1. Endo & Exo-
thermic
Reactions

2. Review: What is a Chem. Rxn?
Chemical
Energy
Chemical
Energy O O
Chemical
Energy
Energy
Pow H H C
Chemical
Energy
Pow
Energy H H
Chemical
Energy O O
Old Bonds Break
New Bonds Form
When reactions occur, the atoms change partners and new bonds form
When this happens, something also causes changes with the energy
Q: What energy is being used to hold the atoms together in molecules?
A: Chemical Energy

3. H C What about the energy? “Chemical Energy” is what makes these atoms
“stick” together. What we call “bonded” H C
These two atoms are bonded
together by the chemical energy
Similar to magnets, there has to be something to hold
the atoms to each other! Without it there is NO BONDING!

4. What is the energy doing?
During a chemical reaction a change in energy occurs. Some reactions absorb energy, other reactions release energy.
Remember that energy can change from one form/type into another! EX: Heat into Chemical or visa versa
Endothermic Reactions: a reaction in which heat
energy is “absorbed” from the surrounding air
Exothermic Reactions: a reaction that “releases”
energy in the form of heat & sometimes light into the air.

5. Overview Exothermic: A reaction that “feels” warm/hot
A change in temperature without purposely adding or
removing heat energy, “indicates” a chemical reaction!
EX: Exothermic Rxn vs. Boiling Water
Exothermic: A reaction that “feels” warm/hot
Endothermic: A reaction that “feels” cool/cold
Reaction
The heat energy leaves and warms up
everything around it (like the thermometer!)
Heat leaves
The new molecules didn’t need as much energy
for their new bonds. So the extra Chemical
energy is turned into heat that
goes somewhere else.
The heat energy is absorbed and the outside
is now cooler (like the thermometer!)
The new molecules need more energy to create
The new bonds so they absorb energy from the
space around the reaction = heat is turned into
Chemical Energy (Bonds)!!!
Reaction
Heat comes in

6. Temperature of an Exothermic Reaction
What would you expect the line to do with this reaction? ?
Temperature
Time

7. Temperature of an Endothermic Reaction
What would you expect the line to do with this reaction? ?
Temperature
Time

8. Exothermic Reaction Reactants must be close enough together O O H H C
Has to have enough energy to make it happen
(Activation Energy) O O
When bonds break and reform we call it a chemical reaction

9. BLAMO! Exothermic Reaction
Release of Energy O O H H C H H O O
Think: What indicators would you
expect with this type of reaction?
Exothermic Reactions give off the extra energy it didn’t need to form the new bonds.