Download Balancing Equations Notes PP

Intro to Balancing Equations
Q: How could you represent this
reaction (rusting bicycle),
besides just describing it in
words?
A: Scientists use a standard method
to represent a chemical reaction,
called a chemical equation.
Fe + O2  Fe2O3
(Rust)
Chemical equations are used
to represent
CHEMICAL REACTIONS
Reactants: Zn + I2
Product: Zn I2
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)
Q: What are the reactants and
products?
Fe + O2 → Fe2O3 (new product - rust)
Reactants Products
The atoms/compounds
you have BEFORE the
reaction occurs.
The atoms/compounds
you have AFTER the
reaction occurs.
When is an equation called
balanced?
When an equation has
the same number of
atoms of the same
kind on both sides.
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
Learning Check
What does “2“ mean in front of LiOH?
Number of molecules
2Li (s) + 2H2O (l)  2LiOH (aq) + H2 (g)
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
Q: What is the chemical equation
for this reaction?
A: The chemical equation is:
C + O 2 → CO 2
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?
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.
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
1C
1C
2O
2O
Is this a balanced
equation?
Count the number of
atoms of each element
found in the reaction.
YES
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.
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.
Learning Check
Why is this reaction correct?
B/C atoms must REARRANGE for a chemical
reaction to take place
+

+
NEW
PRODUCTS
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.
Balancing using shapes
2
1
2
____H
+
____O

____H
2
2
2O
Is this reaction balanced?
Now fix the H on the left.
Double Hydrogen too.
Coefficient ratio: 2:1:2
BALANCED !
Balancing Equations using T-chart
___ H2(g) + ___ O2(g) ---> ___ H2O(l)
1. Count the
number of
atoms of each
element found in
the reaction.
R
H-2
O-2
P
H-2
O-1
What Happened to the Other Oxygen Atom?????
This equation is NOT balanced!
Balancing Equations using T-chart
2 H2(g) + ___
1 O2(g) ---> ___
___
2 H2O(l)
R
2. Add coefficients
to balance the atoms
3. Don’t forget
to recount.
H-2
H-2 4
O-2
O-1 2
Now fix the H on the left.
Don’t forget there is
a 1 in front of O2.
P
4
Now it is
balanced!
Now You Try!
Use shapes or T-chart method
Fe + O2  Fe2O3
2x2 = 4
2
3 2  ____Fe
2
_____Fe
+ ____O
2O3
3x2 = 6
Common multiple
Coefficient ratio: 4:3:2
Easy Balancing
RULE: Balance H and O last
2 Al 1
+
3
O2

R
P
2
Al2O3
4 2 1 Al
2 Al 2 x 2 = 4
2x3=6 2O
3O 3x2=6
2:3 – find a common multiple
Coefficient ratio: 4:3:2