1. Intro to Balancing Equations

2. Q: How could you represent this reaction (rusting bicycle), besides just describing it in words?
Look at this rusty bike wheel. It has been left outside in damp weather too many times, so the iron in the metal parts has rusted. Iron rusts when it combines with oxygen in the air. Iron rusting is an example of a chemical reaction. In a chemical reaction, substances change into entirely different substances. For example, the iron in the bike and the oxygen in the air have changed into rust.

3. Fe + O2  Fe2O3 A: Scientists use a standard method
to represent a chemical reaction,
called a chemical equation.
Fe + O2  Fe2O3
(Rust)
Look at this rusty bike wheel. It has been left outside in damp weather too many times, so the iron in the metal parts has rusted. Iron rusts when it combines with oxygen in the air. Iron rusting is an example of a chemical reaction. In a chemical reaction, substances change into entirely different substances. For example, the iron in the bike and the oxygen in the air have changed into rust.

4. Chemical equations are used to represent CHEMICAL REACTIONS
Reactants: Zn + I2
Product: Zn I2

5. Q: What is the general equation for the reaction in which iron rusts?
Iron + Oxygen → Iron Oxide (RUST)
Or, using symbols:
Fe + O2 → Fe2O3 (rust)

6. Q: What are the reactants and products?
Fe + O2 → Fe2O3 (new product - rust)
Reactants
Products
The atoms/compounds you have AFTER the reaction occurs.
The atoms/compounds you have BEFORE the reaction occurs.

7. When is an equation called balanced?
When an equation has the same number of atoms of the same kind on both sides.

8. Chemical Equations 4 Al (s) + 3 O2 (g) ---> 2 Al2O3 (s)
The numbers in the front are called
coefficients
subscripts
The letters (s), (g), and (l) are the
physical states of compounds.
(aq) – aqueous solution
The arrow  means:
to yield = to produce = to form

9. Learning Check
What does “2“ mean in front of LiOH? 2Li (s) + 2H2O (l)  2LiOH (aq) + H2 (g)
Number of molecules
What does “2“ to the right of “H2” mean?
Number of atoms of H
What does “(aq)“ to the right of LiOH mean?
Aqueous solution
What does ““ mean?
Yield, form, produce

10. Q: What is the chemical equation for this reaction?
A: The chemical equation is:
C + O 2 → CO 2

11. Q: How have the atoms of the reactants been rearranged in the products of the reaction?
C + O 2 → CO 2
A: Carbon atom chemically bonds with
two oxygen atoms.
HOW ? What bonds are broken and
what new bonds have formed?

12. C + O 2 → CO 2
C + O=O →
O=C=O
Bonds
Break
New Bonds Form
A: Bonds between the oxygen atoms in the oxygen molecule have been broken, and new bonds have formed between the carbon atom and the two oxygen atoms.

13. Is this a balanced equation?
The charcoal used in a grill is basically carbon. The carbon reacts with oxygen to yield carbon dioxide. The chemical equation for this reaction,
C + O2  CO2, R P
Count the number of atoms of each element found in the reaction.
1 C
1 C
2 O
2 O
Is this a balanced equation?
YES

14. Learning Check
Q: What needs to happen for a chemical reaction (change) to take place?
A: Bonds between atoms of the reactants
break and NEW bonds form, producing
new substance.

15. Q: Is this chemical reaction correct? Explain why.
Learning Check
Q: Is this chemical reaction correct? Explain why. + 
A: No. There are no “pink atoms” on the
reactant side – NEW matter (pink) on the
product side. Matter canNOT be created.

16. Learning Check +  Why is this reaction correct?
B/C atoms must REARRANGE for a chemical reaction to take place + 
NEW PRODUCTS

17. REMEMBER: To balance a chemical equation, you must add COEFFICIENTS (big numbers) in front of the chemical formulas in the equation. You cannot add or change subscripts!!! Why?
Because otherwise it will alter the identity of the substance. Ex. O2 – oxygen, if a subscript changed to 3, O3 = ozone
The order of the Coefficients in the Chemical Equation is important because this is how you write the COEFFICIENT RATIO.

18. Balancing using shapes
____H2 + ____O2  ____H2O 2 1 2
Is this reaction balanced?
Now fix the H on the left.
Double Hydrogen too.
Coefficient ratio: 2:1:2 BALANCED !

19. Balancing Equations using T-chart
___ H2(g) + ___ O2(g) ---> ___ H2O(l) R P
Count the number of atoms of each element found in the reaction.
H - 2
H - 2
O - 2
O - 1
What Happened to the Other Oxygen Atom?????
This equation is NOT balanced!

20. Balancing Equations using T-chart2 1
___ H2(g) + ___ O2(g) ---> ___ H2O(l) 2
R P
H H - 2
O O - 1
2. Add coefficients
to balance the atoms 4 4 2
3. Don’t forget to recount.
Now it is
balanced!
Now fix the H on the left.
Don’t forget there is a 1 in front of O2.

21. Now You Try! Use shapes or T-chart method
Fe + O2  Fe2O3
2x2 = 4
_____Fe + ____O2  ____Fe2O3 2 3 2
3x2 = 6
Common multiple
Coefficient ratio: 4:3:2

22. RULE: Balance H and O last
Easy Balancing
RULE: Balance H and O last
Al O2  Al2O3 2 3 2 1 R P 4 2
1 Al
2 Al
2 x 2 = 4
2 x 3 = 6
2 O
3 O
3 x 2 = 6
2:3 – find a common multiple
Coefficient ratio: 4:3:2