Chemistry · Ch 4 — Chemical Thermodynamics

Enthalpy (H)

4.8

Enthalpy of a system is the sum of the internal energy of the system and the energy equivalent to $PV$ work:

H = U + PV \qquad \text{...(4.19)}

The change in enthalpy, $\Delta H$ , is also a state function, given by

\Delta H = H_2 - H_1 \qquad \text{...(4.20)}

where $H_1$ and $H_2$ are the enthalpies of the initial and final states, respectively. From Eq. (4.19) we write

H_1 = U_1 + P_1V_1 \quad \text{and} \quad H_2 = U_2 + P_2V_2

With these,

\Delta H = U_2 + P_2V_2 - U_1 - P_1V_1 = (U_2 - U_1) + (P_2V_2 - P_1V_1)

= \Delta U + \Delta (PV) \qquad \text{...(4.21)}

For constant pressure, $P_1 = P_2 = P$ , and

\Delta H = \Delta U + P\,\Delta V \qquad \text{...(4.22)}

If the pressure inside and outside is the same, or $P_{ext} = P$ , Eq. (4.18) gives

Q_p = \Delta U + P\,\Delta V \qquad \text{...(4.23)}

From equations (4.22) and (4.23), …