1. Energy Changes in Reactions

2. What is a Chemical Change?
A chemical change is a change that occurs when a substance reacts and forms one or more new substances.
This happens during a chemical reaction.
Because the chemical properties change, you know a chemical reaction has taken place.

3. When Do Chemical Changes Occur?
Chemical changes occur when a chemical reaction takes place.
Examples of reactions include rusting, burning, and digesting.
burn baby burn

4. There are five easy ways you can tell if a chemical reaction (aka: a chemical change) is happening:
1. A gas is produced
2. It produces a solid (precipitate)
3. It changes color
4. It makes light
5. It gives off or takes in heat energy

5. Parts of a Chemical Reaction
There are two main parts of a chemical reaction.
Reactants – The substances that undergo the change.
Products – The new substances formed as a result of that change.

6. What is Chemical Energy?
Chemical energy is the energy stored in the chemical bonds of a substance.
Chemical reactions involve the breaking of chemical bonds in the reactants and the formation of chemical bonds in the products.

7. Chemical Energy in Grilling?
When you use a propane grill, you spark the propane in the presence of oxygen.
When this occurs, you break the bonds in propane and oxygen molecules and form the bonds in carbon dioxide and water molecules.
C3H8 + 5O2  3CO2 + 4H2O + Heat

8. Breaking Bonds The breaking of chemical bonds requires energy.
This is why propane grills need a spark. The ignition gives enough energy to start the reaction and break bonds.

9. Forming Bonds The formation of chemical bonds releases energy.
This energy is shown in the heat and light given off by the grill.

10. Endothermic Vs. Exothermic
During a chemical reaction, energy is either released or absorbed.
A chemical reaction that releases energy to its surroundings is an exothermic reaction. (feels hot)
A chemical reaction that absorbs energy is an endothermic reaction. (feels cold)

11. Reaction Energy - Exothermic
Energy given off by reaction

12. Reaction Energy - Endothermic
Energy absorbed by reaction

13. What About the Law of Conservation of Energy?
In a chemical reaction, the amount of energy before and after the reaction is the same.
Energy at beginning = Energy at end