We known 1st
law of thermodynamics
dU=dq+dW
If Pext=constant
Then dW= -PdV
and dU=dq- PdV
dq=dU-PdV
dq=(U2-U1)+P(V2-V1)
dq=(U2+PV2)-(U1+PV1)
dQ=H2-H1 ( H2=U2+PV2 and H1=U1+PV1)
dq=dH
hence
The enthalpy of a system is defined as:
H
= U + PV
DH = DU + D(PV)
Where
H is the enthalpy of the system
U is the internal energy of the system
P is the
pressure at the boundary of the system and its environment.
(1) In thermodynamics the quantity U + PV is a new state function and known
as the enthalpy of the system and is denoted by H=U+PV. It represents the total energy stored in the system.
(2) It may be noted that change in
enthalpy is equal to heat exchange at constant pressure.(3) Enthalpy
is also an extensive property as
well as a state function. (4)The absolute value of enthalpy cannot
be determined, however the change in enthalpy can be experimentally determined.
DH = DU + D(PV)
(5) Change in enthalpy is a more useful quantity than its
absolute value.
(6) The unit of measurement for enthalpy (SI) is joule.
(7)The enthalpy is the preferred expression of system energy
changes in many chemical and physical measurements, because it simplifies
certain descriptions of energy transfer. This is because a change in enthalpy
takes account of energy transferred to the environment through the expansion of
the system under study.
(8)The change dH
is positive in endothermic reactions,
and negative in exothermic
processes. dH of
a system is equal to the sum of non-mechanical work done on it and the heat
supplied to it.
(9) For quasi static processes under constant pressure, dH is equal to the change in the
internal energy of the system, plus the work that the system has done on its
surroundings. This means that the change in enthalpy under such conditions is
the heat absorbed (or released) by a chemical reaction.
NOTE:
Transfer of heat at constant volume brings about a change
in the internal energy(DU) of the system whereas that at constant pressure
brings about a change in the
enthalpy (DH) of the system.
For Ideal
gas
H=U+PV and U=f(T)
PV=nRT
H=U+nRT and H=f(T) only for ideal gas
For other
substance and real gas
H=U+PV
U=f(P,V,T)
and H=f(,PV,T)
So H=f(P,T)/ f(V,T)/ f(P,V)
H=f(T,P)
dH=(dH/dT)p dT+(dH/dP)T
dP------------------------------------- (1)
H=f(V,T)
dH=(dH/dV)T dV+(dH/dT)V
dT------------------------------------- (2)
H=f(P,V)
dH=(dH/dV)P dV+(dH/dP)V
dP------------------------------------- (3)
Out of the
above three relation H as function
on of (T,P) Has a greater significance. The above differential equation simplified
for different substance for different condition.
For
isobaric process : dP = 0 (Molar Heat capacity at constant Pressure)
We known
QP=nCpmdT
(Molar Heat capacity at constant Pressure)
Cpm= (dQ/dT)P for 1 mole of gas
dQ=dH at dP=0
then Cpm= (dH/dT)P
For an ideal gas: change in
enthalpy at constant temperature with change in
pressure is zero. i.e.
Continue...........
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