1. Chemical Reactions
Chapter 4 Lesson 1
p. 212

2. I. What Are The Signs Of A Chemical Reaction?
Chemical Reaction- is the process in which atoms are rearranged to produce new substances.
The bonds that hold atoms together may be formed or broken
The properties of the substances produced in a chemical reaction are different than the properties of the original substances

3. A. Signs Include: Color Change Change in odor Formation of precipitate
Appearance of gas bubbles

4. II. How Are Chemical Reactions Modeled?
Chemical Formula- Uses chemical symbols & numbers to represent a given substance
The chemical symbols in a chemical formula tell you what elements make up a substance
Subscripts tell you how much of each atom are in a molecule
Subscripts are always located to the right of the chemical symbol they correspond with

5. A. WITH CHEMICAL EQUATIONS
Chemical Equation- is an expression that uses symbols to show the relationship between the starting substances & the substances that are produced in a chemical reaction
Reactants- substances that participate in a chemical reaction
Products- substances formed in a reaction
Arrow known as a Yields Sign points from reactants to products

6. Chemical Equation Example

7. III. How Do Chemical Equations Show The Law Of Conservation Of Mass?
? What is the Law of Conservation of Mass?
This means a CE must show the same numbers & kinds of atoms on both sides of the arrow
Reactants & products contain the same number of atoms of each element

8. Balancing the Equation
Coefficients are used to balance the equation
A coefficient is a number placed in front of a Chem Formula.
Ex: 3H2O
You must multiply the subscript with the coefficient in the formula
Only coefficients, NOT SUBSCRIPTS, can be changed when balancing an equation
Ex: H2O2

9. Let’s DO THE MATH
Individually complete the Do The Math problem on page 217.
Take about 5 minutes.
Drop your pencils or pens and look up when you are done.

10. IV. What Happens To Energy During Chemical Reactions?
Changes in energy are part of all chemical reactions
CEs will either absorb or release energy
Energy is needed to break the bonds in the reactants
Reactants are described by the overall change in energy b/t the product & reactants

11. A. ENERGY CAN BE ABSORBED
Endothermic Reaction- a chemical reaction that requires an input of energy
Energy taken in is absorbed from the surroundings
Often as heat
This is why endothermic reactions often feels cold
Photosynthesis is an example of an endothermic reaction
Light is the energy input in this example
More energy is absorbed during the process than is released

12. B. ENERGY CAN BE RELEASED
Exothermic Reaction- a chemical reaction in which energy can be released to the surroundings.
Burning is an example
The products of the reaction are lower in energy than the reactants
Often feel warm when heat is released to their surroundings

13. C. ENERGY IS ALWAYS CONSERVED
What is the Law of Conservation of Energy?
The total amount of energy does not change in an exothermic or endothermic reaction
The total amount of all the types of energy is the same before and after every chemical reaction

14. V. What Affects the Rates of Reactions?
The rate of reaction describes how fast the reaction occurs.
For a reaction to occur, particles of the reactants must collide
Reaction rates depend on how often the particles collide
What are the factors that control reaction rates?
Concentration
Surface Area
Temperature
Catalysts

15. A. CONCENTRATION
Higher concentrations = more reactants in a given volume
This means particles are more likely to collide
Reaction rates are higher when reactant concentration is higher

16. B. SURFACE AREA
Crushing or grinding solids increases surface area and reaction rate
Reaction rate increases when more particles are in contact with one another

17. C. TEMPERATURE Reactions usually occur faster at higher temperatures
Particles move faster at higher temperatures
This increases likelihood of collision

18. D. Catalysts
A substance that changes the rate of chemical reactions without being used up or changing very much.
Increase the rate by bring together the reactants
Enzymes are an example of catalysts in your body

19. QUESTIONS!!!
What is happening to atoms as the undergo a chemical reaction?
Use methane (CH4) + two oxygen molecules (2O2)
How are endo & exo thermic reactions different?
How can the formation of Bubbles tell us that a chemical reaction to place?
How is it possible for 2 substances to combine to form bubbles without a Chem reaction taking place?
How are coefficients different from subscripts in a chemical formula?
Lets go back and recap our 5 main questions