Enthalpy (∆H)

Enthalpy of Reactions, ΔH C3H8 + 5 O2  3 CO2 + 4 H2O ∆H = -2221 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. -2221 kJ/1 mol C3H8 or -2221 kJ/5 mol O2 to form the heat ratio You cannot use the heat ratio until your given number has been converted to moles.

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 = -2221 kJ” First we must convert 100 L to moles Then we multiply by the heat ratio (∆ H/coefficient) 100 L C3H8 x 1 𝑚𝑜𝑙 22.4 𝐿 = 4.64 mol C3H8 4.64 mol C3H8 x −2221 𝑘𝐽 1 𝑚𝑜𝑙 𝐶3𝐻8 = -10,305 kJ From balanced equation

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 = -2221 kJ

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 = -2221 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 = -138.8 kJ From balanced equation

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)

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