1. Enthalpy and Thermochemical Equations

2. Direction of heat
If a reaction produces heat, it is called an exothermic reaction.
If a reaction absorbs heat, it is called an endothermic reaction.

3. Enthalpy (H) The heat content of a system at constant pressure.
The heat absorbed or released during a chemical reaction or phase change is equal to the change in enthalpy, ΔH.
qsys = ΔH
ΔH is – for exothermic processes and + for endothermic processes.

4. Enthalpy (Heat) of Reaction (Hrxn)
Energy released or absorbed in a chemical reaction
Let’s look at energy potential diagrams…

7. Thermochemical Equation
A chemical equation that includes the enthalpy change.
Two ways of writing:
Style 1: Energy can be written as a product or reactant
Style 2: ΔH value can be written beside the chemical equation

8. Exothermic Reactions
Style 1: Would the energy be a reactant or product when burning propane (C3H8)?
First, write the balanced equation with states.
C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(g)
Since this is an exothermic process, the energy is a product. This reaction produces 2200 kJ of energy.
C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(g) kJ

9. Exothermic Reactions Style 2: Would ΔH be positive or negative?
C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(g) ΔH= -2200kJ

10. Endothermic Reactions
What about the endothermic reaction of baking soda producing carbon dioxide?
Style 1: Would the energy be a reactant or product?
2NaHCO3(s) + 129kJ  2 Na2O(s) + H2O(g) + CO2(g)
Style 2: Would ΔH be positive or negative?
2NaHCO3(s) 2 Na2O(s) + H2O(g) + CO2(g) ΔH= 129kJ

11. What must be specified?
Must include the physical states of each substance.
The enthalpy for the reaction exactly as written. For the reaction:
2NaHCO3(s) Na2O(s) + H2O(g) + CO2(g) ΔH= 129kJ
129 kJ of energy must be added to react 2 moles of NaHCO3

12. Enthalpy Calculations
Use coefficients and ΔH to make mol/energy or energy/mol ratios
Use the ratios to determine the amount of heat absorbed or given off during a reaction

13. Enthalpy Calculations
C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(g) kJ
If 2.0 moles of C3H8 are completely burned, how much energy is given off?
Answer: 4400kJ of energy are released

14. Enthalpy Calculations
C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(g) kJ
If 1.75 moles of CO2 are produced during the combustion of C3H8, how much energy is given off?
Answer: 1300kJ of energy are given off

15. Enthalpy Calculations
C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(g) kJ
How many moles of C3H8 must be burned to produce 10,500kJ of energy?
Answer: moles C3H8

16. Enthalpy Calculations
C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(g) kJ
How many grams of C3H8 must be burned to produce 15,250kJ of energy?
Answer: 310 g of C3H8