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Enthalpy

07/23/2008 9:47 PM

What is physical meaning of enthalpy? why it is needed to calculate the enthalpy in chemical reactions or in thermodynamics?

why it is defined as H=U+PV?

Enthalpy value shows what?

someone said me that it is a change in heat at constant pressure,agreed--but what will happen to enthalpy if the pressure is not constant?

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#1

Re: Enthalpy

07/23/2008 10:42 PM

Hi jignesh142,

From Wiki:

Enthalpy 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 H was introduced by the Dutch physicist Kamerlingh Onnes in early 20th 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)

If you need further help, please let us know....

Mike

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#2
In reply to #1

Re: Enthalpy

07/24/2008 3:23 AM

ok i know this.

but why we need to calculate enthalpy chane ( delta H) instead of simply heat change (delta Q) in thermodynamics processes?

why it is H=U+PV and that value shows what

please give example for better understanding

jignesh142--i posted without login

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#3
In reply to #2

Re: Enthalpy

07/24/2008 4:25 AM
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#4
In reply to #2

Re: Enthalpy

07/24/2008 11:56 PM

Well, one reason why is that heat change, transfer, etc., may be so small in comparison with enthalpy or enthalpy changes that Q is dropped from the calculation.

For example, consider a nozzle or diffuser: typically, at steady state operation,

0 = Q dot (over a control volume)/m dot plus delta h plus kinetic energy change.

In this example, the heat transfer (Q dot) per mass flow rate (m dot) is negligible, and is dropped from the equation, leaving:

0 = delta h plus the kinetic energy change (Vi squared minus Vf squared, divided by 2).

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#6
In reply to #2

Re: Enthalpy

07/25/2008 8:29 AM

Enthalpy is a mathematical construct that provides for measurement of the changes in the sum of changes in internal energy (of the system) plus the changes in the pressure volume state (of the system). Enthalpy of a system is meaningless apart from the base (reference state) being defined for the system. The change in enthalpy is directly concerned with the changes in energy of the system.

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#5

Re: Enthalpy

07/25/2008 3:13 AM

I have always understood enthalpy as a mixture of 'sensible' heat and 'latent' heat.

The sensible heat is the proportional rise in temperature (of the solid only, liquid only or gas only) due to heat input, whereas the latent heat is the heat input at constant temperature during the stage when the solid turns to liquid, or the liquid turns to gas.

In simplistic terms the temperature of a block of ice will rise in proportion to the heat input until 0C, then stay at 0C until the ice has melted, then the water temperature increases in proportion to the heat input until 100C, when it stays at 100C until all the water has evaporated, then the temperature of the vapour rises again in proportion to the heat input.

Using enthalpy in calculations helps because sensible heat and latent heat are lumped together as a single entity.

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#7

Re: Enthalpy

07/25/2008 11:30 AM

Look at big picture: Energy balance

Any change in energy of the system has two components: external change and internal change

external change: delta W = f (delta V, delta P, delta E_potential, delta E_kinetic, delta E_acceleration ...); delta Q = heat supplied/removed

internal change: delta U ==> properties of the molecular structure and arrangements (no definition exists)

when the system is closed (no mass transfer), all the energy supplied by external change, is used for internal change i.e.

delta U = delta Q + delta W

delta Q = delta U - delta W

When the only work done is mechanically reversible on the closed stationary system:

delta W = - delta (P*V) = -V*delta (P) - P*delta (V)

so depending on what is constant ( P or V), heat transfered is either delta Q or delta H.

i.e. delta Q = delta U + P*delta (V) ….constant volume

And delta H = delta U + V*delta (P) …. Constant pressure

So, ethalpy change tells u heat transferred; the equation is purely convenience shake and mathematical; Regarding when pressure is not constant, as per ethalpy definition, that's not possible.

Why the heck go thru all of these pain? By varying measurable quantities of P,V,H and/or Q, U could be studied. The understanding of U could be used in design and modelling.

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#8

Re: Enthalpy

07/25/2008 11:40 AM

Hi Guys

According with my experience in Thermodynamics

Enthalpy= ∑Energy of Mechanical Source (PV) +∑Energy of Molecular vibration source (U)

Enthalpy= ∑Macromolecular Energy (PV) + ∑Micromolecular Energy Quantum (U)

Enthalpy = Is just a mathematical relation of the total content of energy of a mol in steady state or suffering a chemical reaction, or being transported in a closed system or opened system. Beside is a part of the general energy balance.

Israel Barron

ibarron@sfmex.com

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#9

Re: Enthalpy

07/25/2008 1:57 PM

Addendum

When we are working on analysis of thermal processes is important the ENTHALPY to establish the GENERAL ENERGY BALANCE , part of the thermal design is understand the Cv OR Cp , those are related the enthalpy of substance and its variations relative to the temperature.

Cv = (∂Η⁄∂Τ)v,n...

Israel Barron

ibarron@sfmex.com

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