First Law for Reactive Systems

For the S.S.S.F. process as shown in Fig. 3.4 the first law gives

HR + QC.V. = HP + WC.V.

(3.97)

or,

(3.98)

or,

(3.99)

Fig.3.4 First law for a reaction system

When the states of reactants and products are not in the standard reference state (298K, 1atm), then, as shown in Fig. 3.5,

Fig. 3.5 Enthalpy of reactants and products varying with temperature

(3.100)

(3.101)

where  is the enthalpy of reaction at the standard temperature (298 K). The variation of enthalpy with pressure is not significant.

(3.102)

where  is the internal energy of reaction at 298 K.