1. Chemical Reactions

2. Chemical Reactions Change Substances
Chemical reactions occur when substances undergo chemical changes to form new substances.
Common, every day chemical reactions include: growing, ripening, decomposing, & burning (combustion)
Growing
Ripening
Decomposing
Burning

3. Signs of Chemical Reactions
1. Bubbles appear.
2. A precipitate (solid) forms in the bottom of the test tube.
3. A color change occurs.
4. The temperature changes (either increases or decreases).
5. Light is given off – often happens in combustion reactions.
6. A change in smell or taste occurs.
Bubbles forming in a reaction
Glowing jellyfish: give off light -- bioluminescence

4. Chemical Reactions Rearrange Atoms
Photosynthesis Equation
Reactants
Products
6 CO H2O + sunlight  C6H12O O2
carbon dioxide + water glucose oxygen

5. Energy is Conserved in Chemical Reactions
1. Energy is neither created nor destroyed.
2. Energy is transformed from one for into another.
3. The total amount of energy is the same at the beginning of the reaction as at the end of the reaction.

6. Energy is Transformed in Chemical Reactions
In photosynthesis, light energy is transformed into chemical energy for the plant.
Energy is transformed from one form into another.

7. Energy and Chemical Reactions
1. Exothermic Reactions: reactions that release energy (energy exits to the environment) – they will heat up!
2. Temperature of surroundings increases because energy is released.
3. Examples of exothermic reactions:
a. digesting food
b. decomposition
c. burning fuel
4. Exothermic reactions may give off light
A burning match is an exothermic reaction – it gives off light & heat.

8. Energy and Chemical Reactions
1. Endothermic reactions: absorb energy from the environment
2. Requires energy to form chemical bonds.
3. Energy used may be in the form of light or heat.
4. May (but not always) see a drop in temperature
Photosynthesis is an endothermic reaction.

9. Synthesis 2 or more elements or compounds combine to make one product
A + B  AB

10. Decomposition
One compound is broken down into 2 or more elements or compounds
AB  A + B

11. Single Replacement
Replacing of one element in a compound with another element
AB + C  AC + B

12. Double Replacement
Two compounds exchange elements
AB + CD  AD + CB

13. Combustion
Fuel burns in the presence of oxygen to produce carbon dioxide and water (along with other by-products)
ALWAYS EXOTHERMIC
Fuel + O2  CO2 + H2O

14. Reactants  Products Reactants (always go on the left):
What you want to react
What you mix together
What you start with
Products (always go on the right):
What is produced by the reaction
The chemicals created by the reaction
What you end up with

15. Identify the reactants and products!
H O  H2O
Na Cl  NaCl
KClO  KCl O2
(Disclaimer: The above reactions are not balanced!)

16. What’s wrong with this picture?
H2 + O2  H2O 
Answer: It is not balanced. The number of atoms on each side are not equal

17. A Closer Look:
H O  H2O 
The Law of Conservation of Matter says that matter cannot be created or destroyed.
Is matter being created or destroyed above?

18. Balancing Chemical Equations
YOU CANNOT CHANGE SUBSCRIPTS! Changing the subscript in a chemical formula changes the chemical. Example:
Methane = CH4
Ethane = C2H6
Subscripts

19. Balancing Chemical Reactions
You CAN Change COEFFICIENTS. Changing a coefficient changes the NUMBER of particles of the substance that you have!
CH (one assumed) = 1C and 4H
2CH4 = 2C and 8H
3CH4 = 3C and 12H

20. Counting Atoms
You ALWAYS multiply coefficients by subscripts to get total of a particular element
How many atoms are represented on each side?
4Al + 3Cl2  2Al2Cl3
Al=4
Cl=6

21. A Closer Look
Lets Balance the creation of Water… our original reaction:
H O  H2O
First… List all elements in the same order on both sides 2 2
Next, go down the list and balance one at a time
Whenever you make a change, you should go
through the list again to make sure!
We are BALANCED!
Balanced
Balanced
Unbalanced
H=4
H=2
H=2
H=4
Unbalanced
Balanced
O=2
O=2
O=1

22. Lets Do another! Na + Cl2  NaCl 2 2
1st: List all the elements you have in the same order
2nd: Go down list and balance as you go.
4th: We are BALANCED!.
3rd: You made changes. Balance list again.
Balanced
Balanced
Unbalanced
Na=1
Na=2
Na=1
Na=2
Balanced
Unbalanced
Cl=2
Cl=2
Cl=1

23. X One More! KClO3  KCl + 2 2 3 O2 O3 O2
PLEASE NOTE: Since 2 does not divide into 3, you will need to change two at once to balance oxygen. Both 2 and 3 divide into 6 so this should be your new total!
You must do all chemical reactions as shown before. You can ONLY change coefficients!
So lets get started…
You might be tempted to…
change the subscript
Add coefficients that will give you 6 oxygens on each side.
Step 3: Since you made changes, do the list again!
Finally: We are BALANCED!
Step 2: Balance the list.
Step 1: List all elements in the same order
But as you can see, doing this changes regular oxygen into another substance altogether… Ozone!
Since this reaction NEVER creates ozone, doing this is WRONG!
K=1
K=2
K=2
K=1
Balanced
Unbalanced
Balanced
Notice we balanced this too!
Cl=1
Cl=2
Cl=2
Cl=1
Balanced
O=3
O=6
O=6
O=2
Unbalanced
Balanced
You CAN NEVER change subscripts!

24. Classify & Balance Each Chemical Equation Warm-Up
1. Mg + O2  MgO
2. H2O  H2 + O2
3. Al + CuCl2  AlCl3 + Cu
4. FeCl2 + Na2CO3  NaCl + FeCO3

25. Warm-Up: Check Your Answers
Synthesis
Decomposition
Single replacement
Double replacement
1. 2 Mg + O2  2 MgO
2. 2 H2O  2 H2 + O2
3. 2 Al CuCl2  2 AlCl Cu
4. FeCl2 + Na2CO3  2 NaCl + FeCO3