heat of formation
   HOME

TheInfoList



OR:

In chemistry and
thermodynamics Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of th ...
, the standard enthalpy of formation or standard heat of formation of a
compound Compound may refer to: Architecture and built environments * Compound (enclosure), a cluster of buildings having a shared purpose, usually inside a fence or wall ** Compound (fortification), a version of the above fortified with defensive struc ...
is the change of
enthalpy Enthalpy , a property of a thermodynamic system, is the sum of the system's internal energy and the product of its pressure and volume. It is a state function used in many measurements in chemical, biological, and physical systems at a constant ...
during the formation of 1
mole Mole (or Molé) may refer to: Animals * Mole (animal) or "true mole", mammals in the family Talpidae, found in Eurasia and North America * Golden moles, southern African mammals in the family Chrysochloridae, similar to but unrelated to Talpida ...
of the substance from its constituent elements in their reference state, with all substances in their standard states. The standard
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
value is recommended by
IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
, although prior to 1982 the value 1.00 atm (101.325 kPa) was used. There is no standard temperature. Its symbol is . 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 The ideal gas law, also called the general gas equation, is the equation of state of a hypothetical ideal gas. It is a good approximation of the behavior of many gases under many conditions, although it has several limitations. It was first sta ...
at a pressure of 1 bar # For a gaseous or solid
solute In chemistry, a solution is a special type of homogeneous mixture composed of two or more substances. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent. If the attractive forces between the solvent ...
present in a diluted ideal solution: the hypothetical state of concentration of the solute of exactly one mole per liter (1  M) at a pressure of 1 bar extrapolated from infinite dilution # For a pure substance or a
solvent A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
in a condensed state (a liquid or a solid): the standard state is the pure liquid or solid under a pressure of 1 bar For elements that have multiple allotropes, the reference state usually is chosen to be the form in which the element is most stable under 1 bar of pressure. One exception is
phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...
, for which the most stable form at 1 bar 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 Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
would be the enthalpy of the following reaction under the above conditions: :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 The joule per mole (symbol: J·mol−1 or J/mol) is the unit of energy per amount of substance in the International System of Units (SI), such that energy is measured in joules, and the amount of substance is measured in moles. It is also an ...
(kJ mol−1), but also in kilocalorie per mole,
joule The joule ( , ; symbol: J) is the unit of energy in the International System of Units (SI). It is equal to the amount of work done when a force of 1 newton displaces a mass through a distance of 1 metre in the direction of the force applie ...
per mole or kilocalorie per
gram The gram (originally gramme; SI unit symbol g) is a unit of mass in the International System of Units (SI) equal to one one thousandth of a kilogram. Originally defined as of 1795 as "the absolute weight of a volume of pure water equal to th ...
(any combination of these units conforming to the energy per mass or amount guideline). All elements in their reference states (
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
gas, solid
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...
in the form of
graphite Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on lar ...
, 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 bar, 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 .


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 The standard enthalpy of reaction (denoted \Delta_ H^\ominus or \Delta H_^\ominus) for a chemical reaction is the difference between total reactant and total product molar enthalpies, calculated for substances in their standard states. This can in ...
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

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 Lithium fluoride is an inorganic compound with the chemical formula LiF. It is a colorless solid, that transitions to white with decreasing crystal size. Although odorless, lithium fluoride has a bitter-saline taste. Its structure is analogous to ...
, :Li(s) + 1/2F2(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 In chemistry, the enthalpy of atomization (also atomisation in British English) is the enthalpy change that accompanies the total separation of all atoms in a chemical substance (either an element or a compound). This is often represented by th ...
(or bond energy) of fluorine gas. # , the
electron affinity The electron affinity (''E''ea) of an atom or molecule is defined as the amount of energy released when an electron attaches to a neutral atom or molecule in the gaseous state to form an anion. ::X(g) + e− → X−(g) + energy Note that this is ...
of a fluorine atom. # , the
lattice energy In chemistry, the lattice energy is the energy change upon formation of one mole of a crystalline ionic compound from its constituent ions, which are assumed to initially be in the gaseous state. It is a measure of the cohesive forces that bin ...
of lithium fluoride. The sum of all these enthalpies will give the standard enthalpy of formation () of lithium fluoride: :\Delta H_\text = \Delta H_\text + \text_\text + \frac\text - \text_\text + \text_\text. In practice, the enthalpy of formation of lithium fluoride can be determined experimentally, but the lattice energy cannot be measured directly. The equation is therefore rearranged in order to evaluate the lattice energy: :-U_\text = \Delta H_\text + \text_\text + \frac\text - \text_\text - \Delta H_\text.


Organic compounds

The formation reactions for most organic compounds are hypothetical. For instance, carbon and hydrogen won't directly react to form
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ea ...
(), so that the standard enthalpy of formation cannot be measured directly. However the standard enthalpy of combustion is readily measurable using
bomb calorimetry In chemistry and thermodynamics, calorimetry () is the science or act of measuring changes in '' state variables'' of a body for the purpose of deriving the heat transfer associated with changes of its state due, for example, to chemical react ...
. The standard enthalpy of formation is then determined using Hess's law. The combustion of methane: :CH4 + 2 O2 -> CO2 + 2 H2O is equivalent to the sum of the hypothetical decomposition into elements followed by the combustion of the elements to form
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
() and water (): :CH4 -> C + 2H2 :C + O2 -> CO2 :2H2 + O2 -> 2H2O Applying Hess's law, :\Delta_\text H^\ominus ( \text_4 ) = \Delta_\text H^\ominus (\text_2) + 2 \Delta_\text H^\ominus ( \text_2 \text ) - \Delta_\text H^\ominus (\text_4). Solving for the standard of enthalpy of formation, :\Delta_\text H^\ominus (\text_4) = \Delta_\text H^\ominus (\text_2) + 2 \Delta_\text H^\ominus (\text_2 \text)- \Delta_\text H^\ominus (\text_4). The value of 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, methane 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, ) ''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: :\Delta_ H^ = \sum \nu \Delta_ H^(\text) - \sum \nu \Delta_ H^(\text). If the standard enthalpy of the products is less than the standard enthalpy of the reactants, the standard enthalpy of reaction is negative. This implies that the reaction is exothermic. The converse is also true; the standard enthalpy of reaction is positive for an endothermic reaction. This calculation has a tacit assumption of ideal solution between reactants and products where the
enthalpy of mixing In thermodynamics, the enthalpy of mixing (also heat of mixing and excess enthalpy) is the enthalpy liberated or absorbed from a substance upon mixing. When a substance or compound is combined with any other substance or compound, the enthalpy o ...
is zero. For example, for the combustion of methane, CH4 + 2O2 -> CO2 + 2H2O: :\Delta_ H^ = Delta_ H^(\text_2) + 2\Delta_ H^ (\text_2 \text)- \Delta_ H^(\text_4) + 2\Delta_ H^(\text_2)]. However O2 is an element in its standard state, so that \Delta_ H^(\text_2) = 0, and the heat of reaction is simplified to :\Delta_ H^ = Delta_ H^(\text_2) + 2\Delta_ H^ (\text_2 \text)- \Delta_ H^(\text_4), which is the equation in the previous section for the enthalpy of combustion \Delta_H^.


Key concepts for doing enthalpy calculations

# When a reaction is reversed, the magnitude of Δ''H'' stays the same, but the sign changes. # When the balanced equation for a reaction is multiplied by an integer, the corresponding value of Δ''H'' 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. Allotropy, 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 298.15 K and 1 atm


Inorganic substances


Aliphatic hydrocarbons


Other organic compounds


See also

*
Calorimetry In chemistry and thermodynamics, calorimetry () is the science or act of measuring changes in ''state variables'' of a body for the purpose of deriving the heat transfer associated with changes of its state due, for example, to chemical reac ...
*
Enthalpy Enthalpy , a property of a thermodynamic system, is the sum of the system's internal energy and the product of its pressure and volume. It is a state function used in many measurements in chemical, biological, and physical systems at a constant ...
* Heat of combustion *
Thermochemistry Thermochemistry is the study of the heat energy which is associated with chemical reactions and/or phase changes such as melting and boiling. A reaction may release or absorb energy, and a phase change may do the same. Thermochemistry focuses on ...


References

* {{Cite book, last1=Zumdahl, first1= Steven , year=2009, title= Chemical Principles, edition= 6th , pages=384–387, publisher= Houghton Mifflin, location= Boston. New York, isbn= 978-0-547-19626-8


External links


NIST Chemistry WebBook
Enthalpy Thermochemistry de:Enthalpie#Standardbildungsenthalpie