enthalpy.doc

7/28/2019 enthalpy.doc

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First Law of ThermodynamicsThe first law of thermodynamics is the application of the conservation ofenergy principle to heat and thermodynamic processes:

The first law makes use of the key concepts ofinternal energy,heat, andsystem work. It is used extensively in the discussion ofheat engines.

It is typical for chemistry texts to write the first law as U=Q+W. It is thesame law, of course - the thermodynamic expression of the conservation ofenergy principle. It is just that W is defined as the work done on thesystem instead of work done by the system. In the context of physics, the

common scenario is one of adding heat to a volume of gas and using theexpansion of that gas to do work, as in the pushing down of a piston in aninternal combustion engine. In the context of chemical reactions andprocess, it may be more common to deal with situations where work isdone on the system rather than by it.

Index

Heat

engine

concepts
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EnthalpyFour quantities called "thermodynamic potentials" are useful in the chemical

thermodynamics of reactions and non-cyclic processes. They are internal

energy, the enthalpy, the Helmholtz free energy and the Gibbs free energy.

Enthalpy is defined by

H = U + PVwhere P and V are the pressure and volume, and U is internal energy. Enthalpy

is then a precisely measurable state variable, since it is defined in terms of

three other precisely definable state variables. It is somewhat parallel to the

first law of thermodynamics for a constant pressure system

Q = U + PV since in this case Q=H

It is a useful quantity for tracking chemical reactions. If as a result of an

exothermic reaction some energy is released to a system, it has to show up in

some measurable form in terms of the state variables. An increase in the

enthalpy H = U + PV might be associated with an increase in internal energy

which could be measured by calorimetry, or with work done by the system, or

a combination of the two.

The internal energy U might be thought of as the energy required to create a

system in the absence of changes in temperature or volume. But if the process

changes the volume, as in a chemical reaction which produces a gaseous

product, then workmust be done to produce the change in volume. For a

constant pressure process the work you must do to produce a volume change

V is PV. Then the term PV can be interpreted as the work you must do to

"create room" for the system if you presume it started at zero volume.

Table of enthalpy changes

Index

Internal

energy

concepts

HyperPhysics***** Thermodynamics R NaveGo Back
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System WorkWhen workis done by a thermodynamic system, it is usually a gas that is

doing the work. The work done by a gas at constant pressure is:

Example

For non-constant pressure, the work can be visualized as the area under the

pressure-volume curve which represents the process taking place. The more

general expression for work done is:

Work done by a system decreases the internal energy of the system, as

indicated in the First Law of Thermodynamics. System work is a major focus

in the discussion of heat engines.

Index

Heat

engine

concepts

HyperPhysics***** Thermodynamics R NaveGo Back
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Thermodynamics is a branch of physics which deals with theenergy and work of a system. Thermodynamics deals onlywith the large scale response of a system which we canobserve and measure in experiments. In aerodynamics, weare most interested in thermodynamics in the study ofpropulsion systems and understanding high speed flows.

The state of a gas is defined by several properties includingthe temperature, pressure, and volume which the gasoccupies. From a study of the first law of thermodynamics, wefind that the internal energy of a gas is also a state variable,that is, a variable which depends only on the state of the gasand not on any process that produced that state. We are freeto define additional state variables which are combinations ofexisting state variables. The new variables often make theanalysis of a system much simpler. For a gas, a usefuladditional state variable is the enthalpy which is defined to bethe sum of the internal energy E plus the product of the
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At the bottom of the slide, we have divided by the mass of gasto produce the specific enthalpy equation version.

(h2 - h1) = cp * (T2 - T1)

The specific heat capacity cp is called the specific heat atconstant pressure and is related to the universal gas constantof the equation of state. This final equation is used todetermine values of specific enthalpy for a given temperature.Enthalpy is used in the energy equation for a fluid. Acrossshock waves, the total enthalpy of the gas remains a

constant.

Enthalpy

In thermodynamics and molecular chemistry, the enthalpy or heatcontent(denoted asHor H, or rarely as) is a quotient ordescription ofthermodynamic potential of a system, which can beused to calculate the "useful" work obtainable from a closedthermodynamic system under constant pressure.

The term enthalpy is composed of the prefix en-, meaning to "putinto", plus the Greekword -thalpein, meaning "to heat", althoughthe original definition is thought to have stemmed from the word,"enthalpos" It is often calculated as a differential sum, describingthe changes within exo- and endothermic reactions, which minimizeat equilibrium.

Thermodynamic potentials

Internal energy U(S,V)

Helmholtz freeenergy

A(T,V) = UTS

EnthalpyH(S,p) = U+pV

Gibbs freeenergy

G(T,p) = U+pV TS

edit
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Contents

[hide] 1 History

o

1.1 Original Definition 2 Application and extended formula

o 2.1 Overviewo 2.2 Relationshipso 2.3 Heats of reactiono 2.4 Open systems

3 Standard enthalpy changeso 3.1 Definitions

3.1.1 Examples: Inorganic compounds (at 25 C) 4 Specific enthalpy 5 Notes 6 References 7 See also

8 External links

[edit] 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 Paulmile Clapeyron and Rudolf Clausius through the publishing of theClausius-Clapeyron relation in "The Mollier Steam Tables and

Diagrams" in 1927, but it was later published that the earliestrecording of the word was in 1875, by Josiah Willard Gibbs in thepublication "Physical Chemistry: an Advanced Treatise"[2], although itis not referenced in Gibbs' works directly[3]. In 1909, Keith Landlerdiscussed Gibbs' work on the 'heat function for constant pressure'

and noted that Heike Kamerlingh Onnes had coined its modernname from the Greekword "enthalpos" () meaning "to putheat into." [1]

[edit] Original Definition

This is the heat change which occurs when 1 mol of a substancereacts completely with oxygen to form products at 298K and 1 atm.The functionHwas introduced by the Dutch physicist HeikeKamerlingh Onnes in early 20th century in the following form:
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whereErepresents the energy of the system. In the absence of anexternal 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)

His the enthalpy (joules)

Uis the internal energy, (joules) p is the pressure of the system, (pascals) Vis the volume, (cubic meters)

[edit] Application and extended formula

[edit] Overview

In terms of thermodynamics, enthalpy can be calculated bydetermining the requirements for creating a system from

"nothingness"; the mechanical work required,PVdiffers, basedupon the Constance of conditions present at the creation of thethermodynamic system.

Internal energy, U, must be supplied to remove particles from asurrounding in order to allow space for the creation of a system,providing that environmental variables, such as pressure (p) remainconstant. This internal energy also includes the energy required foractivation and the breaking of bonded compounds into gaseous

species.This process is calculated within enthalpy calculations as U+PV, tolabel the amount of energy or work required to "set aside space for"and "create" the system; describing the work done by both thereaction or formation of systems, and the surroundings. For systemsat constant pressure, the change in enthalpy is the heat received bythe 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 asimple system, with a constant number of particles, the difference in
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enthalpy is the maximum amount of thermal energy derivable from a

thermodynamic process in which the pressure is held constant.

[edit] Relationships

As an expansion of the first law of thermodynamics, enthalpy can berelated to several other thermodynamic formulae. As with theoriginal definition of the first law;

Where, as defined by the law;dUrepresents the infinitesimal increase of the systematic orinternal energy.Q represents the infinitesimal amount of energy attributed oradded to the system.Wrepresents the infinitesimal amount of energy acted out bythe system on the surroundings.

As a differentiating expression, the value of H can be defined as

Where

represents the inexactdifferential.

Uis the internal energy,

Q = TdSis the energy addedby heating during areversible process,

W=pdVis the work doneby the system in areversible process.

dSis the increase in entropy(joules per Kelvin), Pis the constantpressure

dVis an infinitesimal volume Tis the temperature

(Kelvin)

For a process that is not reversible, the second law ofthermodynamics states that the increase in heat Q is less than orequal to the product TdSof temperature Tand the increase inentropy dS; thus
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It is seen that, if a thermodynamic process is isobaric (i.e., occurs atconstant pressure), then dPis zero and thus

The difference in enthalpy is the maximum thermal energy attainablefrom the system in an isobaric process. This explains why it issometimes called the heat content. That is, the integral of dHoverany isobar in state space is the maximum thermal energy attainablefrom the system.

If, in addition, the entropy is held constant as well, i.e., dS= 0, theabove equation becomes:

with the equality holding at equilibrium. It is seen that the enthalpyfor 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 energywith additional terms involving the chemical potential and thenumber of particles of various types. The differential statement fordH is then:

where i is the chemical potential for an i-type particle, andNi is thenumber of such particles. It is seen that, not only must the Vdp termbe set to zero by requiring the pressures of the initial and final statesto be the same, but the idNi terms must be zero as well, by requiringthat the particle numbers remain unchanged. Any further

generalization will add even more terms whose extensive differentialterm must be set to zero in order for the interpretation of theenthalpy to hold.

[edit] Heats of reaction

The total enthalpy of a system cannot be measured directly; theenthalpy change of a system is measured instead. Enthalpy change isdefined by the following equation:
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where

H is the enthalpy changeHfinal 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 exothermicreaction at constant pressure, the system's changein enthalpy is equal to the energy released in the reaction, includingthe energy retained in the system and lost through expansion againstits surroundings. In a similar manner, for an endothermic reaction,the system's change in enthalpy is equal to the energy absorbedin thereaction, including the energy lost by the system andgainedfromcompression from its surroundings. A relatively easy way todetermine whether or not a reaction is exothermic or endothermic isto determine the sign of H. If His positive, the reaction isendothermic, that is heat is absorbed by the system due to theproducts of the reaction having a greater enthalpy than the reactants.The product of an endothermic reaction will be cold to the touch. Onthe other hand if His negative, the reaction is exothermic, that isthe overall decrease in enthalpy is achieved by the generation of heat.The product of an exothermic reaction will be warm to the touch.

Although enthalpy is commonly used in engineering and science, it isimpossible to measure directly, as enthalpy has no datum (referencepoint). Therefore enthalpy can only accurately be used in a closedsystem. However, few real world applications exist in closed isolation,and it is for this reason that two or more closed systems cannot becompared using enthalpy as a basis, although sometimes this is doneerroneously.

[edit] Open systems

In thermodynamicopen systems, matter may flow in and out of thesystem boundaries. The first law of thermodynamics for opensystems states: the increase in the internal energy of a system is equalto 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 isgiven by:
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where Uin is the average internal energy entering the system and Uoutis the average internal energy leaving the system

During steady, continuous operation, an energy balance applied to anopen system equates shaft work performed by the system to heatadded plus net enthalpy added.

The region of space enclosed by open system boundaries is usuallycalled a control volume, and it may or may not correspond tophysical walls. If we choose the shape of the control volume such thatall flow in or out occurs perpendicular to its surface, then the flow ofmatter into the system performs work as if it were a piston of fluid

pushing mass into the system, and the system performs work on theflow of matter out as if it were driving a piston of fluid. There arethen two types of work performed:flow workdescribed above whichis performed on the fluid (this is also often calledPV work) and shaftworkwhich 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,H, permits us to use this thermodynamicpotential to account for both internal energy and PV work in fluidsfor open systems:
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During steady-state operation of a device (see turbine,pump, andengine), the expression above may be set equal to zero. This yields auseful expression for the power generation or requirement for thesedevices in the absence of chemical reactions:

This expression is described by the diagram above.

[edit] Standard enthalpy changes

[edit] Definitions

Standard enthalpy change of combustion

Standard enthalpy of combustion is defined as the enthalpychange observed in a constituent thermodynamic system

when 1 mole of a substance reacts completely with oxygen

understandard conditions.

Standard enthalpy change of hydrogenation

Standard enthalpy of hydrogenation is the enthalpy changeobserved 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

Standard enthalpy change of formation is defined as theenthalpy 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 (denotedH orHo) is theenthalpy change that occurs in a system when 1 equivalent of matteris transformed by a chemical reaction under standard conditions.

A common standard enthalpy change is the standard enthalpychange of formation, which has been determined for a vast number
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of substances. The enthalpy change of any reaction under anyconditions can be computed, given the standard enthalpy change offormation of all of the reactants and products. Other reactions withstandard enthalpy change values include combustion (standard

enthalpy change of combustion) and neutralisation (standardenthalpy change of neutralisation).

[edit] Examples: Inorganic compounds (at 25 C)

ChemicalCompound

Phase(matter)

Chemicalformula

Hf0 inkJ/mol

Ammonia aq NH3 -80.8

Ammonia g NH3 -46.1

Sodium carbonate s Na2CO3 -1131

Sodium chloride(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
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Sodium nitrate aq NaNO3 -446.2

Sodium nitrate s NaNO3 -424.8

Sulfur dioxide g SO2 -297

Sulfuric acid l H2SO4 -814

Silica s SiO2 -911

Nitrogen dioxide g NO2 +33

Nitrogen monoxide g NO +90

Water l H2O -286

Water g H2O -241.8

Carbon dioxide g CO2 -393.5

Hydrogen g H2 0

Fluorine g F2 0

Chlorine g Cl2 0
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Bromine l Br2 0

Bromine g Br2 0

(State: g - gaseous; l - liquid; s - solid; aq = aqueous)

[edit] Specific enthalpy

The specific enthalpy of a working mass is a property of that massused in thermodynamics, defined as where u is thespecific internal energy,p is the pressure, and v is specific volume. Inother words, h =H/ m where m is the mass of the system. The SI unitfor specific enthalpy is joules per kilogram.
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