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

2. 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.

3. 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

4. Brain Break!
5 min

5. 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 + O kJ → 2 N2O
or 2 N2 + O2 → 2 N2O ; ΔH = +164 kJ

6. 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 kJ
or H2 + Cl2 → 2 HCl ; ΔH = -184 kJ

7. 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

8. Practice
Pg. 16 #24-28
20 min