1. Chemical Reactions + -

2. Chemical Reactions Objectives: I will be able to:
describe what happens to the elements in a chemical reaction.
describe how to write a skeleton equation.
describe how the rate of a reaction can be changed.
review chemical change indicators such as precipitate, evolution of gas, color change, and absorption/release of heat.

3. Chemical Reaction
chemical reaction: the process by which a chemical change occurs
Atoms are rearranged, and chemical bonds are broken and reformed.
One or more substances change to produce one or more different substances.
Types of Reactions
Synthesis (creating)
Decomposition (separating)
Combustion (burning)
Single, Displacement/Replacement (switching)
Double Displacement/Replacement (switching)
What does
H2 & O2 gas
create?
Out of the five types of reactions, which would describe the chemical reaction taking place to the right? H2 O2

4. Physical changes are not indicative of a chemical reaction
physical change: a change in shape, size, color, or state without a change in chemical composition
Physical change is a change that is reversible.
Examples
tearing paper
cutting your hair
change in state
Physical changes are not indicative of a chemical reaction

5. mixing vinegar & baking soda
Chemical Change
chemical change: a change in which a substance becomes another substance having different properties and chemical composition
a change that is not reversible using ordinary physical means
Changes that usually cause production of, heat, sound, light, odor, fizzing/foaming, color changes
You usually need more than one of the above characteristics to be considered a chemical change!
Examples
mixing vinegar & baking soda
burning a piece of wood
soured milk
A chemical analysis is the only 100% way to know a
chemical change has occurred.

6. Chemical Equations
chemical equations: shorthand form for writing what reactants are used and what products are formed in a chemical reaction
Sometimes shows whether energy is produced or absorbed
Examples:
2H2 + O2  2H2O
CH4 + 2O2   CO2  +  2H2O
C6H12O6 + 6O2  6CO2 + 6H2O + energy

7. Chemical Equations
chemical formula: a combination of symbols that indicates the chemical composition of a substance
coefficient: the number in front of a chemical formula that tells us how many molecules of a given formula are present
subscript: a number after an element’s chemical symbol that tells us how many atoms there are within a compound

8. Components of a Chemical Equation
Formulas
Chemical
Formula 
2H2 + O2
2H2O
Subscript
Coefficient
Coefficient
Subscripts
(Yield)
(Reactants)
(Products)
Sometimes you will see a “yields” sign that looks like this.
What do you think it means?

9. Energy and Chemical Reactions
Exothermic Reaction
A chemical reaction in which thermal energy is released.
The products have greater bond energy than the reactants
C6H12O6 + 6O2  6CO2 + 6H2O + energy
(respiration)
Endothermic Reaction
A chemical reaction in which thermal energy is absorbed.
The products have lower bond energies than the reactants
6CO2 + 6H2O + energy  C6H12O6 + 6O2
(photosynthesis)
Chemistry Comes Alive! Sample Movies
TeacherTube - Videos
Chemistry Demonstration Videos
Can you think of other reactions where
energy is gained or released?

10. Rates of Chemical Reactions
The rates at which chemical reactions can take place are
based on the interaction (collisions) between the different
particles. These rates can be impacted by the following:
Temperature: a measure of the average kinetic energy of the particles in a sample of matter
Ex. Increasing the temperature when cooking
Surface area: amount of material that comes into contact with the reactants
Ex. Cutting a potato into smaller pieces when cooking
Concentration: amount of substance per volume
Ex. Turning the valve on a gas stove to increase the concentration of methane molecules
Catalysts (enzymes): organic substances that help speed up chemical reactions, but are not consumed in the reaction

11. Law of Conservation of Mass
Proposed by Antoine Lavoisier
In a chemical reaction, atoms are neither created nor destroyed
All atoms present in the reactants are also present in the products
Chemical equations must account for/show the conservation of mass  balancing equations
In its present form, does this chemical equation show a
conservation of mass? 2
H2 + O2  2
H2O
Reactants
Products
How would you balance this equation to
show the conservation of mass?
H  2
O  2 4 2
H  2
O  1 4 2

12. Hints For Balancing Equations
Count the atoms
List the number of atoms of each element to see which elements must be balanced
Use a coefficient to add atoms to one side of the equation
Start with the reactant or product that has the greatest number of different elements
Add a coefficient to another reactant or product
Make sure that the coefficients in your balanced equation are the smallest whole numbers possible (they should have no common factor other than one)
Tutorial on Balancing Equations

14. Chemical Reactions https://www.youtube.com/watch?v=CgWHbpMVQ1U
Objectives:
Predict products of chemical reactions such as synthesis, decomposition, replacement, and combustion.
Balance chemical equations with adherence to law of conservation of matter.
Review chemical change indicators such as precipitate, evolution of gas, color change, and absorption/release of heat.