Standard enthalpy of formation - wikipedia Standard enthalpy of formation 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 substance from its constituent elements, with all substances in their standard states, and at a pressure of 1 bar ( 100 kPa ). There is no standard temperature. Its symbol is Δ H. The superscript Plimsoll 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 : For a gas : the hypothetical state it would have assuming it obeyed the ideal gas equation at a pressure of 1 atm For a solute present in an ideal solution : a concentration of exactly one mole per liter ( 1 M ) at a pressure of 1 atm 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 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. 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 ( g ) → CO ( 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 ), but also in kilocalorie 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 ( eV ), where 1 eV corresponds to 96.485 kJ mol. 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 Δ H ⊖ 298 K. Contents 1 Hess 's law 2 Ionic compounds : Born - Haber cycle 3 Organic compounds 4 Use in calculation for other reactions 5 Key concepts for doing enthalpy calculations 6 Examples : standard enthalpies of formation at 25 ° C 6.1 Inorganic substances 6.2 Aliphatic hydrocarbons 6.3 Other organic compounds 7 See also 8 References 9 External links Hess 's law For many substances, the formation reaction may be considered as the sum of a number of simpler reactions, either real or fictitious. The enthalpy of reaction can then be analyzed by applying Hess 's Law, which states that the sum of the enthalpy changes for a number of individual reaction steps equals the enthalpy change of the overall reaction. This is true because enthalpy is a state function, whose value for an overall process depends only on the initial and final states and not on any intermediate states. Examples are given in the following sections. Ionic compounds : Born - Haber cycle Standard enthalpy change of formation in Born -- Haber diagram for lithium fluoride. For ionic compounds, the standard enthalpy of formation is equivalent to the sum of several terms included in the Born -- Haber cycle. For example, the formation of lithium fluoride, Li ( s ) + ⁄ F ( g ) → LiF ( s ) may be considered as the sum of several steps, each with its own enthalpy ( or energy, approximately ) : The standard enthalpy of atomization ( or sublimation ) of solid lithium The first ionization energy of gaseous lithium The standard enthalpy of atomization ( or bond energy ) of fluorine gas The electron affinity of a fluorine atom The lattice energy of lithium fluoride The sum of all these enthalpies will give the standard enthalpy of formation of lithium fluoride. Δ H f = Δ H sub + IE Li + 1 2 B ( F -- F ) − EA F + U L ( displaystyle Delta ( \text ( H ) ) _ ( \text ( f ) ) = Delta ( \text ( H ) ) _ ( \text ( sub ) ) + ( \text ( IE ) ) _ ( \text ( Li ) ) + ( \frac ( 1 ) ( 2 ) ) ( \text ( B ( F -- F ) ) ) - ( \text ( EA ) ) _ ( \text ( F ) ) + ( \text ( U ) ) _ ( \text ( L ) ) ) In practice, the enthalpy of formation of lithium fluoride can be determined experimentally, but the lattice energy can not be measured directly. The equation is therefore rearranged in order to evaluate the lattice energy. U L = Δ H sub + IE Li + 1 2 B ( F -- F ) − EA F + Δ H f ( displaystyle ( \text ( U ) ) _ ( \text ( L ) ) = Delta ( \text ( H ) ) _ ( \text ( sub ) ) + ( \text ( IE ) ) _ ( \text ( Li ) ) + ( \frac ( 1 ) ( 2 ) ) ( \text ( B ( F -- F ) ) ) - ( \text ( EA ) ) _ ( \text ( F ) ) + Delta ( \text ( H ) ) _ ( \text ( f ) ) ) Organic compounds The formation reactions for most organic compounds are hypothetical. For instance carbon and hydrogen will not directly react to form methane ( CH ), so that the standard enthalpy of formation can not be measured directly. However the standard enthalpy of combustion is readily mesurable using bomb calorimetry. The standard enthalpy of formation is then determined using Hess 's law. The combustion of methane ( CH + 2 O → CO + 2 H O ) is equivalent to the sum of the hypothetical decomposition into elements followed by the combustion of the elements to form carbon dioxide and water : CH → C + 2 H C + O → CO 2 H + O → 2 H O Applying Hess 's law, Δ H ( CH ) = ( Δ H ( CO ) + 2 Δ H ( H O ) ) -- Δ H ( CH ) Solving for the standard of enthalpy of formation, Δ H ( CH ) = ( Δ H ( CO ) + 2 Δ H ( H O ) ) -- Δ H ( CH ) The value of Δ H ( CH ) is determined to be -- 74.8 kJ mol. The negative sign 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 heats of formation for simple unstrained organic compounds with the Heat of formation group additivity method. Use in calculation for other reactions The standard enthalpy change of any reaction can be calculated from the standard enthalpies of formation of reactants and products using Hess 's law. A given reaction is considered as the decomposition of all reactants into elements in their standard states, followed by the formation of all products. The heat of reaction is then minus the sum of the standard enthalpies of formation of the reactants ( each being multiplied by its respective stoichiometric coefficient, ν ) plus 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 : Δ H = Σν Δ H ( products ) − Σν Δ H ( 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. This calculation has a tacit assumption of ideal solution between reactants and products where the enthalpy of mixing is zero. For example, for the combustion of methane, CH + 2 O → CO + 2 H O : Δ H = ( Δ H ( CO ) + 2 Δ H ( H O ) ) -- ( Δ H ( CH ) + 2 Δ H ( O ) ) However O is an element in its standard state, so that Δ H ( O ) = 0 and the heat of reaction is simplified to Δ H = ( Δ H ( CO ) + 2 Δ H ( H O ) ) -- Δ H ( CH ), which is the equation in the previous section for the enthalpy of combustion Δ H. Key concepts for doing enthalpy calculations When a reaction is reversed, the magnitude of ΔH stays the same, but the sign changes. 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. The change in enthalpy for a reaction can be calculated from the enthalpies of formation of the reactants and the products 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. Examples : standard enthalpies of formation at 25 ° C Thermochemical properties of selected substances at 298 K and 1 atm Inorganic substances Species Phase Chemical formula Δ H / ( kJ / mol ) Aluminium Aluminium Solid Al 0 Aluminium chloride Solid AlCl − 705.63 Aluminium oxide Solid Al O − 1669.8 Aluminium hydroxide Solid Al ( OH ) − 1277 Aluminium sulphate Solid Al ( SO ) − 3440 Ammonia ( ammonium hydroxide ) Aqueous NH ( NH OH ) − 80.8 Ammonia Gas NH − 46.1 Ammonium nitrate Solid NH NO − 365.6 Barium Barium chloride Solid BaCl − 858.6 Barium carbonate Solid BaCO − 1213 Barium hydroxide Solid Ba ( OH ) − 944.7 Barium oxide Solid BaO − 548.1 Barium sulfate Solid BaSO − 1473.2 Beryllium Beryllium Solid Be 0 Beryllium hydroxide Solid Be ( OH ) − 902.9999 Beryllium oxide Solid BeO − 609.4 ( 25 ) Boron Boron trichloride Solid BCl − 402.96 Bromine Bromine Liquid Br 0 Bromide ion Aqueous Br − 121 Bromine Gas Br 111.884 Bromine Gas Br 30.91 Bromine trifluoride Gas BrF − 255.60 Hydrogen bromide Gas HBr − 36.29 Cadmium Cadmium Solid Cd 0 Cadmium oxide Solid CdO − 258 Cadmium hydroxide Solid Cd ( OH ) − 561 Cadmium sulfide Solid CdS − 162 Cadmium sulfate Solid CdSO − 935 Calcium Calcium Solid Ca 0 Calcium Gas Ca 178.2 Calcium ( II ) ion Gas Ca 1925.90 Calcium carbide Solid CaC − 59.8 Calcium carbonate ( Calcite ) Solid CaCO − 1206.9 Calcium chloride Solid CaCl − 795.8 Calcium chloride Aqueous CaCl − 877.3 Calcium phosphate Solid Ca ( PO ) − 4132 Calcium fluoride Solid CaF − 1219.6 Calcium hydride Solid CaH − 186.2 Calcium hydroxide Solid Ca ( OH ) − 986.09 Calcium hydroxide Aqueous Ca ( OH ) − 1002.82 Calcium oxide Solid CaO − 635.09 Calcium sulfate Solid CaSO − 1434.52 Calcium sulfide Solid CaS − 482.4 Wollastonite Solid CaSiO − 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 Carbon ( Graphite ) Solid 0 Carbon ( Diamond ) Solid 1.9 Carbon Gas 716.67 Carbon dioxide Gas CO − 393.509 Carbon disulfide Liquid CS 89.41 Carbon disulfide Gas CS 116.7 Carbon monoxide Gas CO − 110.525 Carbonyl chloride ( Phosgene ) Gas COCl − 218.8 Carbon dioxide ( un -- ionized ) Aqueous CO ( aq ) − 419.26 Bicarbonate ion Aqueous HCO − 689.93 Carbonate ion Aqueous CO − 675.23 Chlorine Monatomic chlorine Gas Cl 121.7 Chloride ion Aqueous Cl − 167.2 Chlorine Gas Cl 0 Chromium Chromium Solid Cr 0 Copper Copper Solid Cu 0 Copper ( II ) oxide Solid CuO − 155.2 Copper ( II ) sulfate Aqueous CuSO − 769.98 Fluorine Fluorine Gas 0 Hydrogen Monatomic hydrogen Gas 218 Hydrogen Gas 0 Water Gas H O − 241.818 Water Liquid H O − 285.8 Hydrogen ion Aqueous 0 Hydroxide ion Aqueous OH − 230 Hydrogen peroxide Liquid H O − 187.8 Phosphoric acid Liquid H PO − 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 0 Iodine Gas 62.438 Iodine Aqueous 23 Iodide ion Aqueous − 55 Iron Iron Solid Fe 0 Iron carbide ( Cementite ) Solid Fe C 5.4 Iron ( II ) carbonate ( Siderite ) Solid FeCO − 750.6 Iron ( III ) chloride Solid FeCl − 399.4 Iron ( II ) oxide ( Wüstite ) Solid FeO − 272 Iron ( II, III ) oxide ( Magnetite ) Solid Fe O − 1118 Iron ( III ) oxide ( Hematite ) Solid Fe O − 824.2 Iron ( II ) sulfate Solid FeSO − 929 Iron ( III ) sulfate Solid Fe ( SO ) − 2583 Iron ( II ) sulfide Solid FeS − 102 Pyrite Solid FeS − 178 Lead Lead Solid Pb 0 Lead dioxide Solid PbO − 277 Lead sulfide Solid PbS − 100 Lead sulfate Solid PbSO − 920 Lead ( II ) nitrate Solid Pb ( NO ) − 452 Lead ( II ) sulfate Solid PbSO − 920 Magnesium Magnesium Solid Mg 0 Magnesium ion Aqueous Mg − 466.85 Magnesium carbonate Solid MgCO − 1095.797 Magnesium chloride Solid MgCl − 641.8 Magnesium hydroxide Solid Mg ( OH ) − 924.54 Magnesium hydroxide Aqueous Mg ( OH ) − 926.8 Magnesium oxide Solid MgO − 601.6 Magnesium sulfate Solid MgSO − 1278.2 Manganese Manganese Solid Mn 0 Manganese ( II ) oxide Solid MnO − 384.9 Manganese ( IV ) oxide Solid MnO − 519.7 Manganese ( III ) oxide Solid Mn O − 971 Manganese ( II, III ) oxide Solid Mn O − 1387 Permanganate Aqueous MnO − − 543 Mercury Mercury ( II ) oxide ( red ) Solid HgO − 90.83 Mercury sulfide ( red, cinnabar ) Solid HgS − 58.2 Nitrogen Ammonia Aqueous NH − 80.8 Ammonia Gas NH − 45.90 Ammonium chloride Solid NH Cl − 314.55 Nitrogen dioxide Gas NO 33.2 Nitrous oxide Gas N O 82.05 Nitric oxide Gas NO 90.29 Dinitrogen tetroxide Gas N O 9.16 Dinitrogen pentoxide Solid N O − 43.1 Dinitrogen pentoxide Gas N O 11.3 Oxygen Monatomic oxygen Gas O 249 Oxygen Gas O 0 Ozone Gas O 143 Phosphorus White phosphorus Solid 0 Red phosphorus Solid -- 17.4 Black phosphorus Solid -- 39.3 Phosphorus trichloride Liquid PCl − 319.7 Phosphorus trichloride Gas PCl − 278 Phosphorus pentachloride Solid PCl − 440 Potassium Potassium bromide Solid KBr − 392.2 Potassium carbonate Solid K CO − 1150 Potassium chlorate Solid KClO − 391.4 Potassium chloride Solid KCl − 436.68 Potassium fluoride Solid KF − 562.6 Potassium oxide Solid K O − 363 Potassium perchlorate Solid KClO − 430.12 Silicon Silicon Gas Si 368.2 Silicon carbide Solid SiC − 73.22 Silicon tetrachloride Liquid SiCl − 640.1 Silica ( Quartz ) Solid SiO − 910.86 Silver Silver bromide Solid AgBr − 99.5 Silver chloride Solid AgCl − 127.01 Silver iodide Solid AgI − 62.4 Silver oxide Solid Ag O − 31.1 Silver sulfide Solid Ag S − 31.8 Sodium Sodium Solid Na 0 Sodium Gas Na + 107.5 Sodium bicarbonate Solid NaHCO − 950.8 Sodium carbonate Solid Na CO − 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 NaNO − 446.2 Sodium nitrate Solid NaNO − 424.8 Sodium oxide Solid Na O − 414.2 Sulfur Sulfur ( monoclinic ) Solid 0.3 Sulfur ( rhombic ) Solid 0 Hydrogen sulfide Gas − 20.63 Sulfur dioxide Gas SO − 296.84 Sulfur trioxide Gas SO − 395.7 Sulfuric acid Liquid H SO − 814 Tin Titanium Titanium Gas Ti 468 Titanium tetrachloride Gas TiCl − 763.2 Titanium tetrachloride Liquid TiCl − 804.2 Titanium dioxide Solid TiO − 944.7 Zinc Zinc Gas Zn 130.7 Zinc chloride Solid ZnCl − 415.1 Zinc oxide Solid ZnO − 348.0 Zinc sulfate Solid ZnSO − 980.14 Aliphatic hydrocarbons Formula Name Δ H / ( kcal / mol ) Δ H / ( kJ / mol ) Straight - chain CH Methane − 17.9 − 74.9 Ethane − 20.0 − 83.7 Ethylene 12.5 52.5 Acetylene 54.2 226.8 Propane − 25.0 − 104.6 n - Butane − 30.0 − 125.5 n - Pentane − 35.1 − 146.9 n - Hexane − 40.0 − 167.4 n - Heptane − 44.9 − 187.9 n - Octane − 49.8 − 208.4 n - Nonane − 54.8 − 229.3 n - Decane − 59.6 − 249.4 C Alkane branched isomers Isobutane ( methylpropane ) − 32.1 − 134.3 C Alkane branched isomers Neopentane ( dimethylpropane ) − 40.1 − 167.8 Isopentane ( methylbutane ) − 36.9 − 154.4 C Alkane branched isomers 2, 2 - Dimethylbutane − 44.5 − 186.2 2, 3 - Dimethylbutane − 42.5 − 177.8 2 - Methylpentane ( isohexane ) − 41.8 − 174.9 3 - Methylpentane − 41.1 − 172.0 C Alkane branched isomers 2, 2 - Dimethylpentane − 49.2 − 205.9 2, 2, 3 - Trimethylbutane − 49.0 − 205.0 3, 3 - Dimethylpentane − 48.1 − 201.3 2, 3 - Dimethylpentane − 47.3 − 197.9 2, 4 - Dimethylpentane − 48.2 − 201.7 2 - Methylhexane − 46.5 − 194.6 3 - Methylhexane − 45.7 − 191.2 3 - Ethylpentane − 45.3 − 189.5 C Alkane branched isomers 2, 3 - Dimethylhexane − 55.1 − 230.5 2, 2, 3, 3 - Tetramethylbutane − 53.9 − 225.5 2, 2 - Dimethylhexane − 53.7 − 224.7 2, 2, 4 - Trimethylpentane ( isooctane ) − 53.5 − 223.8 2, 5 - Dimethylhexane − 53.2 − 222.6 2, 2, 3 - Trimethylpentane − 52.6 − 220.1 3, 3 - Dimethylhexane − 52.6 − 220.1 2, 4 - Dimethylhexane − 52.4 − 219.2 2, 3, 4 - Trimethylpentane − 51.9 − 217.1 2, 3, 3 - Trimethylpentane − 51.7 − 216.3 2 - Methylheptane − 51.5 − 215.5 3 - Ethyl - 3 - Methylpentane − 51.4 − 215.1 3, 4 - Dimethylhexane − 50.9 − 213.0 3 - Ethyl - 2 - Methylpentane − 50.4 − 210.9 3 - Methylheptane − 60.3 − 252.5 4 - Methylheptane ? ? 3 - Ethylhexane ? ? C Alkane branched isomers ( selected ) 2, 2, 4, 4 - Tetramethylpentane − 57.8 − 241.8 2, 2, 3, 3 - Tetramethylpentane − 56.7 − 237.2 2, 2, 3, 4 - Tetramethylpentane − 56.6 − 236.8 2, 3, 3, 4 - Tetramethylpentane − 56.4 − 236.0 3, 3 - Diethylpentane − 55.7 − 233.0 Other organic compounds Species Phase Chemical formula Δ H / ( kJ / mol ) Acetone Liquid C H O − 248.4 Benzene Liquid 48.95 Benzoic acid Solid C H O − 385.2 Carbon tetrachloride Liquid CCl − 135.4 Carbon tetrachloride Gas CCl − 95.98 Ethanol Liquid C H OH − 277.0 Ethanol Gas C H OH − 235.3 Glucose Solid C H O − 1271 Isopropanol Gas C H OH − 318.1 Methanol ( methyl alcohol ) Liquid CH OH − 238.4 Methanol ( methyl alcohol ) Gas CH OH − 201.0 Methyl linoleate ( Biodiesel ) Gas C H O − 356.3 Sucrose Solid C H O − 2226.1 Trichloromethane ( Chloroform ) Liquid CHCl − 134.47 Trichloromethane ( Chloroform ) Gas CHCl − 103.18 Vinyl chloride Solid C H Cl − 94.12 See also Calorimetry Enthalpy Heat of combustion Thermochemistry References Jump up ^ Oxtoby, David W ; Pat Gillis, H ; Campion, Alan ( 2011 ). Principles of Modern Chemistry. p. 547. ISBN 0 - 8400 - 4931 - 5. Jump up ^ Moore, Stanitski, and Jurs. Chemistry : The Molecular Science. 3rd edition. 2008. ISBN 0 - 495 - 10521 - X. pages 320 - 321. Jump up ^ http://www.science.uwaterloo.ca/~cchieh/cact/c120/heatreac.html ^ Jump up to : Housecroft, C.E. ; Sharpe, A.G. ( 2004 ). Inorganic Chemistry ( 2nd ed. ). Prentice Hall. p. 392. ISBN 978 - 0130399137. Zumdahl, Steven ( 2009 ). Chemical Principles ( 6th ed. ). Boston. New York : Houghton Mifflin. pp. 384 -- 387. ISBN 978 - 0 - 547 - 19626 - 8. External links NIST Chemistry WebBook Retrieved from `` https://en.wikipedia.org/w/index.php?title=Standard_enthalpy_of_formation&oldid=808920788 '' Categories : Enthalpy Thermochemistry Talk Contents About Wikipedia Afrikaans Български Català Čeština Deutsch Ελληνικά Español فارسی Français Galego Հայերեն हिन्दी Bahasa Indonesia Italiano Қазақша Magyar Nederlands 日本 語 Norsk Polski Português Română Русский Slovenčina Slovenščina کوردی Svenska Українська 中文 Edit links This page was last edited on 6 November 2017, at 01 : 19. About Wikipedia

Question: why is the enthalpy of formation of an element zero
Answer: there is no change involved in their formation