1. Chemistry – Chapter 2
Chemical Reactions

2. Section 1: Chemical Formulas and Equations

3. Vocabulary Words Physical Changes Chemical Changes Chemical Reaction
Reactants
Products
Chemical Equation
Word Equation
Law of Conservation of Mass
Endothermic Reactions
Exothermic Reactions

4. Physical or Chemical Change
Matter can undergo two kinds of changes:
Physical Changes
Only affects physical properties
Same chemical formula and properties
Chemical Changes
Produce new substances with different properties
Different chemical formula and properties
Chemical Reaction: process that produces a chemical change

5. Recognizing a Physical Change
Change in size
Change in shape
Change in state of matter
Solid, liquid, or gas

6. Practice Identifying Physical Changes
How can you tell a physical change has taken place in each photo?

7. Recognizing a Chemical Change
Light produced
Sound produced
Temperature change
Gas produced (bubbles)
Flavor changes
Color changes
Smell/odor changes

8. Practice Identifying Chemical Changes
How can you tell a chemical change has taken place in each photo?

9. A penny is added to a test tube of nitric acid
How do you know the penny is undergoing a chemical reaction?
Color change
Gas Produced - Bubbles
Can the penny ever change back? No

10. Chemical Equations
Reactants: Substances that exist before the reaction begins
Products: Substances that form as a result of the reaction
In a chemical reaction, the reactants change into the products.

11. Describing What Happens
Chemical Equation: Shorthand form of writing what reactants are used and what products are created
Ex. HC2H3O2 + NaHCO3 → NaC2H3O2 + CO2 + H2O
(baking soda + vingear) → (sodium acetate + carbon dioxide + water)

12. Using Words Reactants Products
Charcoal + Oxygen → Ash + CO2 (gas) + Heat
Iron + Oxygen + Water → Rust + Heat
Silver + Hydrogen Sulfide (gas) → Black Tarnish + H2 (gas)
Natural gas from stove + Oxygen → CO2(gas) + Heat
Note: The arrow reads as “produces” or “yields”.

14. Using Chemical Names
Word Equation: Chemical equation that uses chemical names instead of formulas
Ex. Sodium Bicarbonate + Acetic Acid → Sodium Acetate + Carbon Dioxide + Dihydrogen Monoxide
Use chemical names instead of common names
Ex. Baking Soda + Vinegar → Sodium Salt + Carbon Dioxide + Water

15. Chemical vs. Word Equations
Faster
Show chemical formulas
Word
Longer
Use more familiar names

16. Conservation of Mass
Law of Conservation of Mass: Matter cannot be created or destroyed
Created by Antoine Lavoisier
Means that you must have the same number of atoms on both sides of an equation
I created Law of Conservation of Mass, named Oxygen and Hydrogen, wrote the first extensive list of elements, and introduced the metric system!
Because of all of these accomplishments I am known as the Father of Chemistry!

18. Balancing Chemical Equations
When you write a chemical equation, you must observe the Law of Conservation of Mass
Every atom that appears on the left, must appear on the right!
If there are 5 atoms of Oxygen at the start, then there must be five at the finish!

19. For Example…
There are 4 Hydrogen and 2 Oxygen in the reactants
There are still 4 Hydrogen and 2 Oxygen in the Products
2H O2  2H2O

20. Count the Atoms Ag + H2S → Ag2S + H2 (gas) (black tarnish) (gas)
silver + Hydrogen sulfide Silver sulfide + Hydrogen
(gas) (black tarnish) (gas)
You must count the atoms on each side.
How many Silver atoms in the reactants?
How many Silver atoms in the products? 1 2
This is IMPOSSIBLE!
WHY???

21. Ag + H2S → Ag2S + H2
silver + Hydrogen sulfide Silver sulfide + Hydrogen
(gas) (black tarnish) (gas)
Place a 2 in front of the Ag in the reactants and see if they balance now.
2Ag + H2S → Ag2S + H2
Ag = Ag = 2
H = H = 2
S = S = 1
It’s Balanced!
NOTE: You will never add a new subscript!
Only a coefficient!

22. Try this one! CH4 + O2 → CO2 + H2O + Heat Reactants Products C=1 C=1
Methane and oxygen react to form carbon dioxide, water, and heat.
CH4 + O2 → CO2 + H2O + Heat
Reactants Products
C=1 C=1
H= H=2 (You need 2 more H)
O=2 O=3
- Place a 2 in front on the H2O
CH4 + O2 → CO2 + 2H2O + Heat
Now, multiply 2x2 = 4H and 2x1=2O

23. Energy in Chemical Reactions
When a chemical reaction takes place bonds break in the reactants and new bonds form in the products

24. Energy released Products are more stable
Bonds have less energy than those of the reactants
Extra energy is released in the form of light/sound/heat
Exothermic Reactions: release heat → temperature increases ↑
Ex. Burning

25. Energy absorbed Products are more stable
Bonds have less energy than those of the reactants
Endothermic Reactions: absorb heat → temperature decreases ↓
Ex. Ice pack

27. Energy in the Equation
Energy is often written in equations as a reactant or a product
Energy as a reactant = necessary for the reaction to occur
Energy as a product = energy released during the reaction
Ex. Cooking food on your gas stove:
CH4 + 2O2 → CO2 + 2H2O + energy

28. Section 2: Rates of Chemical Reactions

29. Vocabulary Words Activation Energy Rate of Reaction Concentration
Inhibitor
Catalyst
Enzyme
Proteases

30. How Fast? Reaction speeds vary greatly
Fireworks explode in a few seconds
Copper pennies darken slowly as they react with substances they touch over a number of years

31. Activation Energy
Before a reaction can start, molecules must collide with each other
This collision will cause old bonds to break and allow new bonds to form
Molecules must collide with a certain amount of energy or the reaction will not occur
Activation Energy: Minimum amount of energy needed to start a chemical reaction

32. Activation Energy Continued….
Reactions that release and absorb energy need activation energy to begin
Ex. Burning gasoline
Won’t burn on its own it needs to be ignited

33. Reaction Rate Rate of Reaction: How fast a reaction occurs Helpful in
Measured in 2 ways:
How fast the reactants disappear
How fast the products appear
Helpful in
Determining how fast a product can be made
Measuring time before food/medicine will spoil

35. Affecting Reaction Rate
6 Factors that affect reaction rate:
Temperature
Concentration
Surface Area
Inhibitors
Catalysts
Enzymes

36. 1. Temperature Decreasing Temperature
Decreases rate of reaction (slows down) 
Food in the fridge lasts longer because bacteria grows more slowly in colder temperatures
Atoms move slower at lower temperatures
The more slowly an atom moves the fewer collisions it will have

37. Increasing Temperature
Increases rate of reaction (speeds up) 
Heating foods while cooking them speeds up the molecules
Higher temperatures cook faster than lower ones
Atoms move faster at higher temperatures
The faster an atom moves the more collisions it will have

38. 2. Concentration
The more molecules present the more likely a collision will be
Ex. You’re more likely to bump into someone in a crowded hallway than an empty one

39. Concentration: Amount of substance present in a certain volume
Decrease Concentration
Decreases rate of reaction (slows down) 
Fewer atoms = less collisions
Increase Concentration
Increases rate of reaction (speeds up) 
More atoms = more collisions

40. 3. Surface Area
The exposed surface of reactant particles affects how fast a reaction can occur
Only molecules in the outer layer can touch other reactants and react
Ex. Small twigs of wood have more surface area for their size than large logs and would be easier to start a fire with because more wood molecules can react

41. Decreases rate of reaction (slows down)  Increase Surface Area
Decrease Surface Area
Decreases rate of reaction (slows down) 
Fewer surface atoms = fewer reactions
Increase Surface Area
Increases rate of reaction (speeds up) 
More surface atoms = More reactions

44. 4. Inhibitors Inhibitor: Substance that slows down a chemical reaction
Adding an Inhibitor
Decreases rate of reaction (slows down) 
Used to increase the shelf life of foods and medicines

45. 5. Catalysts Catalyst: Substance that speeds up a chemical reaction
Reduces the level of activation energy needed to start a reaction by providing a surface for the reaction to take place
A reaction using a catalyst will not produce more product, it will just produce the same amount of product in shorter time

46. Increases rate of reaction (speeds up) 
Adding a catalyst
Increases rate of reaction (speeds up) 
Ex. Catalytic Converter in a car speeds up the reactions that change harmful substances like carbon monoxide into harmless substances like water

48. 6. Enzymes
Enzymes: Protein molecules that speed up or slow down reactions needed for your cells to work properly
Adding enzymes make it possible for your body to function properly  or 
Convert food to fuel
Build bone and muscles
Convert extra energy into fat
Produce other enzymes

50. Proteases: Enzymes outside of the body that specialize in protein reactions
Ex.
Meat tenderizer: breaks down proteins to make meat more tender
Contact lens cleaner: breaks down proteins from your eyes that could collect on your lens and cloud your vision