|
t In thermodynamics and molecular chemistry, the enthalpy or heat content (denoted as H or ΔH, or rarely as χ) is a quotient or description of thermodynamic potential of a system, which can be used to calculate the "useful" work obtainable from a closed thermodynamic system under constant pressure. Thermodynamics (from the Greek θεÏμη, therme, meaning heat and δυναμις, dunamis, meaning power) is a branch of physics that studies the effects of changes in temperature, pressure, and volume on physical systems at the macroscopic scale by analyzing the collective motion of their particles using statistics. ...
It has been suggested that the central science be merged into this article or section. ...
In thermodynamics, four quantities, measured in units of energy, are called thermodynamic potentials: where T = temperature, S = entropy, p = pressure, V = volume Differential definitions The following differential relations hold for the four potentials: If we write the above four equations generally as Then it is seen that yielding expressions for...
Thermodynamics (Greek: thermos = heat and dynamic = change) is the physics of energy, heat, work, entropy and the spontaneity of processes. ...
In thermodynamics, a closed system, as contrasted with an isolated system, can exchange heat and work, but not matter, with its surroundings. ...
Thermodynamics (Greek: thermos = heat and dynamic = change) is the physics of energy, heat, work, entropy and the spontaneity of processes. ...
The term enthalpy is composed of the prefix en-, meaning to "put into", plus the Greek word -thalpein, meaning "to heat", although the original definition is thought to have stemmed from the word, "enthalpos" (ἐνθάλπος).[1] It is often calculated as a differential sum, describing the changes within exo- and endothermic reactions, which minimize at equilibrium. In thermodynamics, the word exothermic describes a process or reaction that releases energy in the form of heat. ...
In thermodynamics, the word endothermic describes a process or reaction that absorbs energy in the form of heat. ...
In physics, heat, symbolized by Q, is defined as transfer of thermal energy [1] Generally, heat is a form of energy transfer associated with the different motions of atoms, molecules and other particles that comprise matter when it is hot and when it is cold. ...
History Over the history of thermodynamics, several symbols have been used to denote what is now known as the enthalpy of a system. Originally, it is thought that the word "enthalpy" was created by Benoit Paul Émile Clapeyron and Rudolf Clausius through the publishing of the Clausius-Clapeyron relation in "The Mollier Steam Tables and Diagrams" in 1927, but it was later published that the earliest recording of the word was in 1875, by Josiah Willard Gibbs in the publication "Physical Chemistry: an Advanced Treatise"[2], although it is not referenced in Gibbs' works directly[3]. In 1909, Keith Landler discussed Gibbs' work on the 'heat function for constant pressure' and noted that Heike Kamerlingh Onnes had coined its modern name from the Greek word "enthalpos" (ενθαλπος) meaning "to put heat into." [1] Savery Engine [1698] The history of thermodynamics is a core strand in the history of physics and an important one in the history of science. ...
Benoit Paul Émile Clapeyron Benoit Paul Émile Clapeyron (February 26, 1799 - January 28, 1864) was a French engineer and physicist, one of the founders of thermodynamics. ...
Rudolf Clausius - physicist and mathematician Rudolf Julius Emanuel Clausius (January 2, 1822 – August 24, 1888), was a German physicist and mathematician. ...
The Clausius-Clapeyron relation, in thermodynamics, is a way of characterizing the phase transition between two states of matter, such as solid and liquid. ...
Year 1927 (MCMXXVII) was a common year starting on Saturday (link will display full calendar) of the Gregorian calendar. ...
1875 (MDCCCLXXV) was a common year starting on Friday (see link for calendar). ...
Josiah Willard Gibbs (February 11, 1839 New Haven – April 28, 1903 New Haven) was one of the very first American theoretical physicists and chemists. ...
1909 (MCMIX) was a common year starting on Friday (see link for calendar). ...
Heike Kamerlingh Onnes (September 21, 1853 – February 21, 1926) was a Dutch physicist. ...
Original Definition This is the heat change which occurs when 1 mol of a substance reacts completely with oxygen to form products at 298K and 1 atm. The function H was introduced by the Dutch physicist Heike Kamerlingh Onnes in early 20th century in the following form: Heike Kamerlingh Onnes (September 21, 1853 – February 21, 1926) was a Dutch physicist. ...
 where E represents the energy of the system. In the absence of an external field, the enthalpy may be defined, as it is generally known, by:  but can also be surmised into one formula as;  where (all units given in SI) Look up si, Si, SI in Wiktionary, the free dictionary. ...
The joule (IPA pronunciation: or ) (symbol: J) is the SI unit of energy. ...
In thermodynamics, the internal energy of a thermodynamic system, or a body with well-defined boundaries, denoted by U, or sometimes E, is the total of the kinetic energy due to the motion of molecules (translational, rotational, vibrational) and the potential energy associated with the vibrational and electric energy of...
The use of water pressure - the Captain Cook Memorial Jet in Lake Burley Griffin in Canberra, Australia. ...
The pascal (symbol: Pa) is the SI derived unit of pressure or stress (also: Youngs modulus and tensile strength). ...
The volume of a solid object is the three-dimensional concept of how much space it occupies, often quantified numerically. ...
The cubic meter (symbol m³) is the SI derived unit of volume. ...
Application and extended formula
Overview In terms of thermodynamics, enthalpy can be calculated by determining the requirements for creating a system from "nothingness"; the mechanical work required, PV differs, based upon the constance of conditions present at the creation of the thermodynamic system. Thermodynamics (Greek: thermos = heat and dynamic = change) is the physics of energy, heat, work, entropy and the spontaneity of processes. ...
Internal energy, U, must be supplied to remove particles from a surrounding in order to allow space for the creation of a system, providing that environmental variables, such as pressure (p) remain constant. This internal energy also includes the energy required for activation and the breaking of bonded compounds into gaseous species. The sparks generated by striking steel against a flint provide the activation energy to initiate combustion in this Bunsen burner. ...
This process is calculated within enthalpy calculations as U + PV, to label the amount of energy or work required to "set aside space for" and "create" the system; describing the work done by both the reaction or formation of systems, and the surroundings. For systems at constant pressure, the change in enthalpy is the heat received by the system plus the non-mechanical work that has been done.
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 difference in enthalpy is the maximum amount of thermal energy derivable from a thermodynamic process in which the pressure is held constant.
Relationships As 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; The first law of thermodynamics, a generalized expression of the law of the conservation of energy, states: // Description Essentially, the First Law of Thermodynamics declares that energy is conserved for a closed system, with heat and work being the forms of energy transfer. ...
 - Where, as defined by the law;
- dU represents the infinitesimal increase of the systematic or internal energy.
- δQ represents the infinitesimal amount of energy attributed or added to the system.
- δW 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 Thermodynamics (Greek: thermos = heat and dynamic = change) is the physics of energy, heat, work, entropy and the spontaneity of processes. ...
   Where | | | | | - δW = pdV is the work done by the system in a reversible process.
| - dS is the increase in entropy (joules per kelvin),
| - P is the constant pressure
| | | - T is the temperature (kelvins)
| For a process that is not reversible, the second law of thermodynamics states that the increase in heat δQ is less than or equal to the product TdS of temperature T and the increase in entropy dS; thus In physics, an inexact differential, as contrasted with an exact differential, of a function f is denoted: ; as is true of point functions. ...
In thermodynamics, the internal energy of a thermodynamic system, or a body with well-defined boundaries, denoted by U, or sometimes E, is the total of the kinetic energy due to the motion of molecules (translational, rotational, vibrational) and the potential energy associated with the vibrational and electric energy of...
In thermodynamics, a reversible process (or reversible cycle if the process is cyclic) is a process that can be reversed by means of infinitesimal changes in some property of the system. ...
Ice melting - classic example of entropy increasing[1] described in 1862 by Rudolf Clausius as an increase in the disgregation of the molecules of the body of ice. ...
In mathematics, an infinitesimal, or infinitely small number, is a number that is smaller in absolute value than any positive real number. ...
Ice melting - classic example of entropy increasing[1] described in 1862 by Rudolf Clausius as an increase in the disgregation of the molecules of the body of ice. ...
 It is seen that, if a thermodynamic process is isobaric (i.e., occurs at constant pressure), then dP is zero and thus isobaric (meaning of the same weight or pressure) may refer to: in thermodynamics, an isobaric process, i. ...
 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 heat content. That is, the integral of dH over any isobar in state space is the maximum thermal energy attainable from the system.
If, in addition, the entropy is held constant as well, i.e., dS = 0, the above equation becomes: Ice melting - classic example of entropy increasing[1] described in 1862 by Rudolf Clausius as an increase in the disgregation of the molecules of the body of ice. ...
 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 equilbrium.
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 dH is then: In thermodynamics and chemistry, chemical potential, symbolized by μ, is a term introduced in 1876 by the American mathematical physicist Willard Gibbs, which he defined as follows: Gibbs noted also that for the purposes of this definition, any chemical element or combination of elements in given proportions may be considered a...
 where μi is the chemical potential for an i-type particle, and Ni is the number of such particles. It is seen that, not only must the Vdp term be set to zero by requiring the pressures of the initial and final states to be the same, but the μidNi terms must be zero as well, by requiring that the particle numbers remain unchanged. Any further generalization will add even more terms whose extensive differential term must be set to zero in order for the interpretation of the enthalpy to hold.
Heats 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: In thermodynamics, a thermodynamic system is defined as that part of the universe that is under consideration. ...
 where - ΔH is the enthalpy change
- Hfinal is the final enthalpy of the system, measured in joules. In a chemical reaction, Hfinal is the enthalpy of the products.
- Hinitial is the initial enthalpy of the system, measured in joules. In a chemical reaction, Hinitial 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 absorbed in the reaction, including the energy lost by the system and gained from compression from its surroundings. A relatively easy way to determine whether or not a reaction is exothermic or endothermic is to determine the sign of ΔH. If ΔH 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. The product of an endothermic reaction will be cold to the touch. On the other hand if ΔH is negative, the reaction is exothermic, that is the overall decrease in enthalpy is achieved by the generation of heat. The product of an exothermic reaction will be warm to the touch. In chemistry, an exothermic reaction is one that releases heat . ...
Vapours of hydrogen chloride in a beaker and ammonia in a test tube meet to form a cloud of a new substance, ammonium chloride A chemical reaction is a process that results in the interconversion of chemical substances. ...
The use of water pressure - the Captain Cook Memorial Jet in Lake Burley Griffin in Canberra, Australia. ...
In thermodynamics, the word endothermic describes a process or reaction that absorbs energy in the form of heat. ...
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. In thermodynamics, a closed system, as contrasted with an isolated system, can exchange heat and work, but not matter, with its surroundings. ...
Open systems In thermodynamic open systems, matter 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 matter flowing in and by heating, minus the amount lost by matter flowing out and in the form of work done by the system. The first law for open systems is given by: Thermodynamics (Greek: thermos = heat and dynamic = change) is the physics of energy, heat, work, entropy and the spontaneity of processes. ...
In thermodynamics, an open system is one whose border is permeable to both energy and mass. ...
 where Uin is the average internal energy entering the system and Uout is the average internal energy leaving the system
 During steady, continuous operation, an energy balance applied to an open system equates shaft work performed by the system to heat added plus net enthalpy added. 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: flow work described above which is performed on the fluid (this is also often called PV work) and shaft work which may be performed on some mechanical device. Image File history File links Download high-resolution version (946x652, 7 KB) Rasterized First_law_open_system. ...
Image File history File links Download high-resolution version (946x652, 7 KB) Rasterized First_law_open_system. ...
An unit operation is considered to be at steady-state with respect to an operation variable if that variable does not change with time. ...
It has been suggested that this article or section be merged with Control volume. ...
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, H, permits us to use this thermodynamic potential to account for both internal energy and PV work in fluids for open systems: In thermodynamics, four quantities, measured in units of energy, are called thermodynamic potentials: where T = temperature, S = entropy, p = pressure, V = volume Differential definitions The following differential relations hold for the four potentials: If we write the above four equations generally as Then it is seen that yielding expressions for...
 During steady-state operation of a device (see turbine, pump, and engine), the expression above may be set equal to zero. This yields a useful expression for the power generation or requirement for these devices in the absence of chemical reactions: An unit operation is considered to be at steady-state with respect to an operation variable if that variable does not change with time. ...
A Siemens steam turbine with the case opened. ...
An electrically driven pump (electropump) for waterworks near the Hengsteysee, Germany. ...
An engine is something that produces an effect from a given input. ...
In physics, power (symbol: P) is the rate at which work is performed or energy is transferred. ...
 This expression is described by the diagram above.
Standard enthalpy changes
Definitions Standard enthalpy change of combustion Standard enthalpy of Combustion is the Enthalpy change when one mole of a substance is totally combusted in oxygen, and is measured at 298K and 1 atmospheric pressure. ...
| “ | Standard enthalpy of combustion is defined as the enthalpy change observed in a constituent thermodynamic system when 1 mole of a substance reacts completely with oxygen under standard conditions. | ” | Standard enthalpy change of hydrogenation Look up mole in Wiktionary, the free dictionary. ...
Temperature and air pressure can vary from one place to another on the Earth, and can also vary in the same place with time. ...
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 100 kPa...
| “ | Standard enthalpy of hydrogenation is the enthalpy change observed in a constituent thermodynamic system, when one mole of an unsaturated compound reacts completely with an excess of hydrogen under standard conditions to form a saturated compound. | ” | 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...
| “ | Standard enthalpy change of formation is defined as the enthalpy change observed in a constituent thermodynamic system when a compound is formed from its elementary antecedents under standard conditions. | ” | The standard enthalpy change of reaction (denoted H° or Ho) is the enthalpy change that occurs in a system when 1 equivalent of matter is transformed by a chemical reaction under standard conditions. Vapours of hydrogen chloride in a beaker and ammonia in a test tube meet to form a cloud of a new substance, ammonium chloride A chemical reaction is a process that results in the interconversion of chemical substances. ...
Temperature and air pressure can vary from one place to another on the Earth, and can also vary in the same place with time. ...
A common standard enthalpy change is the standard enthalpy change of formation, which has been determined for a vast number of substances. The enthalpy change of any reaction under any conditions can be computed, given the standard enthalpy change of formation of all of the reactants and products. Other reactions with standard enthalpy change values include combustion (standard enthalpy change of combustion) and neutralisation (standard enthalpy change of neutralisation). 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...
Standard enthalpy of Combustion is the Enthalpy change when one mole of a substance is totally combusted in oxygen, and is measured at 298K and 1 atmospheric pressure. ...
The standard enthalpy change 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(298k and 1atm), i. ...
Examples: Inorganic compounds (at 25 °C) - (State: g - gaseous; l - liquid; s - solid; aq = aqueous)
In the physical sciences, a phase is a set of states of a macroscopic physical system that have relatively uniform chemical composition and physical properties (i. ...
This article or section does not cite any references or sources. ...
Ammonia is a compound with the formula NH3. ...
Sodium carbonate (also known as washing soda or soda ash), Na2CO3, is a sodium salt of carbonic acid. ...
Jordanian and Israeli salt evaporation ponds at the south end of the Dead Sea Sodium chloride, also known as common salt, table salt, or halite, is a chemical compound with the formula NaCl. ...
Sodium hydroxide (NaOH), also known as lye or caustic soda, is a caustic metallic base. ...
Made of Porn and sex things Inhalation respiratory irritation Skin May cause irritation. ...
Sulfur dioxide (or Sulphur dioxide) has the chemical formula SO2. ...
Sulfuric (or sulphuric) acid, H2SO4, is a strong mineral acid. ...
The chemical compound silicon dioxide, also known as silica, is the oxide of silicon, chemical formula SiO2. ...
[1] R-phrases , S-phrases , , , , , Supplementary data page Structure and properties n, εr, etc. ...
The chemical compound nitric oxide is a gas with chemical formula NO. It is an important signaling molecule in the body of mammals including humans, one of the few gaseous signaling molecules known. ...
This page provides supplementary chemical data on water. ...
Carbon dioxide is a chemical compound composed of one carbon and two oxygen atoms. ...
General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ...
General Name, Symbol, Number fluorine, F, 9 Chemical series halogens Group, Period, Block 17, 2, p Appearance Yellowish brown gas Atomic mass 18. ...
General Name, Symbol, Number chlorine, Cl, 17 Chemical series halogens Group, Period, Block 17, 3, p Appearance yellowish green Standard atomic weight 35. ...
General Name, Symbol, Number bromine, Br, 35 Chemical series halogens Group, Period, Block 17, 4, p Appearance gas/liquid: red-brown solid: metallic luster Atomic mass 79. ...
Specific enthalpy The specific enthalpy of a working mass is a property of that mass used in thermodynamics, defined as  where u is the specific internal energy, p is the pressure, and v is specific volume. In other words, h = H / m where m is the mass of the system. The SI unit for specific enthalpy is joules per kilogram. Working mass is a mass against which a system operates in order to produce acceleration. ...
Thermodynamics (from the Greek θεÏμη, therme, meaning heat and δυναμις, dunamis, meaning power) is a branch of physics that studies the effects of changes in temperature, pressure, and volume on physical systems at the macroscopic scale by analyzing the collective motion of their particles using statistics. ...
The International System of Units (symbol: SI) (for the French phrase Système International dUnités) is the most widely used system of units. ...
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.
Josiah Willard Gibbs (February 11, 1839 New Haven – April 28, 1903 New Haven) was one of the very first American theoretical physicists and chemists. ...
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.
See also The world’s first ice-calorimeter, used in the winter of 1782-83, by Antoine Lavoisier and Pierre-Simon Laplace, to determine the heat evolved in various chemical changes; calculations which were based on Joseph Black’s prior discovery of latent heat. ...
A calorimeter is a device used for calorimetry, the science of measuring the heat of chemical reactions or physical changes as well as heat capacity. ...
In thermodynamics, a departure function is defined for any thermodynamic property as the difference between the property as computed for an ideal gas and the property of the species as it exists in the real world, for a specified temperature T and pressure P. Common departure functions include those for...
A visual representation of the law in action. ...
This does not adequately cite its references or sources. ...
Thermodynamic databases contain information about thermodynamic properties for substances, the most important being enthalpy, entropy, and Gibbs free energy. ...
External links |
Feeds |
Contact