1. Exothermic and Endothermic Reactions
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2. Energy and Chemical Reactions
Energy is involved in chemical reactions in two ways:
to break bonds between atoms in the reactants, or
to form new bonds between atoms to make new products

3. Endothermic and Exothermic Reactions
Step 1: Energy must be SUPPLIED to break chemical bonds.
Step 2: Energy is RELEASED when new chemical bonds are made.
A reaction is EXOTHERMIC if more energy is RELEASED than SUPPLIED.
If more energy is SUPPLIED than is RELEASED, the reaction is ENDOTHERMIC
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4. Two Types of Chemical Reactions
We classify chemical reactions based on how the energy of the reactants compares to the energy of the products.
Either energy is given off or energy is required.

5. Whenever chemical reactions occur, energy is transferred to or from the surroundings.
An exothermic reaction is one which transfers heat energy to the surroundings
An endothermic reaction is one which takes heat energy from the surroundings

6. heat taken in and temperature drops
Measuring Heat
reaction
Endothermic
heat taken in and temperature drops
energy is “trapped” in new bonds

7. Reactants + Energy as Heat  Products

8. Combustion reactions are exothermic.
Exothermic chemical reactions release heat. There is more energy than needed to make the new bonds so the “extra” energy is released as heat.
Combustion reactions are exothermic.

9. Exothermic chemical reactions give off heat and the temperature rises.
Heat Released
Exothermic chemical reactions give off heat and the temperature rises.
reaction

10. Reactants  Products + Energy as Heat

11. Exothermic Chemical Reactions
Once started, exothermic reactions tend to keep going as each reaction releases more energy to fuel neighboring molecules.
What is we wanted to take water apart instead?

12. Exothermic vs Endothermic:
EXOTHERMIC – more energy is given out than is taken in
ENDOTHERMIC – energy is taken in but not given out
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13. EXOTHERMIC ENDOTHERMIC Chemical reactions where HEAT…
is transferred from the surroundings into the reactants …
indicated by a DROP in temperature
Chemical reactions where HEAT…
is transferred to the surroundings…
indicated by a RISE in temperature

14. Examples Exothermic Endothermic A candle flame Rusting iron
Burning sugar
Glow sticks
Fire crackers
Endothermic
photosynthesis
cooking food

15. Activation Energy
Activation energy is the energy needed to begin a reaction and break chemical bonds in the reactants.

16. Activation Energy
A flammable material such as gasoline does not burn without a spark. The spark provides the activation energy.