1. Exothermic & Endothermic REACTIONS
Energy in Reactions

2. Energy in Reactions
Some chemical reactions release heat to the surroundings
These are called EXOTHERMIC reactions
Heat EXITS the reaction
Some chemical reactions absorb heat from the surroundings
These are called ENDOTHERMIC reactions
Heat ENTERS the reaction

3. Energy in Reactions All substances have a type of chemical potential:
The total energy (J or kJ) contained in a system
Chemical potential energy in a substance
The symbol for enthalpy is H
ΔH = Hproducts – Hreactants

4. Exothermic Reactions
An exothermic reaction converts the enthalpy stored in a substance into HEAT which is released to the surroundings N2 + 3H2  2 NH3+ heat
Has higher enthalpy
Has lower enthalpy

5. Exothermic Enthalpy Diagram
Products
Reactants
Enthalpy (H)
Reaction Proceeds
ΔH is Negative

6. Exothermic Reactions
1. The “heat term” written on the PRODUCTS side N2 + 3H2  2 NH kJ 2. ΔH written beside the equation is NEGATIVE. N2 + 3H2  2 NH3 ΔH = kJ

7. In Summary… Exothermic Reactions: ΔH is negative
Products are lower than reactants on the enthalpy diagram
Heat is released to the surroundings

8. Endothermic Reactions
An endothermic reaction absorbs heat from the surroundings and converts into enthalpy
2C + 2 H2 + heat  C2H4
Has lower enthalpy
Has higher enthalpy

9. Endothermic Enthalpy Diagram
Products
Reactants
Enthalpy (H)
Reaction Proceeds
ΔH is POSITIVE

10. Endothermic Reactions
1. The “heat term” written on the REACTANTS side. 2 C + 2H kJ  C2H4 2. ΔH written beside the equation is POSITIVE. 2 C + 2 H2  C2H4 ΔH = kJ

11. In Summary… Endothermic Reactions: ΔH is positive
Products are higher than reactants on the enthalpy diagram
Heat is absorbed from the surroundings

14. Please collect the Endo/Exo Worksheet from the front of the room, finish this worksheet by next class
Next Class: Begin Stoich
In Class Work