What is the defination of enthalpy? dont give the mathematical defination.what actually enthalpy is??

Answer 1

In thermodynamics, the quantity enthalpy, symbolized by H, also called heat content, is the sum of the internal energy of a thermodynamic system plus the energy associated with work done by the system on the atmosphere which is the product of the pressure times the volume. The term enthalpy is composed of the prefix en-, meaning to "put into", plus the Greek suffix -thalpein, meaning "to heat".

Answer 2

Its been awhile since I had thermodynamics, but as I recall, as a practical matter, enthalpy seems to be another way of stating the law of the conservation of energy. The amount of energy (or work) that you get out of a system cannot be more than the amount of energy (or work) that you put into it.

In other words, a motor or generator can never be more than 100 percent efficient. If this seems obvious, tell that to the numerious inventors that submit applications to the Patent Office yearly claiming to have invented hyper efficient motors.

For example, the watts of mechanical energy used to drive a generator must be equal to the watts of electrical energy output by the same (Or greater than, because of work lost to internal friction. Often a engine is usually about 30-40 percent efficient because of internal losses.).

Answer 3

Enthalpy is the sum of the heat energy and the energy due to compression. It's a thermodynamics concept that is useful in calculating the energy balance in power machinery such as internal combustion engines and turbines.

Answer 4

Enthalpy isn't a real quantity, you can't measure it. It's actually a made-up of cenvention used for convenience. No one has even seen a unit of enthalpy.

Just use it when you need to and try not to think about it too much.

Answer 5

you're genuinely no longer as smart as you imagine of, new baby. i am going to twist numbers too. operating party, you're conserving there are an infinite variety of plausible gods by the time of historic previous? good, take those infinite gods vs no god for atheists, by using very reality they do no longer have the diverse possibility diverse than no god or gods of any type. Now do the mathematics. i'd say theres an infinity to at least a million possibility that Athiests do now no longer comprehend mathematical possibility, and are thoroughly incorrect. of course my numbers do now no longer make any journey, yet neither do yours.you'll make a good toddler-kisser sometime.

Answer 6

Main Entry: en·thal·py
Pronunciation: 'en-"thal-pE, en-'
Function: noun
Etymology: en- + Greek thalpein to heat
: the sum of the internal energy of a body and the product of its volume multiplied by the pressure

Answer 7

History
The function H was introduced by the Dutch physicist Kammerlingh Onnes in late 19th century in the following form:


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:


where (all units given in SI)

H is the enthalpy
U is the internal energy, (joule)
P is the pressure of the system, (pascal)
V is the volume, (cubic metre)
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Overview
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. A possible interpretation of enthalpy is as follows. Imagine we are to create the system out of nothing, then, in addition to supplying the internal energy U for the system, we need to do work to push the atmosphere away in order to make room for the system. Assuming the environment is at some constant pressure P, this mechanical work required is just PV where V is the volume of the system. Therefore, colloquially, enthalpy is the total amount of energy one needs to provide to create the system and then place it in the atmosphere. Conversely, if the system is annihilated, the energy extracted is not just U, but also the work done by the atmosphere as it collapses to fill the space previously occupied by the system, which is PV.

Enthalpy is a thermodynamic potential, and is useful particularly for nearly-constant pressure processes, where any energy input to the system must go into internal energy or the mechanical work of expanding the system. 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. In other words, when considering change in enthalpy, one can ignore the compression/expansion mechanical work. Therefore, 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.

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Some useful relationships
From the first law of thermodynamics:


And differentiating the expression for H we have:




where
U is the internal energy,
is the energy added by heating during a reversible process,
is the work done by the system in a reversible process.
T is the Temperature, dS is the increase in entropy,
P is the constant pressure,
dV is an infintesimal volume, and
δ represents the inexact differential.

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


It is seen that, if a thermodynamic process is isobaric (i.e., occurs at constant pressure), then dP = 0 and thus


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:


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:


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.

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Applications
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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:


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 enthlapy 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.

Although Enthalpy is commonly used in engineering and science, being impossible to measure directly, 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 two or more closed systems cannot be compared using enthalpy as a basis, although it is sometimes erroneously.

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Open systems

A first law energy balance applied to an open system equates changes in the enthalpy flowing through the system to heat added and shaft work performed.Open systems provide additional possibilities for performing work—by rotating a steam turbine for example. This "shaft work" is separate from work done on the fluid itself (called PV work):


The incorporation of the PV term into enthalpy is very useful for these systems. From the first law:


and the definition of enthalpy:


we obtain a version of the first law for shaft work in open systems with no chemical reaction:


This expression, like the first law expressed in terms of U, is not limited to reversible processes or any assumptions about the thermodynamic path taken by the process.

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Standard enthalpy
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.

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).

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Specific enthalpy
The specific enthalpy of a working mass is a property of that mass used in thermodynamics, defined as h = u + P * v 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/kilogram.

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See also
Calorimetry
Calorimeter
Isenthalpic process
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External links
Enthalpy - Eric Weisstein's World of Physics
Enthalpy - Georgia State University
Enthalpy - KnowAllAbout.com
Enthalpy (example calculations) - Texas A&M University (Chemistry Department)
Retrieved from "http://en.wikipedia.org/wiki/Enthalpy"
Categories: Enthalpy

Answer 8

the sum of the internal energy of a body and the product of its volume multiplied by the pressure

Answer 9

The measure of heat in a system.

Answer 10

heat