First Law for Reacting Systems

The first law of thermodynamics for a reacting system under SSSF process with negligible changes in kinetic energy and potential energy is given as 

The enthalpy of combustion is the difference in total enthalpy of the products and total enthalpy of the reactants at a given temperature and pressure and given as 

Separating terms for the enthalpy of formation and change of enthalpy. Equation 3.88b is written as 

Comparing Equation 3.82 and the first law equation for reacting system with no work, we can see that enthalpy of combushon is equivalent to the heat transfer rate across the control volume surrounding the combustion chamber. 

Similarly, the internal energy of combustion is given as the difference in internal energy of the products and the internal energy of reactants 

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