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## Enthalpy
The term
## HistoryOver the history of thermodynamics, several terms have been used to denote what is now known as the ## Original definitionThis is the heat change which occurs when 1 mol of a substance reacts completely with oxygen to form products at 298 K and 1 atm.
The function where where (all units given in SI) *H*is the enthalpy (joules)*U*is the internal energy, (joules)*p*is the pressure of the system, (pascals)*V*is the volume, (cubic metres)
## Application and extended formula## OverviewIn terms of thermodynamics, enthalpy can be calculated by determining the requirements for creating a system from "nothingness"; the mechanical work required, Internal energy, This process is calculated within enthalpy calculations as Therefore, the change in enthalpy can be devised or represented without the need for compressive or expansive mechanics; for a simple system, with a constant number of particles, The term ## RelationshipsAs an expansion of the first law of thermodynamics, enthalpy can be related to several other thermodynamic formulae. As with the original definition of the first law; - Where, as defined by the law;
- d
*U*represents the infinitesimal increase of the systematic or internal energy. - δ
*Q*represents the infinitesimal amount of energy attributed or added to the system. - δ
*W*represents the infinitesimal amount of energy acted out by the system on the surroundings.
As a differentiating expression, the value of H can be defined as Where
For a process that is not reversible, the second law of thermodynamics states that the increase in heat δ It is seen that, if a thermodynamic process is isobaric (i.e., occurs at constant pressure), then d The difference in enthalpy is the maximum thermal energy attainable from the system in an isobaric process. This explains why it is sometimes called the If, in addition, the entropy is held constant as well, i.e., d with the equality holding at equilibrium. It is seen that the enthalpy for a general system will continuously decrease to its minimum value, which it maintains at equilibrium. In a more general form, the first law describes the internal energy with additional terms involving the chemical potential and the number of particles of various types. The differential statement for d where μ ## Heats of reactionThe total enthalpy of a system cannot be measured directly; the where - Δ
*H*is the*enthalpy change*
*H*_{final}is the final enthalpy of the system, measured in joules. In a chemical reaction,*H*_{final}is the enthalpy of the products.
*H*_{initial}is the initial enthalpy of the system, measured in joules. In a chemical reaction,*H*_{initial}is the enthalpy of the reactants.
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 lost through expansion against its surroundings. In a similar manner, for an endothermic reaction, the system's change in enthalpy is equal to the energy Although enthalpy is commonly used in engineering and science, it is impossible to measure directly, as enthalpy has no datum (reference point). Therefore enthalpy can only accurately be used in a closed system. However, few real world applications exist in closed isolation, and it is for this reason that two or more closed systems cannot be compared using enthalpy as a basis, although sometimes this is done erroneously. ## Open systemsIn thermodynamic open systems, matter may flow in and out of the system boundaries. The first law of thermodynamics for open systems states: where U is the average internal energy leaving the system
The region of space enclosed by open system boundaries is usually called a control volume, and it may or may not correspond to physical walls. If we choose the shape of the control volume such that all flow in or out occurs perpendicular to its surface, then the flow of matter into the system performs work as if it were a piston of fluid pushing mass into the system, and the system performs work on the flow of matter out as if it were driving a piston of fluid. There are then two types of work performed: _{out}flow work described above which is performed on the fluid (this is also often called ) and pV workshaft work which may be performed on some mechanical device.
These two types of work are expressed in the equation: Substitution into the equation above for the control volume The definition of enthalpy, During steady-state operation of a device ( This expression is described by the diagram above. ## Standard enthalpy changes## Definitions
A common Other types of standard enthalpy change include combustion (standard enthalpy change of combustion), neutralisation (standard enthalpy change of neutralisation), melting (standard enthalpy change of fusion), vaporisation/condensation (standard enthalpy change of vaporisation), atomisation (standard enthalpy change of atomisation), mixing (standard enthalpy change of mixing), dissolution (standard enthalpy change of solution), and denaturation (standard enthalpy change of denaturation). ## Examples: Inorganic compounds (at 25 °C)
- (State: g = gaseous; l = liquid; s = solid; aq = aqueous)
## Specific enthalpyThe specific enthalpy of a working mass is a property of that mass used in thermodynamics, defined as where ## Notes- ^
^{a}^{b}*The World of Chemistry*noted that whilst ruminating on the origin being credited to Gibbs, the original word was created by Onnes, who had specified its derivation. **^***Physical Chemistry: An Advanced Treatise*States that the original creator of the word was Josiah Willard Gibbs, who noted "*the familiar definition of enthalpy as introduced by Gibbs in 1875 (‘heat function for constant pressure’)”*"**^***The Collected Works of J. Willard Gibbs, Vol. I*do not contain reference to the word enthalpy, but rather reference the heat function for constant pressure.
## References- Haase, R. In
*Physical Chemistry: An Advanced Treatise*; Jost, W., Ed.; Academic: New York, 1971; p 29. - Gibbs, J. W. In
*The Collected Works of J. Willard Gibbs, Vol. I*; Yale University Press: New Haven, CT, reprinted 1948; p 88. - Laidler, K.
*The World of Physical Chemistry*; Oxford University Press: Oxford, 1995; p 110. - C.Kittel, H.Kroemer In
*Thermal Physics*; W.H.Freeman and Company, New York, 1980; p246
## See also- Calorimetry
- Calorimeter
- Departure function
- Hess's law
- Isenthalpic process
- Thermodynamic databases for pure substances
Categories: Thermodynamics | Enthalpy |
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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Enthalpy". A list of authors is available in Wikipedia. |