Standard enthalpy change of formation

The standard enthalpy of formation or standard heat of formation of a compound is the change of enthalpy

Enthalpy

Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.Enthalpy is a...

 that accompanies the formation of 1 mole

Mole (unit)

The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...

 of a substance in its standard state

Standard state

In chemistry, the standard state of a material is a reference point used to calculate its properties under different conditions. In principle, the choice of standard state is arbitrary, although the International Union of Pure and Applied Chemistry recommends a conventional set of standard states...

 from its constituent elements in their standard state

Standard state

In chemistry, the standard state of a material is a reference point used to calculate its properties under different conditions. In principle, the choice of standard state is arbitrary, although the International Union of Pure and Applied Chemistry recommends a conventional set of standard states...

s (the most stable form of the element at 1 bar

Bar (unit)

The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...

 of pressure and the specified temperature, usually 298.15 K or 25 degrees Celsius). Its symbol is ΔH_f^O or Δ_fH^O.

For example, the standard enthalpy of formation of carbon dioxide

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 would be the enthalpy of the following reaction under the conditions above:

C_(s,graphite) + O_2(g) → CO_2(g)

The standard enthalpy of formation is measured in units of energy per amount of substance. Most are defined in kilojoules per mole

Mole (unit)

The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...

 (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

Physics

Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...

 the energy per particle is often expressed in electronvolt

Electronvolt

In physics, the electron volt is a unit of energy equal to approximately joule . By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt...

s which corresponds to about 100 kJ mol⁻¹.

All elements in their standard states (oxygen

Oxygen

Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 gas, solid carbon

Carbon

Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

 in the form of graphite

Graphite

The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...

, etc.) have a standard enthalpy of formation of zero, as there is no change involved in their formation.

Mechanics

The standard enthalpy of formation is equivalent to the sum of many separate processes included in the Born-Haber cycle

Born-Haber cycle

The Born–Haber cycle is an approach to analyzing reaction energies. It was named after and developed by the two German scientists Max Born and Fritz Haber....

 of synthesis reactions. For example, to calculate the standard enthalpy of formation of sodium chloride

Sodium chloride

Sodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...

,
we use the following reaction:

Na_(s) + (1/2)Cl_2(g) → NaCl_(s)

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

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

Hess's law

Hess' law is a relationship in physical chemistry named for Germain Hess, a Swiss-born Russian chemist and physician.The law states that the enthalpy change for a reaction that is carried out in a series of steps is equal to the sum of the enthalpy changes for the individual steps.The law is an...

 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

Hess's law

Hess' law is a relationship in physical chemistry named for Germain Hess, a Swiss-born Russian chemist and physician.The law states that the enthalpy change for a reaction that is carried out in a series of steps is equal to the sum of the enthalpy changes for the individual steps.The law is an...

. That it is negative shows that the reaction, if it were to proceed, would be exothermic

Exothermic

In thermodynamics, the term exothermic describes a process or reaction that releases energy from the system, usually in the form of heat, but also in the form of light , electricity , or sound...

; 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

Heat of formation group additivity

Heat of formation group additivity methods in thermochemistry enable the calculation and prediction of heat of formation of organic compounds based on additivity. This method was pioneered by S. W...

 method.

Standard enthalpy of reaction

The standard enthalpy of formation is used in thermochemistry

Thermochemistry

Thermochemistry is the study of the energy and heat associated with chemical reactions and/or physical transformations. A reaction may release or absorb energy, and a phase change may do the same, such as in melting and boiling. Thermochemistry focuses on these energy changes, particularly on the...

 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 × ΔH_f°) (products) - Σ(ν × ΔH_f°) (reactants)

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

ΔH_r° = [(1 × ΔH_f°_(CO2)) + (2 × ΔH_f°_(H2O))] (products) - [(1 × ΔH_f°_(CH4)) + (2 × ΔH_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.

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
  Enthalpy of sublimation
  The enthalpy of sublimation, or heat of sublimation, is defined as the heat required to sublime one mole of the substance at a given combination of temperature and pressure, usually standard temperature and pressure...
  
  , 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
  Saturation (chemistry)
  In chemistry, saturation has six different meanings, all based on reaching a maximum capacity...
  
   compound reacts with an excess of hydrogen to become fully saturated under standard conditions, as previously defined.

Examples: Inorganic compounds (at 25 °C)

Chemical Compound	Phase (matter) Phase (matter) In the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, and chemical composition...	Chemical formula Chemical formula A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....	Δ Hf0 in kJ/mol
Ammonia Ammonia Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...  (Ammonium Hydroxide)	aq	NH3 (NH4OH)	-80.8
Ammonia	g	NH3	-46.1
Copper (II) sulfate	aq	CuSO4	-769.98
Sodium carbonate Sodium carbonate Sodium carbonate , Na2CO3 is a sodium salt of carbonic acid. It most commonly occurs as a crystalline heptahydrate, which readily effloresces to form a white powder, the monohydrate. Sodium carbonate is domestically well-known for its everyday use as a water softener. It can be extracted from the...	s	Na2CO3	-1131
Sodium chloride Sodium chloride Sodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...  (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 Sodium nitrate Sodium nitrate is the chemical compound with the formula NaNO3. This salt, also known as Chile saltpeter or Peru saltpeter to distinguish it from ordinary saltpeter, potassium nitrate, is a white solid which is very soluble in water...	aq	NaNO3	-446.2
Sodium nitrate	s	NaNO3	-424.8
Sulfur dioxide Sulfur dioxide Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...	g	SO2	-297
Sulfuric acid Sulfuric acid Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...	l	H2SO4	-814
Silica	s	SiO2	-911
Nitrogen dioxide Nitrogen dioxide Nitrogen dioxide is the chemical compound with the formula it is one of several nitrogen oxides. is an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced each year. This reddish-brown toxic gas has a characteristic sharp, biting odor and is a prominent...	g	NO2	+33
Nitrogen monoxide	g	NO	+90
Water Water (data page) This page provides supplementary data of the properties of water.Further comprehensive authoritative data can be found at the page on thermophysical properties of fluids.-Structure and properties:-Thermodynamic properties:-Liquid physical properties:...	l	H2O	-285.8
Water	g	H2O	-241.82
Carbon dioxide Carbon dioxide Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...	g	CO2	-393.5
Hydrogen Hydrogen Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...	g	H2	0
Fluorine Fluorine Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...	g	F2	0
Chlorine Chlorine Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...	g	Cl2	0
Bromine Bromine Bromine ") is a chemical element with the symbol Br, an atomic number of 35, and an atomic mass of 79.904. It is in the halogen element group. The element was isolated independently by two chemists, Carl Jacob Löwig and Antoine Jerome Balard, in 1825–1826...	l	Br2	0
Bromine	g	Br2	+31
Iodine Iodine Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....	s	I2	0
Iodine	g	I2	+62
Zinc sulfate	aq	ZnSO4	-980.14

See also

• Thermochemistry
  Thermochemistry
  Thermochemistry is the study of the energy and heat associated with chemical reactions and/or physical transformations. A reaction may release or absorb energy, and a phase change may do the same, such as in melting and boiling. Thermochemistry focuses on these energy changes, particularly on the...
  
  
• Enthalpy
  Enthalpy
  Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.Enthalpy is a...
  
  
• Calorimetry
  Calorimetry
  Calorimetry is the science of measuring the heat of chemical reactions or physical changes. Calorimetry is performed with a calorimeter. The word calorimetry is derived from the Latin word calor, meaning heat...
  
  
• Standard enthalpy change of formation (data table)
  Standard enthalpy change of formation (data table)
  These tables include heat of formation data gathered from a variety of sources, including the primary and secondary literature, as well as the NIST Chemistry WebBook...