1. The Law of Mass Conservation & Energy in Reactions

2. The Law of Mass Conservation states matter can not be created or destroyed during a chemical reaction.
This means that all of the atoms present before the reaction must be present after the reaction (they are just “rearranged” when bonds break/form)

3. Each atom present at the beginning of a reaction will be present at the end of a reaction, just as part of a different substance.

4. Overall, the total mass of the reactants should equal the total mass of the products
Ex/

5. Open –v- Closed Systems
In an open system, matter may escape into the environment, but mass is still conserved
Ex/ Burning a log – gas escapes
In a closed system, matter can not escape into the environment
Ex/ A reaction in a sealed container

6. Balanced Equations
To show mass is conserved during a reaction, a chemical equation must be balanced.
In a balanced equation, the total number of atoms of each element on the reactant side of the equation, must be the same as the number of atoms of that element on the product side of the reaction.

7. Example 2 H2O  2 H2 + O2 Reactants Products H = 4 O = 2
This equation is balanced, and follows the law of mass conservation

8. Representing balanced chemical reactions using diagrams…

9. Energy in Reactions
When a chemical reaction occurs, the chemical bonds of the reactants break and/or new chemical bonds form.
Chemical bonds contain a form of energy called chemical energy
The reaction requires energy to break the bonds of the reactants
The formation of a chemical bond releases energy to the surroundings

10. Exothermic Reactions
Exothermic reactions release more energy than they absorb.
The energy released when the products bond is greater than the energy required to break the bonds
You feel this as heat, and the temperature increases

11. Endothermic Reactions
Endothermic reactions absorb more energy than they release.
The energy required to break the bonds is greater than the energy released when the products bond.
This causes the reaction to feel cold, and the temperature decreases.

12. What else impacts reactions?
The amount of thermal energy absorbed or released by a reactions can be impacted by factors such as the amount of each substance used or the concentration of a given substance.

13. Homework Complete 4 Square Vocabulary Boxes for the following words:
Law of Mass Conservation
Endothermic Reaction
Exothermic Reaction