1. Energy Balance on Reactive Processes

2. Content Introduction Heat of Reaction or Enthalpy of Reaction
Relationship Between Enthalpy Change and Heat of Reaction
Properties of Heat of Reaction
Hess’s Law
Heats of Formation
Heats of Combustion
Energy Balance on Reactive Processes

3. Introduction In any reaction:
Energy is required to break the reactant chemical bond
Energy is released when product chemical bond is formed
Exothermic reaction
If energy required to break the reactant chemical bond is less than energy released when product chemical bond is formed
the product molecules have lower internal energies than the reactants at the same T and P (i.e ΔH=-ve)
Heat of reaction must be released as heat or work to maintain the operation temperature
Endothermic reaction
If energy required to break the reactant chemical bond is larger than energy released when product chemical bond is formed
the product molecules have higher internal energies than the reactants at the same T and P (i.e ΔH=+ve)
Energy is need by the process to maintain the operation temperature

4. Heat of Reaction or Enthalpy of Reaction: ΔĤr (T,P)
Enthalpy change for a process in which stoichiometric quantities of reactant at T & P reacted completely in single reaction to form a products at the same T & P.
Stoichiometric quantities of reactant means molar amount of the reactant numerically equal to their stoichiometric coefficient.
In simple word;
Reactants and products: stoichiometric quantities
Complete Reaction
Reactants are fed at T,P
Products are emerging at T,P

5. Heat of Reaction : Per mole of what ?
2A + B  3C ΔĤr (100C, 1 atm) =-50 kJ/mol
Meaning that:
If 150 mol C/s is generated, enthalpy change is

6. Relationship Between Enthalpy Change and Heat of Reaction
Although the Heat of Reaction is defined so, the actual enthalpy change of the reaction depends on how many moles of reactant has been consumed (Extent of reaction). Therefore:
Where:
vA - stoichiometric coefficient
ξ - extent of reaction
nA,r - moles of A consumed or generated

7. Properties of Heat of Reaction
Standard heat of reaction (ΔĤr°) - heat of reaction when both reactants and products are at reference conditions (usually 25 C and 1 atm)
At low and moderate pressure, ΔĤr is nearly independent of pressure
Exothermic (ΔĤr= -ve) and Endothermic (ΔĤr= +ve)
ΔĤr depends on how the stoichiometric equation is written
CH4 (g) + 2O2(g)  CO2(g) + 2H2O(l)
ΔĤr1 (25C)= kJ/mol  for 1 CH4
2CH4 (g) + 4O2(g)  2CO2(g) + 4H2O(l)
ΔĤr2 (25C)= kJ/mol  for 2 CH4
5. ΔĤr depends on the states of aggregation (gas, liquid, or solid)
ΔĤr1 (25C)= kJ/mol
CH4 (g) + 2O2(g)  CO2(g) + 2H2O(g)
ΔĤr2 (25C)= kJ/mol

8. Internal Energy of Reaction
For a reaction takes place in a closed reactor or constant volume
Example for the reaction
C6H14 (l) + 19/2 O2 (g)  6 CO (g) + 7 H2O (v)

9. Hess’s Law (Cal’tion of Reaction Heat
Normal procedure using calorimeter, however has a limitation
If the stoichiometric equation for reaction 1 can be obtained by algebraic operations (multiplication by constant, addition, and subtraction) on stoichiometric equation for reaction 2,3….., then the heat of reaction ΔĤr1 can be obtained by performing the same operations on the heats of reactions ΔĤr2 , ΔĤr3 ….
C + 1/2O2(g)  CO (incomplete combustion)
Alternative method
C + O2  CO2 ΔHr1 = kJ/mol
CO + ½ O2  CO2 ΔHr2 = kJ/mol
C + ½ O2 (+ ½ O2)
CO (+ ½ O2)
CO2

10. Class Discussion
Example 9.1-1
Example 9.1-2
Example 9.2-1

11. Heats of Formation
Formation reaction
– reaction in which the compound is formed from its elemental constituents as they normally occur in nature (e.g. O2 rather than O)
standard heat of formation (ΔĤ°f)
Enthalpy change associated with the formation of 1 mole of compound at a reference temperature (25C) and pressure (1 atm)
Standard heat of formation are listed in Table B.1.
Standard heat of formation for elemental species (e.g O2) is zero
Relationship between standard heat of formation and heat of reaction based on Hess’s Law

12. Example 9.3-1
Determine the standard heat of reaction for the combustion of liquid n-pentane assuming H2O (l) is a combustion product.

13. Heats of Combustion Standard heat of combustion,
heat of combustion of that substance with oxygen to yield specified products (e.g. CO2, H2O) with both reactant and products at 25C and 1 atm.
Several value are listed in Table B.1
Relationship between heat of reaction and heat of combustion

14. Example 9.4-1
Calculate the standard heat of reaction for the dehydrogenation of ethane
C2H6  C2H4 + H2

15. Energy Balance on Reactive Processes
Method 1: Heat of Reaction Method
preferable when there is a single reaction for which
ΔĤ°r is known
Reactants
Tin
Products
Tout
T=25 oC
ΔHro ΔH
ΔH1
ΔH2

16. Method 1: Heat of Reaction Method
Complete the material balance
Choose reference states for specific enthalpy changes
- reactant and products species at 25C and 1 atm for which ΔĤ°r is known
- For nonreacting species at any convenient temperature, such as reactor inlet or outlet
For a single reaction in a continuous process, calculate the extent of reaction
-choose as species A any reactant or product for which the feed and product flow rates are known
Prepare inlet-outlet enthalpy table
Calculate each unknown stream component enthalpy
Calculate for the reactor ; use following eq.
Substitute calculated value in the energy balance equation and complete the required calculations.

17. Energy Balance on Reactive Processes
Method 2: Heat of Formation Method
preferable when there is a multiple reaction and single reaction for which ΔĤ°r is unknown
Reactants
Tin
Products
Tout ΔH
Elements
25 oC
ΔH1
ΔH2

18. Method 2: Heat of Formation Method
Complete the material balance
Choose reference states for specific enthalpy changes
- elemental species that constitute the reactants and products in the states in which the elements are found at 25C and 1 atm
- For nonreacting species at any convenient temperature
Prepare inlet-outlet enthalpy table
Calculate each unknown stream component enthalpy
Calculate for the reactor for single or multiple reaction. Note that heat of reaction terms are not required if the element are chosen as references ; use following eq.
Substitute calculated value in the energy balance equation and complete the required calculations.

19. Example 9.5-1
The standard heat of reaction for the oxidation of ammonia is given below:
4 NH3 (g) + 5 O2 (g)  4 NO (g) + 6 H2O (v)
ΔĤ°r= kJ/mol
100 mol NH3/s and 200 mol O2/s at 25C are fed into a reactor in which the ammonia is completely consumed. The products gas emerges at 300C. Calculate the rate at which heat must be transferred to or from the reactor, assuming operation at approximately 1 atm.

20. Class Discussion
Example 9.5-2
Example 9.5.3
Example 9.5.4

21. GOOD LUCK FOR YOUR FINAL EXAM