Enthalpy

Enthalpy is a property of a thermodynamic system, and is defined as 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 pressure, that is conveniently provided by the large ambient atmosphere. The pressure–volume term expresses the work required to establish the system's physical dimensions, i.e. to make room for it by displacing its surroundings. As a state function, enthalpy depends only on the final configuration of internal energy, pressure, and volume, not on the path taken to achieve it.
The unit of measurement for enthalpy in the International System of Units (SI) is the joule. Other historical conventional units still in use include the calorie and the British thermal unit (BTU).
The total enthalpy of a system cannot be measured directly because the internal energy contains components that are unknown, not easily accessible, or are not of interest in thermodynamics. In practice, a change in enthalpy is the preferred expression for measurements at constant pressure, because it simplifies the description of energy transfer. When matter transfer into or out of the system is also prevented, at constant pressure the enthalpy change equals the energy exchanged with the environment by heat. 
In chemistry, the standard enthalpy of reaction is the enthalpy change when reactants in their standard states (, ) change to products in their standard states. This quantity is the standard heat of reaction at constant pressure and temperature, but it can be measured by calorimetric methods in which the temperature does vary, provided that the initial and final pressure and temperature correspond to the standard state. The value does not depend on the path from initial to final state since enthalpy is a state function.
Calibration of enthalpy changes requires a reference point. Enthalpies for chemical substances at constant pressure usually refer to standard state: most commonly pressure. Standard state does not strictly specify a temperature, but expressions for enthalpy generally reference the standard heat of formation at . For endothermic (heat-absorbing) processes, the change is a positive value; for exothermic (heat-releasing) processes it is negative.
The enthalpy of an ideal gas is independent of its pressure, and depends only on its temperature, which correlates to its internal energy. Real gases at common temperatures and pressures often closely approximate this behavior, which simplifies practical thermodynamic design and analysis.
Definition.
The enthalpy of a thermodynamic system is defined as the sum of its internal energy and the work required to achieve its pressure and volume:
where is pressure, and is the volume of the system.
Enthalpy is an extensive property; it is proportional to the size of the system (for homogeneous systems). As intensive properties, the specific enthalpy is referenced to a unit of mass of the system, and the molar enthalpy , where is the number of moles. For inhomogeneous systems the enthalpy is the sum of the enthalpies of the composing subsystems:
where
A closed system may lie in thermodynamic equilibrium in a static gravitational field, so that its pressure varies continuously with altitude, while, because of the equilibrium requirement, its temperature is invariant with altitude. (Correspondingly, the system's gravitational potential energy density also varies with altitude.) Then the enthalpy summation becomes an integral:
where
The enthalpy of a closed homogeneous system is its energy function , with natural state variables its entropy and its pressure . A differential relation for it can be derived as follows. We start from the first law of thermodynamics for closed systems for an infinitesimal process:
where
In a homogeneous system in which only reversible, or quasi-static, processes are considered, the second law of thermodynamics gives , with the absolute temperature and the infinitesimal change in entropy of the system. Furthermore, if only work is done, . As a result,
Adding to both sides of this expression gives
or
So
Other expressions.
The above expression of in terms of entropy and pressure may be unfamiliar to some readers. However, there are expressions in terms of more familiar variables such as temperature and pressure:
Here is the heat capacity at constant pressure and is the coefficient of (cubic) thermal expansion:
With this expression one can, in principle, determine the enthalpy if and are known as functions of and .
Note that for an ideal gas, , so that
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 then becomes
where is the chemical potential per particle for an -type particle, and is the number of such particles. The last term can also be written as (with the number of moles of component added to the system and, in this case, the molar chemical potential) or as (with the mass of component added to the system and, in this case, the specific chemical potential).
Characteristic functions.
The enthalpy, , expresses the thermodynamics of a system in the "energy representation". As a function of state, its arguments include both one intensive and several extensive state variables. The state variables , , and are said to be the "natural state variables" in this representation. They are suitable for describing processes in which they are determined by factors in the surroundings. For example, when a virtual parcel of atmospheric air moves to a different altitude, the pressure surrounding it changes, and the process is often so rapid that there is too little time for heat transfer. This is the basis of the so-called adiabatic approximation that is used in meteorology.
Conjugate with the enthalpy, with these arguments, the other characteristic function of state of a thermodynamic system is its entropy, as a function, , of the same list of variables of state, except that the entropy, , is replaced in the list by the enthalpy, . It expresses the "entropy representation". The state variables , , and are said to be the "natural state variables" in this representation. They are suitable for describing processes in which they are experimentally controlled. For example, and can be controlled by allowing heat transfer, and by varying only the external pressure on the piston that sets the volume of the system.
Physical interpretation.
The term can be interpreted as the energy required to create the system, and the term as the work that would be required to "make room" for the system if the pressure of the environment remained constant. When a system, for example, moles of a gas of volume at pressure and temperature , is created or brought to its present state from absolute zero, energy must be supplied equal to its internal energy plus , where is the work done in pushing against the ambient (atmospheric) pressure.
In basic physics and statistical mechanics it may be more interesting to study the internal properties of the system and therefore the internal energy is used. In basic chemistry, experiments are often conducted at constant atmospheric pressure, and the pressure–volume work represents an energy exchange with the atmosphere that cannot be accessed or controlled, so that is the expression chosen for the heat of reaction.
For a heat engine a change in its internal energy is the difference between the heat input and the pressure–volume work done by the working substance while a change in its enthalpy is the difference between the heat input and the work done by the engine:
where the work done by the engine is:
Relationship to heat.
In order to discuss the relation between the enthalpy increase and heat supply, we return to the first law for closed systems, with the physics sign convention: , where the heat is supplied by conduction, radiation, and Joule heating. We apply it to the special case with a constant pressure at the surface. In this case the work term can be split into two contributions, the so-called work, given by (where here is the pressure at the surface, is the increase of the volume of the system), and the so-called isochoric mechanical work , such as stirring by a shaft with paddles or by an externally driven magnetic field acting on an internal rotor. Cases of long range electromagnetic interaction require further state variables in their formulation, and are not considered here. So we write . In this case the first law reads:
Now,
So
With sign convention of physics, , because isochoric shaft work done by an external device on the system adds energy to the system, and may be viewed as virtually adding heat. The only thermodynamic mechanical work done by the system is expansion work, .
If the system is under constant pressure (). Consequently, the increase in enthalpy of the system is equal to the added heat and virtual heat:
This is why the now-obsolete term "heat content" was used in the 19th century.
Applications.
In thermodynamics, one can calculate enthalpy by determining the requirements for creating a system from "nothingness"; the mechanical work required, , differs based upon the conditions that obtain during the creation of the thermodynamic system.
Energy must be supplied to remove particles from the surroundings to make space for the creation of the system, assuming that the pressure remains constant; this is the term. The supplied energy must also provide the change in internal energy, , which includes activation energies, ionization energies, mixing energies, vaporization energies, chemical bond energies, and so forth. Together, these constitute the change in the enthalpy . For systems at constant pressure, with no external work done other than the work, the change in enthalpy is the heat received by the system.
For a simple system, with a constant number of particles, the difference in enthalpy is the maximum amount of thermal energy derivable from a thermodynamic process in which the pressure is held constant.
Heat of reaction.
The total enthalpy of a system cannot be measured directly; the "enthalpy change" of a system is measured instead. Enthalpy change is defined by the following equation:
where
For an exothermic reaction at constant pressure, the system's change in enthalpy equals 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 "absorbed" in the reaction, including the energy "lost by" the system and "gained" from compression from its surroundings. If is positive, the reaction is endothermic, that is heat is absorbed by the system due to the products of the reaction having a greater enthalpy than the reactants. On the other hand, if is negative, the reaction is exothermic, that is the overall decrease in enthalpy is achieved by the generation of heat.
From the definition of enthalpy as , the enthalpy change at constant pressure . However for most chemical reactions, the work term is much smaller than the internal energy change which is approximately equal to . As an example, for the combustion of carbon monoxide 2 CO(g) + O2(g) → 2 CO2(g), = −566.0 kJ and = −563.5 kJ. Since the differences are so small, reaction enthalpies are often loosely described as reaction energies and analyzed in terms of bond energies.
Specific enthalpy.
The specific enthalpy of a uniform system is defined as where is the mass of the system. The SI unit for specific enthalpy is joule per kilogram. It can be expressed in other specific quantities by , where is the specific internal energy, is the pressure, and is specific volume, which is equal to , where is the density.
Enthalpy changes.
An enthalpy change describes the change in enthalpy observed in the constituents of a thermodynamic system when undergoing a transformation or chemical reaction. It is the difference between the enthalpy after the process has completed, i.e. the enthalpy of the products, and the initial enthalpy of the system, namely the reactants. These processes are specified solely by their initial and final states, so that the enthalpy change for the reverse is the negative of that for the forward process.
A common standard enthalpy change is the enthalpy of formation, which has been determined for a large number of substances. Enthalpy changes are routinely measured and compiled in chemical and physical reference works, such as the CRC Handbook of Chemistry and Physics. The following is a selection of enthalpy changes commonly recognized in thermodynamics.
When used in these recognized terms the qualifier "change" is usually dropped and the property is simply termed "enthalpy of 'process"'. Since these properties are often used as reference values it is very common to quote them for a standardized set of environmental parameters, or standard conditions, including:
For such standardized values the name of the enthalpy is commonly prefixed with the term "standard", e.g. "standard enthalpy of formation".
Chemical properties:
Physical properties:
Open systems.
In thermodynamic open systems, mass (of substances) may flow in and out of the system boundaries. The first law of thermodynamics for open systems states: The increase in the internal energy of a system is equal to the amount of energy added to the system by mass flowing in and by heating, minus the amount lost by mass flowing out and in the form of work done by the system:
where is the average internal energy entering the system, and is the average internal energy leaving the system.
The region of space enclosed by the boundaries of the open system 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 mass 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 mass out as if it were driving a piston of fluid. There are then two types of work performed: "flow work" described above, which is performed on the fluid (this is also often called " work"), and "shaft 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 (cv) yields:
The definition of enthalpy, , permits us to use this thermodynamic potential to account for both internal energy and work in fluids for open systems:
If we allow also the system boundary to move (e.g. due to moving pistons), we get a rather general form of the first law for open systems. In terms of time derivatives it reads:
with sums over the various places where heat is supplied, mass flows into the system, and boundaries are moving. The terms represent enthalpy flows, which can be written as
with the mass flow and the molar flow at position respectively. The term represents the rate of change of the system volume at position that results in power done by the system. The parameter represents all other forms of power done by the system such as shaft power, but it can also be, say, electric power produced by an electrical power plant.
Note that the previous expression holds true only if the kinetic energy flow rate is conserved between system inlet and outlet. Otherwise, it has to be included in the enthalpy balance. During steady-state operation of a device ("see turbine, pump, and engine"), the average may be set equal to zero. This yields a useful expression for the average power generation for these devices in the absence of chemical reactions:
where the angle brackets denote time averages. The technical importance of the enthalpy is directly related to its presence in the first law for open systems, as formulated above.
Diagrams.
The enthalpy values of important substances can be obtained using commercial software. Practically all relevant material properties can be obtained either in tabular or in graphical form. There are many types of diagrams, such as diagrams, which give the specific enthalpy as function of temperature for various pressures, and diagrams, which give as function of for various . One of the most common diagrams is the temperature–specific entropy diagram ( diagram). It gives the melting curve and saturated liquid and vapor values together with isobars and isenthalps. These diagrams are powerful tools in the hands of the thermal engineer.
Some basic applications.
The points a through h in the figure play a role in the discussion in this section.
Points e and g are saturated liquids, and point h is a saturated gas.
Throttling.
One of the simple applications of the concept of enthalpy is the so-called throttling process, also known as Joule–Thomson expansion. It concerns a steady adiabatic flow of a fluid through a flow resistance (valve, porous plug, or any other type of flow resistance) as shown in the figure. This process is very important, since it is at the heart of domestic refrigerators, where it is responsible for the temperature drop between ambient temperature and the interior of the refrigerator. It is also the final stage in many types of liquefiers.
For a steady state flow regime, the enthalpy of the system (dotted rectangle) has to be constant. Hence
Since the mass flow is constant, the specific enthalpies at the two sides of the flow resistance are the same:
that is, the enthalpy per unit mass does not change during the throttling. The consequences of this relation can be demonstrated using the diagram above. Point c is at 200 bar and room temperature (300 K). A Joule–Thomson expansion from 200 bar to 1 bar follows a curve of constant enthalpy of roughly 425 kJ/kg (not shown in the diagram) lying between the 400 and 450 kJ/kg isenthalps and ends in point d, which is at a temperature of about 270 K. Hence the expansion from 200 bar to 1 bar cools nitrogen from 300 K to 270 K. In the valve, there is a lot of friction, and a lot of entropy is produced, but still the final temperature is below the starting value.
Point e is chosen so that it is on the saturated liquid line with = 100 kJ/kg. It corresponds roughly with = 13 bar and = 108 K. Throttling from this point to a pressure of 1 bar ends in the two-phase region (point f). This means that a mixture of gas and liquid leaves the throttling valve. Since the enthalpy is an extensive parameter, the enthalpy in f () is equal to the enthalpy in g () multiplied by the liquid fraction in f () plus the enthalpy in h () multiplied by the gas fraction in f . So
With numbers: , so = 0.64. This means that the mass fraction of the liquid in the liquid–gas mixture that leaves the throttling valve is 64%.
Compressors.
A power is applied e.g. as electrical power. If the compression is adiabatic, the gas temperature goes up. In the reversible case it would be at constant entropy, which corresponds with a vertical line in the diagram. For example, compressing nitrogen from 1 bar (point a) to 2 bar (point b) would result in a temperature increase from 300 K to 380 K. In order to let the compressed gas exit at ambient temperature , heat exchange, e.g. by cooling water, is necessary. In the ideal case the compression is isothermal. The average heat flow to the surroundings is . Since the system is in the steady state the first law gives
The minimal power needed for the compression is realized if the compression is reversible. In that case the second law of thermodynamics for open systems gives
Eliminating gives for the minimal power
For example, compressing 1 kg of nitrogen from 1 bar to 200 bar costs at least . With the data, obtained with the diagram, we find a value of 476 kJ/kg.
The relation for the power can be further simplified by writing it as
With , this results in the final relation
History.
The term "enthalpy" was coined relatively late in the history of thermodynamics, in the early 20th century. Energy was introduced in a modern sense by Thomas Young in 1802, while entropy was coined by Rudolf Clausius in 1865. "Energy" uses the root of the Greek word ("ergon"), meaning "work", to express the idea of capacity to perform work. "Entropy" uses the Greek word ("tropē") meaning "transformation". "Enthalpy" uses the root of the Greek word ("thalpos") "warmth, heat"
The term expresses the obsolete concept of "heat content", as refers to the amount of heat gained in a process at constant pressure only, but not in the general case when pressure is variable.
Josiah Willard Gibbs used the term "a heat function for constant pressure" for clarity.
Introduction of the concept of "heat content" is associated with Benoît Paul Émile Clapeyron and Rudolf Clausius (Clausius–Clapeyron relation, 1850).
The term "enthalpy" first appeared in print in 1909. It is attributed to Heike Kamerlingh Onnes, who most likely introduced it orally the year before, at the first meeting of the Institute of Refrigeration in Paris.
It gained currency only in the 1920s, notably with the "Mollier Steam Tables and Diagrams", published in 1927.
Until the 1920s, the symbol was used, somewhat inconsistently, for "heat" in general.
The definition of as strictly limited to enthalpy or "heat content at constant pressure" was formally proposed by Alfred W. Porter in 1922.