When Doing Energy Balance Around a Reactor, We Use the Heat-Of-Formation Method and Not

Overall Standard Heat of Reactions

When doing an energy balance around a reactor, we use the heat-of-formation method and not the heat-of-reaction method. In the heat-of-formation method, the molar enthalpy of any mixture is calculated by starting with the heat of formations for each pure compound. Because of this fact, the heat-of-formation method automatically accounts for multiple reactions that may be occurring within the system boundary.

A chemical reaction can be either exothermic (giving up energy) or endothermic (requiring energy). For multiple reactions, how do you determine the overall energy change for the combined reactions?

You only need to use the standard heat of reaction () for each chemical reaction at 25°C and 1 atm to calculate the overall standard heat of reactions. If you have multiple reactions, four possibilities exist for the overall standard heat of reactions as follows:

for each reaction
Rxn I: / - / + / - / +
Rxn II: / - / + / + / -
Rxn III: / - / + / - / -
etc.

For the first column, the overall would be exothermic.

For the second column, the overall would be endothermic.

For the third column, the overall could be exothermic or endothermic.

For the fourth column, the overall could be endothermic or exothermic.

For the third and fourth column, the overall will depend upon how many Rxn I's are occurring, how many Rxn II's are occurring, how many Rxn III's are occurring, etc. You will not know that until you solve for the extent of reactions (’s) in the material balances.

After finding the ’s, you can then calculate the overall standard heat of reactions as follows:

Overall

If the overall ΔH of reaction is negative, than it is exothermic; otherwise, it is endothermic.

where / nc / is / number of chemical compounds within the system boundary.
nr / is / the number of chemical reactions within the system boundary.
/ is / the number of reaction events, often called the extent of reaction, for the l-th reaction in units of (mass-reaction of l )/time. Some example units are: g-rxn/s, kg-rxn/h, and lb-rxn/min.
/ is / the standard heat-of-reaction for the l-th chemical reaction at the standard condition of 25ºC and 1 atm. The units are (energy of l/mass-rxn of l).
/ is / the stoichiometric coefficient for the j-th component in the l-th reaction. It is positive (+) for a product and negative (−) for a reactant. It has units of (mass-mole of j)/(mass-reaction of l ), like g-mol/g-rxn, kg-mol/kg-rxn, or lb-mol/lb-rxn.
/ is / the standard heat-of-formation for pure component j-th at the standard condition of 25ºC, 1 atm, and stable phase () of j-th. The units are (energy of j/mass-mole of j).