1. Endothermic & Exothermic Reactions

2. Endothermic process
Heat flows into the system from the surroundings in an endothermic process.
Surroundings
Cold pack
Energy
Temperature falls

3. Exothermic process
Heat flows into the surroundings from the system in an exothermic process.
Hot pack
Energy
Surroundings
Temperature rises

4. Energy Level Diagrams Endothermic reactions products reactants
Enthalpy (ΔH)
is positive.
reactants
The products are higher in energy than the reactants
time of reaction

5. Energy Level Diagrams Exothermic reactions reactants products
Enthalpy (ΔH)
is negative.
energy
reactants
products
Time of reaction
The products are lower in energy than the reactants

6. But we use the unit called the JOULE
Units of Energy
James Joule
1 calorie = heat required to raise temp. of 1.00 g of H2O by 1.0 oC.
1 kcal = 1 Calorie (a food “calorie”)
But we use the unit called the JOULE
1 cal = joules

7. Breaking & Making Bonds
Breaking Bonds
always requires energy, so the process is endothermic
Making Bonds
gives off energy, so the process is exothermic

8. Energy (in Joules) CH4 + 2O2 CO2 + 2H2O C 4H 4O Step 1 Step 2
Represents energy diagram
Step 1: Breaking Bonds (Endothermic)
Step 2: Making Bonds (Exothermic)