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Thermodynamics II 25
2, we know that the internal energy of a system can be expressed as:
AU=q+w+q=AU-w+q=AU+pAV. This means that q
(or more precisely, qp, at constant pressure) is the change in enthalpy,
AH.

Change in Enthalpy, AH
The amount of heat, qp, exchanged when the work done by the system
is expansion work at constant pressure is termed AH, the change in
enthalpy (AH = qp) * AH = Hfinal - Hinitial = (Ufinal + PVfina1)-
(Uinitial +pViniti,l). Since the pressure is a constant, AH = (Ufinal -
Uinitial) + P( Vfinal - Vinitial) = AU + PA V, where PA V is related
to AugRT, from the equation of state of an ideal gas, Chapter 1
(pAV = AugRT), where Avg = change in the coefficients of
gaseous reagents,

i.e. Aug = C[u(Gaseous products)] - C[u(Gaseous reactants)]

For example, in the reaction, C~HQ) + 3.502,,) -+ 3H,O,l) + 2C02,,,,
Aug = (2) - (3.5 + 1) = -2.5. If the system is at both constant
pressure and constant volume (A V = 0), AH = AU +PA V = A U.

Summary: AH = A U + PA V = A U + Aug RT at constant pressure;
AH= AU at constant volume, since AV = 0.

Under standard state conditions, i.e. 1 bar pressure and 25 "C
I
(298 K), AH= AH". AH:, can be easily determined, using the
expression:
I ATxn = C [Aq (Products)] - C [Aq (Reactants)] I

i.e. for the reaction, UAA + uBB + u& + vDD, where UA, uB, vc and
UD are the stoichiometry factors, AHRn = [(vcAH~,)) + (uDAH;(~))]
- [hAH)(A)) + (mAH;iB))I-

Changes in the Enthalpy of an Element
The standard molar enthalpy of formation of an element in its most
stable state (solid, liquid or gas) is zero, since the formation of an