Welcome to Chem Zipper.com......: ENTHALPY (H) INTRODUCTION

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Friday, September 28, 2018


We known 1st law of thermodynamics   
  If Pext=constant
                Then dW= -PdV
     and     dU=dq- PdV
                dQ=H2-H1                    ( H2=U2+PV2  and H1=U1+PV1)
The enthalpy of a system is defined as:
                  H = U + PV
                  DH = DU + D(PV)
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.

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)
                    H=U+nRT   and H=f(T) only for ideal gas

For other substance and real gas
                     U=f(P,V,T) and H=f(,PV,T)
       So         H=f(P,T)/ f(V,T)/ f(P,V)

           dH=(dH/dT)p dT+(dH/dP)T dP------------------------------------- (1)

           dH=(dH/dV)T dV+(dH/dT)V dT------------------------------------- (2)

           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.


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