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anilkumarm • Jun 25, 2009

# thermodynamics

what is enthalphy
skipper • Jun 25, 2009
Enthalpy is just the difference in heat between the activation-energy in a chemical reaction, and the heat absorbed or emitted by the reaction itself. It's a stoichiometric measurement of the entropy contributed or 'removed' by the reaction process, relative to how much energy it takes to start the process, after accounting for initial and final species, or reactants + energy -> products (+ or - energy).
din_Power Engg • Jun 26, 2009
Enthalpy!!

its a bigger concept...it has various versions depending on at which place, conditions and which thermodynamic process we are applying it. But i shall give here the simpler and basic version used in a const. pressure process...

the defn is, Enthalpy (H) is defined as the summation of work done by the system and the energy supplied to the system, to perform that work. it can be equated as,

H = U + pv, where U is the internel enery supplied/consumed and pv is the work done in the process......

But, in actual practice Enthalpy as a arbitary term is nott used. Only the change in Enthalpy (dH) is used. so the Eqn. becomes...

dH = dU + (pdV + vdP).....where dU is change in the energy supplied and (pdV + vdP) is the corresponding change in the work done.
Rohan_sK • Jun 26, 2009
I would like to make little correction to the definition of ENTHALPY.

ENTHALPY is defined as the TOTAL ENERGY CONTENT of a Thermodynamic system.

Enthalpy is thus the measure of the INTERNAL ENERGY content of the sytem and the Energy the system has convereted to Pressure-Volume type WORK ie DISPLACEMENT WORK, after the system has been supplied External Energy ( usually Heat).

Note that the parameter U is NOT the internal energy CONSUMED but it is the internal energy REMAINING ( increased value than the one before heat addition) after heat addition externally and work output taken from the system.

Thus we have ,

ENTHALPY = INTERNAL ENERGY + PRESSURE-VOLUME TYPE WORK DONE

H = U + P(dV)
pramod.pal007 • Jun 27, 2009
enthalpy is a property of thermodynamic system that shows at a particular state, how much energy it contains; during a change how much energy will be available to generate work.......... (first law)
skipper • Jun 27, 2009
Don't confuse enthalpy with entropy; heat released or absorbed by a system is related to the work done generating pressure + volume as noted. Entropy is the total energy change, as well as what everyone laughingly calls "disorder". The order/disorder is a symmetry relation of the ordering in solids, liquids and gases and the different kinds of exchanges at each symmetry point.

Gases are highly "disordered" relative to a crystal's structure, but gases are more symmetric in terms of "freedom" for individual atoms/molecules that solids are not allowed to have, being ordered differently (Pauli's exclusion principle and boson spin are involved, as well as Coulomb scattering). Spin is a way to order certain gases using strong magnetic fields which can "heat" atoms etc.
balamech123 • Jun 28, 2009
enthalpy is amount of heat present in the body. there are various formula to find the enthalpy depend upon the situation and condition .
jhbalaji • Jul 25, 2009
In simple words
It's a thermodynamic quantity equal to the internal energy of a system plus the product of its volume and pressure ...