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Encyclopedia > Standard enthalpy change of formation

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 atmosphere of pressure and the specified temperature, usually 298 K or 25 degrees Celsius). Its symbol is ΔHfO. Enthalpy (symbolized H, also called heat content) is the sum of the internal energy of matter and the product of its volume multiplied by the pressure. ... The mole (symbol: mol) is the SI term identifying the number of particles in a given amount of matter. ... In chemistry, the standard state of a material is its state at 1 bar (100 kilopascals) and 25 degrees Celsius (298. ... In chemistry, the standard state of a material is its state at 1 bar (100 kilopascals) and 25 degrees Celsius (298. ...

The standard enthalpy change of formation is measured in units of energy per amount of substance. Most are defined in kilojoules per mole, or 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) 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 mole (symbol: mol) is the SI term identifying the number of particles in a given amount of matter. ... A calorie refers to a unit of energy. ... The joule (symbol J, also called newton metre, or coulomb volt) is the SI unit of energy and work. ... For other meanings of gram, see gram (disambiguation). ... For other meanings of gram, see gram (disambiguation). ... General Name, Symbol, Number oxygen, O, 8 Chemical series Chalcogens Group, Period, Block 16, 2, p Appearance colorless Atomic mass 15. ... Graphite (named by Abraham Gottlob Werner in 1789, from the Greek γραφειν: to draw/write, for its use in pencils) is one of the allotropes of carbon. ...

The standard enthalpy change 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 from the sum of the standard enthalpies of formation of the products, as shown in the equation below.

ΔHreactionO = ΣΔHfO (Products) - ΣΔHfO (Reactants)

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: The Born-Haber cycle is a famous cycle of chemical reactions, first devleoped by Max Born and the German chemist Fritz Haber in 1917. ... Flash point Non-flammable R/S statement R: none S: none RTECS number VZ4725000 Supplementary data page Structure and properties n, Îµr, etc. ...

Na(s) + (1/2)Cl2(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. The enthalpy of atomization is is the enthalpy change that accompanies the total separation of all atoms in a chemical substance (either an elemental substance or a compound). ... The first ionization energy is the amount of energy required to remove a single electron from the outermost energy level of a neutral atom. ...

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. Hesss Law Hesss Law is used to predict the enthalpy change for a reaction Î”Hr in enthalpy changes. ...

This leads us to the point that at times chemists may define a standard enthalpy of formation for a reaction that can be (and often is) hypothetical. For instance we cannot combine carbon and hydrogen in the laboratory to make methane, yet we define the standard enthalpy of formation for methane as -74.8 kJ mol-1. The fact that this value is neagative shows that methane is an exothermic compound, that is, it is more stable than the elements which it contains. Results from FactBites:

 enthalpy - definition of enthalpy - Labor Law Talk Dictionary (548 words) Enthalpy is a quantifiable state function, and the total enthalpy of a system cannot be measured directly; the enthalpy change of a system is measured instead. Enthalpy is most useful when pressure is held constant through exposure to the surroundings, to analyse reactions that increase the volume of the system, causing it to do mechanical work on the surroundings and lose energy. For an exothermic reaction at constant pressure, the system's change in enthalpy is equal to the energy released in the reaction, including the energy retained in the system and that lost through expansion against the surroundings.
 ENTHALPY - GoGoSearch.com (1030 words) 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... Enthalpy is a quantifiable state function, and the total enthalpy of a system cannot be measured directly; the enthalpy change of a system is measured instead.
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