Enthalpy Of Evaporation

Enthalpy of evaporation or latent heat (hfg)

This is the amount of heat required to change the state of water at its boiling temperature, into steam. It involves no change in the temperature of the steam/water mixture, and all the energy is used to change the state from liquid (water) to vapour (saturated steam).

The old term latent heat is based on the fact that although heat was added, there was no change in temperature. However, the accepted term is now enthalpy of evaporation.

Like the phase change from ice to water, the process of evaporation is also reversible. The same amount of heat that produced the steam is released back to its surroundings during condensation, when steam meets any surface at a lower temperature.

This may be considered as the useful portion of heat in the steam for heating purposes, as it is that portion of the total heat in the steam that is extracted when the steam condenses back to water.Reference:Google Search

Enthalpy

Enthalpy
A few 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.
But lets consider enthalpy and internal energy
Enthalpy is defined by

H = U + PV
where 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 = ΔU + PΔV since in this case Q=ΔH

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 work must be done to produce the change in volume. For a constant pressure process the work you must do to produce a volume change ΔV is PΔV. 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.

Reference: Google Search

Entropy

Source http://www.engineeringtoolbox.com/steam-entropy-d_99.html

Entropy is the quantitative measure of disorder in a system. The concept comes out of thermodynamics, which deals with the transfer of heat energy within a system. Instead of talking about some form of "absolute entropy," physicists generally talk about the change in entropy that takes place in a specific thermodynamic process.
Calculating Entropy
In an isothermal process, the change in entropy (delta-S) is the change in heat (Q) divided by the absolute temperature (T):

delta-S = Q/T
In any reversible thermodynamic process, it can be represented in calculus as the integral from a processes initial state to final state of dQ/T.
The SI units of entropy are J/K (joules/degrees Kelvin).

Entropy as Time's Arrow



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Reversible Process

A reversible process ia a process that once taken place can be reversed ao that the system and surroundings are returned back to original conditions. In reality, there is no reversible process but for analysis purpose, reversible is use to make the analysis simpler.

Reference :http://www.engineersedge.com/thermodynamics/reversible_process.htm

Irriversible Process

Irreversible Process is a process that cannot return both the system and surroundings to their original condition, even though the process is reversed. Example an engine does not return back the fuel it consumed during climbing the hill,when it coasting back down the hill.There are a number of factors that contribute the the irreversibility of a process, among them are frictions,unrestrained expansion of fluid,heat transfer through a finite temperature change, mixing of two substances.

Reference:http://www.engineersedge.com/thermodynamics/irreversible_process.htm

Polytropic

From roymech.co.uk.

From Wikipedia
A polytropic process is a thermodynamic process that obeys the relation:

PVn = C,
where P is pressure, V is volume, n is any real number (the polytropic index), and C is a constant. This equation can be used to accurately characterize processes of certain systems, notably the compression or expansion of a gas, but in some cases, possibly liquids and solids.

Under standard conditions, most gases can be accurately characterized by the ideal gas law. This construct allows for the pressure-volume relationship to be defined for essentially all ideal thermodynamic cycles, such as the well-known Carnot cycle. (Note however that there may be instances where a polytropic process occurs in a non-ideal gas.)

For certain indices n, the process will be synonymous with other processes:

if n = 0, then PV0=P=const and it is an isobaric process (constant pressure)
if n = 1, then for an ideal gas PV=NkT=const and it is an isothermal process (constant temperature)
if n = γ = cp/cV, then for an ideal gas it is an adiabatic process (no heat transferred)
Note that , since (see: adiabatic index)
if n = , then it is an isochoric process (constant volume)
When the index n occurs between any two of the former values (0,1,gamma, or infinity), it means that the polytropic curve will lie between the curves of the two corresponding indices.

The equation is a valid characterization of a thermodynamic process assuming that:

The process is quasistatic
The values of the heat capacities,cp and cV, are almost constant when 'n' is not zero or infinity. (In reality, cp and cV are a function of temperature, but are nearly linear within small changes of temperature).

Reference:From Google Search

Isothermal

From http://commons.wikimedia.org/wiki/File:Isothermal_process.png
An isothermal process is a change in which the temperature of the system stays constant: ΔT = 0. This typically occurs when a system is in contact with an outside thermal reservoir (heat bath), and the change occurs slowly enough to allow the system to continually adjust to the temperature of the reservoir through heat exchange. An alternative special case in which a system exchanges no heat with its surroundings (Q = 0) is called an adiabatic process. In other words, in an isothermal process, the value ΔT = 0 but Q ≠ 0, while in an adiabatic process, ΔT ≠ 0 but Q = 0.

From Wikipedia

Reference :From Google Search