Enthalpy and Thermochemical Equations

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.

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.

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

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

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) + 2200 kJ

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

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

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

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

Enthalpy Calculations C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(g) + 2200 kJ If 2.0 moles of C3H8 are completely burned, how much energy is given off? Answer: 4400kJ of energy are released

Enthalpy Calculations C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(g) + 2200 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

Enthalpy Calculations C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(g) + 2200 kJ How many moles of C3H8 must be burned to produce 10,500kJ of energy? Answer: 4.78 moles C3H8

Enthalpy Calculations C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(g) + 2200 kJ How many grams of C3H8 must be burned to produce 15,250kJ of energy? Answer: 310 g of C3H8