1. Enthalpy (∆H)

2. Enthalpy of Reactions, ΔH
C3H8 + 5 O2  3 CO2 + 4 H2O ∆H = kJ
Sign of ∆H tells you if the reaction is endothermic (+) or exothermic (-)
The value (#) of ∆H goes with the balanced mole coefficients
i.e kJ/1 mol C3H8 or kJ/5 mol O2 to form the heat ratio
You cannot use the heat ratio until your given number has been converted to moles.

3. First we must convert 100 L to moles
Example 1: “How much heat is released by the combustion of 100 L of propane, C3H8, in the reaction below?
C3H8 + 5 O2  3 CO2 + 4 H2O ∆H = kJ”
First we must convert 100 L to moles
Then we multiply by the heat ratio (∆ H/coefficient)
100 L C3H8 x 1 𝑚𝑜𝑙 𝐿 = 4.64 mol C3H8
4.64 mol C3H8 x −2221 𝑘𝐽 1 𝑚𝑜𝑙 𝐶3𝐻8 = -10,305 kJ
From balanced equation

4. Example 2: How much heat is released by the combustion of 10 g of oxygen in the reaction below?
C3H8 + 5 O2  3 CO2 + 4 H2O ∆H = kJ

5. First we must convert 10 g oxygen to moles
Example 2: How much heat is released by the combustion of 10 g of oxygen in the reaction below?
C3H8 + 5 O2  3 CO2 + 4 H2O ∆H = kJ
First we must convert 10 g oxygen to moles
Then we can multiply by heat ratio
10 g O2 x 1 𝑚𝑜𝑙 32 𝑔 𝑂2 = 0.31 mol O2
0.31 mol O2 x −2221 𝑘𝐽 5 𝑚𝑜𝑙 𝑂2 = kJ
From balanced equation

6. Potential Energy Diagram
AKA: Reaction pathway diagram
Shows visually if a reaction is endothermic or exothermic (just compare the energy at the start and end)
Energy goes up = endothermic (+ΔH)
Energy goes down = exothermic (- Δ H)

7. ∆H = energy of products – energy of reactants
Students should copy this diagram and know how to explain everything that is labeled.