium carbonate s CaCO3 −1207.6
Carbon s C (graphite) 0
Carbon s C (diamond) +1.88
Carbon monoxide g CO −110.5
Carbon dioxide g CO2 −393.5
Chlorine g Cl2 0
Chlorine g Cl +121.3
Hydrogen chloride g HCl −92.3
Copper(II) sulfate aq CuSO4 −769.98
Ethane g C2H6 −84.68
Ethanol l C2H5OH −277.6
Ethylene g C2H4 +52.4
Fluorine g F2 0
Fluorine g F +79.38
Hydrogen fluoride g HF −273.3
Glucose s C6H12O6 −1273.3
Hydrogen g H +216
Hydrogen g H2 0
Iodine s I2 0
Iodine g I2 +62
Isopropanol g C3H7OH −318.1
Methane g CH4 −74.87
Methanol l CH3OH −238.6
Nitrogen g N2 0
Nitrogen dioxide g NO2 +33.2
Nitric oxide g NO +91.3
Oxygen g O +249
Oxygen g O2 0
Ozone g O3 +142.7
Propane g C3H8 −103.85
Sodium s Na 0
Sodium g Na +107.5
Sodium bicarbonate s NaHCO3 −950.8
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
Sodium hydroxide s NaOH −426.7
Sodium nitrate aq NaNO3 −446.2
Sodium nitrate s NaNO3 −424.8
Sucrose s C12H22O11 −2226.1
Sulfur (monoclinic) s S8 0.3
Sulfur (rhombic) s S8 0
Sulfur dioxide g SO2 −296.8
Sulfur trioxide g SO3 −395.7
Sulfuric acid l H2SO4 −814
Silica s SiO2 −911
Silver s Ag 0
Silver chloride s AgCl −127.0
Water l H2O −285.8
Water vapor g H2O −241.82
Zinc sulfate s ZnSO4 −980.14
(State: g = gaseous; l = liquid; s = solid; aq = aqueous)

See also[edit]

External links[edit]

References[edit]

  1. ^ Oxtoby, David W; Pat Gillis, H; Campion, Alan (2011). Principles of Modern Chemistry. p. 547. ISBN 0-8400-4931-5. 
  2. ^ http://www.science.uwaterloo.ca/~cchieh/cact/c120/heatreac.html

Zumdahl, Steven (2009). Chemical Principles (6th ed.). Boston. New York: Houghton Mifflin. pp. 384–387. ISBN 978-0-547-19626-8.