Unit 1: Reaction Kinetics Lesson 4: Enthalpy Changes in Chemical Reactions

Bond Energy Bond energy is the amount of energy required to break a bond between two atoms. To break a bond, an amount of energy equal to the bond energy must be added. When a bond is formed, an amount of energy equal to the bond energy is released.

Enthalpy Enthalpy (H) is the total kinetic and potential energy that exists in a chemical system at constant pressure. ΔH = Hproducts – Hreactants = the change in enthalpy during the course of a reaction 5 min

Brain Break! 5 min

Endothermic Reactions Heat ENters the system in an ENdothermic reaction, so energy is gained, and the change in enthalpy (ΔH) is positive. 5 min Written in one of two ways: 2 N2 + O2 + 164 kJ → 2 N2O or 2 N2 + O2 → 2 N2O ; ΔH = +164 kJ

Exothermic Reactions Heat EXits the system in an EXothermic reaction, so energy is lost, and the change in enthalpy (ΔH) is negative. 5 min Written in one of two ways: H2 + Cl2 → 2 HCl + 184 kJ or H2 + Cl2 → 2 HCl ; ΔH = -184 kJ

Warm (and Cold) Feelings In an exothermic reaction, the heat lost from the system is gained by the surroundings, so that they feel warmer. In an endothermic reaction, the heat gained by the system is lost from the surroundings, so that they feel colder. 5 min

Practice Pg. 16 #24-28 20 min