1. Chemical Reactions
PESS 1

2. Observing Chemical Change
Review:
Physical properties: a characteristic that can be observed without changing the substance
Chemical properties: a characteristic of a substance that describes a substance’s ability to change
Physical change: a change that alters the appearance of a substance, but does not change the substance
Chemical change: a change that occurs when the substance does change into a new substance (AKA chemical reaction)

3. Observing Chemical Change
Chemical changes occur when bonds break and new bonds form the result is a new substance is produced
Evidence that a chemical reactions has occurred
The substance might change form
A solid may appear when two solutions are mixed together (this is called a precipitate)
A gas might be produced from solids or liquids
Be careful because these changes might also occur in a physical change

4. Observing Chemical Change
Changes in energy might also occur in a chemical reaction
Energy can either be absorbed or released
Endothermic: energy is absorbed & feels cool to the touch
Exothermic: energy is released & feels warm to the touch

5. Describing Chemical Reactions
Chemical equations
A short, easy way to show a chemical reaction
Symbols and numbers are used in placed of words
Formulas in equations
CO2, H2O, C6H12O6, NaCl, H2O2, NaHCO3
These are the formulas for common substances

6. Describing Chemical Reactions
Structure of an Equation
Reactants are the substances before a chemical reaction takes place
Products are the substances after a chemical reaction takes place
Reactant + Reactant  Product + Product
CaCO3  CaO + CO2

7. Describing Chemical Reactions
Conservation of Mass
Review: Law of Conservation of Mass (LCM) – matter cannot be created or destroyed
So this means that the same number of atoms that enter a chemical reaction must be the same after the chemical reaction occurs
Open systems: matter can escape into the surroundings (e.g. fire place)
Closed systems: matter cannot escape into the surroundings (e.g. baking bread)

8. Describing Chemical Reactions
Balancing equations
Due to the LCM in a chemical equation the number of atoms that in the reactants must be the same in the products
First step: write the equation
Second step: identify which atoms and how many are used in the equation
Third step: Use coefficients (NOT subscripts) to even out the reactants and the products

9. Practice

10. Answer

11. Describing Chemical Reactions
Classifying Chemical Reactions
Synthesis: 2 or more substances combine to form a more complex substance
H2 + O2  2H2O
Decomposition: a more complex substance breaks apart to 2 or more simpler substances
2H2O2  2H2O + O2
Replacement: an uncombined element replaces an combined element
2Cu2O + C  4Cu + CO2
Double replacement: elements in one compound trade places with elements in another compound
FeS + 2HCl  FeCl2 + H2S

12. Examples

13. Controlling Chemical Reactions
Activation energy
Remember: motion needs a force to get it to move, the same goes for chemical reaction
Activation energy is the “push” that gets chemical reactions going
Examples: a spark is needed for combustion, heat is needed to bake, etc.

14. Controlling Chemical Reactions
Rates of chemical reactions
Chemical reactions can be controlled by changing such factors as: surface area, temperature, concentration levels and using substances called catalysts or inhibitors
Surface area: the smaller the surface area the quicker the chemical reaction (that is why we chew our food)

15. Controlling Chemical Reactions
Temperature: the greater the temperature the quicker the chemical reaction (that is why we put food in the refrigerator because it slows bacteria growth)
Concentration: the greater the concentration the quicker the chemical reaction
Catalysts: a substance that increase the reaction rate by lowering the activation energy needed to get the reaction started
Inhibitors: this is used to decrease the reaction rate by preventing the reactants from combining (preservatives are examples of inhibitors)