Basically, entropy and enthalpy
are thermodynamic functions.
Enthalpy (H):
Enthalpy (H):
A thermodynamic quantity
equivalent to the total heat content of a system. It is equal to the internal
energy of the system plus the product of pressure and volume.
or Enthalpy is a sum of internal energy and flow work
H=U+PV
Explanation:
Consider an ideal gas in a closed
vessel, having a volume V. The energy it contains is a measured by internal
energy ‘u’.
Now if the gas is forced out of
the vessel, the gas has to do some work against the atmosphere at pressure P.
It does this work by pushing the
atmosphere along a boundary which encloses the volume V, (which the gas occupied).
So, apart from internal energy U,
the gas has another component energy, which is the flow work done given by PV.
So the total energy the gas
contains is given by U+PV.
For the sake of convenience, both
these are clubbed together in one term, called ‘enthalpy’.
Enthalpy (H) is given by H = U + PV
U is a pure function of
temperature,
U = m*CVV*T,
Cvv = specific
heat at constant volume,
m= mass of
the gas.
H = m*Cvv*T + PV
For an ideal gas, we have ideal
gas relation
PV = mRT ,
R is the
characteristic gas constant.
H = m*Cvv*T + mRT
= mT*(Cvv + R)
H = m*Cpp*T,
where Cpp = specific heat at
constant pressure.
For an ideal gas, enthalpy is a
pure function of temperature, just like internal energy.
For real gases, the following
relation holds true -
dh = cppdT - [T [ ∂ν∂T∂ν∂T]pp -
νν]dp
(This is per kg mass basis)
Entropy (s):
Clausius introduced the concept of entropy as a precise way of expressing the second law of thermodynamics (heat energy flows from high temperature body to low temperature body)
The Clausius form of the second law states that,
spontaneous change for an irreversible process in an isolated system (ie., one
that does not exchange heat or work with its surroundings) always proceeds in
the direction of increasing entropy.
For example: A block of ice and the stove constitute two parts of an isolated system
for which total entropy increases as the ice melts.
By the Clausius definition,
If an amount of heat Q
flows into a large heat reservoir at temperature T above absolute zero (temperature having above 273.15oC), then the
entropy increase is ΔS
= Q/T.
ΔS = Q/T.
But the above relation was only true if the
change was carried out reversibly.
For an irreversible change, the following relation
is considered.
ds > δQ/T
ds = δQ/T + I