Heat and the Enthalpy of Reaction Ice calorimeter used by Antoine Lavoisier Heat and the Enthalpy of Reaction

Law of Conservation of Energy The Law of Conservation of Energy states that the total energy of a closed system remains constant. This can also be stated as Energy is not created or destroyed. The many different energy conversions that can occur during a chemical reaction can often make this more difficult to verify than conservation of mass.

When a chemical reaction takes place, energy is either absorbed or released. This is because chemical bonds require energy to be formed, and when they are broken, energy is released. This energy is most often absorbed and released in the form of heat and light.

Law of Conservation of Energy The important thing for you to remember is that if it appears energy was lost, then it must have left in the form of heat. Likewise, if it appears energy was gained, then it was absorbed from its surroundings in the form of heat.

Thermochemistry 2H2 + O2 2H2O + HEAT If a chemical reaction releases energy to its surroundings, then we call it an exothermic reaction. Energy is released during chemical reactions when more bonds are broken than made. This means the products have fewer bonds and less stored energy than the reactants. 2H2 + O2 2H2O + HEAT

Thermochemistry H2 + Cl2 + HEAT 2HCl If a chemical reaction absorbs energy from its surroundings, then we call it an endothermic reaction. Energy is absorbed during chemical reactions to make more chemical bonds. This means that the products have more stored chemical energy than the reactants. H2 + Cl2 + HEAT 2HCl

Thermochemistry All chemical reactions, whether they release or absorb energy, require a certain amount of energy to begin. Propane combusts in the presence of oxygen to release large amount of energy, but the fire doesn’t begin as soon as propane and oxygen meet. There first has to be a spark. The energy required to start a chemical reaction is known as the activation energy (EA).

Thermochemistry The enthalpy (H) of a system is the total energy of a system, including the total internal energy plus the work needed to displace its environment (P × ΔV). The change in enthalpy (ΔH) is simply equal to the heat absorbed or released by the system during a chemical process.

Thermochemistry The enthalpy of formation ( ) is the change in enthalpy required to make 1 mole of a substance under standard conditions (this is what the superscript (o) means). Because the change in enthalpy is equal to the heat absorbed or released, this is also referred to as the heat of formation.

2C2H6 (g) + 7O2 (g) → 6H2O(g) + 4CO2 (g) Calculating the Enthalpy of a Reaction 2C2H6 (g) + 7O2 (g) → 6H2O(g) + 4CO2 (g) Reactants Products To calculate the enthalpy of reaction, you simply add up the enthalpy of the products and subtract it from the enthalpy of the reactants. (These numbers will always be provided in a table.)

Important Facts These values are also given in kilojoules per mole (kJ/mol). This means that you will need to multiply each value by the coefficient in the equation. The state of matter MATTERS! Be sure to select the correct value. The heat of formation of ALL elements, including diatomic elements, is ZERO (0 kJ/mol)! Because of this, they may not appear in the table.

2C2H6 (g) + 7O2 (g) → 6H2O(g) + 4CO2 (g) ΔHf = -83.85 kJ/mol H2O (g) ΔHf = -241.818 kJ/mol H2O (l) ΔHf = -285.830 kJ/mol CO2 (g) ΔHf = -393.509 kJ/mol Products = 6(-241.818 kJ/mol) + 4(-393.509 kJ/mol) Products = -3024.94 kJ Reactants = 2(-83.85 kJ/mol) + 7(0 kJ/mol) Reactants = -167.7 kJ

2C2H6 (g) + 7O2 (g) → 6H2O(g) + 4CO2 (g) Products = -3024.94 kJ Reactants = -167.7 kJ ΔH = Products - Reactants ΔH = (-3024.94 kJ) – (-167.7 kJ) ΔH = -2857.2 kJ

Calculate the Enthalpy of a Reaction C3H8 (l) + 5O2 (g) → 4H2O(g) + 3CO2 (g) Products = 4(-241.818 kJ/mol) + 3(-393.509 kJ/mol) Products = -2147.799 kJ Reactants = -104.7 kJ/mol + 5(0 kJ/mol) Reactants = -104.7 kJ ΔH = Products – Reactants = (-2147.799 kJ) – (-104.7 kJ) ΔH = -2043.1 kJ

Calculate the Enthalpy of a Reaction 2H2O (l) → 2H2 (g) + O2 (g) Products = 2(0 kJ/mol) + 0 kJ/mol Products = 0 kJ Reactants = 2(-285.830 kJ/mol) Reactants = -571.66 kJ ΔH = Products – Reactants = (0) – (- 571.66 kJ) ΔH = + 571.66 kJ

Thermochemistry A negative enthalpy (ΔH < 0) means a chemical reaction releases energy. This is an exothermic reaction. The products have less stored energy than the reactants, so products – reactants is a negative number when energy is released.

Thermochemistry A positive enthalpy (ΔH > 0) means a chemical reaction absorbs energy. This is an endothermic reaction. The products have more stored energy than the reactants, so products – reactants is a positive number when energy is absorbed.