Transcript Document

Review from yesterday’s lesson

We are learning how to balance equations
because the conservation of mass says
that the mass before a reaction must be
the same as the mass after a reaction.
Atoms cannot be created nor destroyed
H2 + O2 ---> H2O
Review from yesterday’s lesson

We must know how to count the number of
atoms in a substance before we can balance
 6Na2O3
Coefficient = 6
 Subscript Na= 2
 Subscript O= 3


Total number of Na= 6 X 2= 12

Total number of O= 6 X 3= 18
Is the blue number a coefficient or
a subscript?
3Li
Na2CO3
4K2PO4 6H 0
2Mg(OH)
2 2
2H3PO4 6LiCl
4
Count the number of atoms
5H2CO3
How many H atoms?
5 X 2= 10
How many C atoms?
5 X 1= 5
How many O atoms
5 x 3= 15
Click here to go
to balancing
equations steps
Balancing Equations
Are you up for the
challenge?
Why do you need to balance
equations?
 Let’s
read page 80-81
 The Law of Conservation of Mass
says that the mass of the reactants
must equal the mass of the products.
 In this example: H2 + O2 ---> H2O
There are 2 hydrogen’s on the left
and 2 Oxygen’s, but there is only 1
oxygen on the right.
H2 + O2 ---> H2O
The reactants do not equal the products!
 Where did the extra oxygen go?
 Atoms can’t be created nor destroyed.
 They didn’t go anywhere, you just need to
add numbers so the sides become equal
 An equation is balanced by changing
coefficients in a somewhat trial-and-error
fashion

Important Vocabulary- Coefficient
 Coefficient-
Large number located in
front of an element or compound in a
chemical formula. 2H= 2 hydrogen's
4H20
Coefficient
is 4
H2O H2O
H2O
H2O
Important Vocabulary- Subscript
 Subscript-
The small number that is
in the lower right corner of an
element
H20
 Subscript
is 2
 2 hydroden’s
H
H
 If
there is no coefficient or subscriptassume it is 1
1 C1 S2
Coefficient=1
C’s
subscript= 1
S’s subscript= 2
Let’s practice

2Na2 O
 4 H 3PO 4

5H 2 O 2

3S 8
 6 K20
Math Review

Distributive Property- The coefficient times
the subscript gives the total number of
atoms.
4H201

H= 4 X 2= 8 Hydrogen’s

O= 4 X 1= 4 Oxygen’s
Math Review
4Mg(OH)
2
1
 Mg=
4 X 1= 4
 OH=
4 x 2= 8
3KOH

How many K atoms do I have?
Coefficient 3
Subscript 1
3 X1= 3

How many O atoms do I have?
Coefficient 3
Subscript 1
3 X 1= 3

How many H atoms do I have?
Coefficient 3
Subscript 1
3 X 1= 3
3K2O
 How
many K atoms do I have?
 Coefficient
3
 Subscript 2
 3 X 2= 6
 How
many O atoms do I have?
 Coefficient
3
 Subscript 1
 3 X 1= 3
2H2O
 How
many H atoms do I have?
 Coefficient=
2
 Subscript= 2
 2 X 2= 4
 How
many O atoms do I have?
 Coefficient=
2
 Subscript= 1
 2 X 1= 2
K3PO4

How many K atoms do I have?
 Coefficient=
1
 Subscript= 3
 1 X 3= 3

How many P atoms do I have?
 Coefficient=
1
 Subscript= 1
 1 X 1= 1

How many O atoms do I have?
 Coefficient=
1
 Subscript= 4
 1 X 4= 4
Mg3(PO4)2

How many Mg atoms do I have?
 Coefficient=
1
 Subscript= 3
 1 X 3= 3

How many P atoms do I have?
 Coefficient=
1
 Subscript=1 X 2= 2
 1 X 2= 2

How many O atoms do I have?
 Coefficient=
1
 Subscript= 4 X 2=8
 1 X 8= 8
Steps for balancing equations
 1.
Draw boxes around all the
chemical formulas.
 Never,
ever, change anything inside the
boxes. Ever. Really. If you do, you're
guaranteed to get the answer wrong.
H2 +
O2 --->
H2 O
Steps for balancing equations
 2.
Make an element inventory.
 How
are you going to know if the equation
is balanced if you don't actually make a list
of how many of each atom you have? You
won't. You have to make an inventory of
how many atoms of each element you have,
and then you have to keep it current
throughout the whole problem.
 See
example on next slide
H2 +
O2 --->
H2O
Reactants
Products
Hydrogen
(H)
2
2
Oxygen (O)
2
1
But what if you have more than one formula
on each side of the equation? Let’s practice
with the REACTANTS!
 NaOH + H2CO3
Na2CO3 + H2O
How many Na atoms do I have?
1 X 1 =1
How many O atoms do I have?
1 X 1= 1
1 X 3=3
1+3=4
How many H atoms do I have?
1 X 1= 1
1 X 2= 2
1 + 2= 3
How many C atoms do I have?
1 X 1= 1
Steps for balancing equations
 3.
Write numbers in front of
each of the boxes until the
inventory for each element is the
same both before and after the
reaction.
 Whenever
you change a number, make sure
to update the inventory - otherwise, you run
the risk of balancing it incorrectly. When all
the numbers in the inventory balance, then
the equation can balance,
2 H2 +
O2 ---> 2 H2O
Reactants
Products
Hydrogen
(H)
2
2
4
Oxygen
(O)
2
1
2
4
Notice- I put
the
coefficient in
front of the
box- NOT
inside!
Steps for balancing equations
 4.
Find the elements which
appear in the fewest numbers of
molecules and balance these
first.
 Continue
in sequence until you balance the
element which appears in the most
molecules last.
 Tip:
Start by balancing an element
that appears in only one reactant
and product.
Step 1: Write formula and draw boxes
2
H2 + O2
2 H2O
Step 2: Make an element inventory
R
P
H
2 4 2
4
O
2
2
1
Step 3: Add coefficients to make the
inventory balance. Don’t forget to change the
inventory as you go!
Rules
You cannot change a subscript.
 You cannot place a coefficient in the middle
of a formula. H2 + O2 ---> H22O
 Make sure that your final set of coefficients
are all whole numbers with no common
factors other than one.
 For example, this equation is balanced:

4 H2 + 2 O2 ---> 4 H2O
 However, all the coefficients have the
common factor of two. Divide through to
eliminate common factors like this.
If you run into problems trying to
figure out the answer…

Sometimes, you will need to find the lowest
common multiple in order to make an
inventory balance!
The lowest
R
P
common multiple
H
2
2
between 2 and 7
is 14
O
2
7
14
14
If you run into problems trying to
figure out the answer…
What happens when the only way you can
get a problem to work out is to make one
of the numbers a decimal or fraction?
 When this happens, find the largest
molecule in the equation and stick a "2" in
front of it.
 Then start the problem over.
 Will this work all the time? Well, no. But it
will work sometimes, and give you a new
strategy for hard problems.

Step 1: Write formula and draw boxes
4
C + S8
4
CS2
Step 2: Make an element inventory
R
P
C
1 4 1
4
S
8
8
2
How do I get the
S’s in the
products to equal
8?
Step 3: Add coefficients to make the
inventory balance. Don’t forget to change the
inventory as you go!
Step 1: Write formula and draw boxes
4
Na + O2
2 Na2O
Step 2: Make an element inventory
R
P
Na
1 4 2
4
O
2
2
1
Step 3: Add coefficients to make the
inventory balance. Don’t forget to change the
inventory as you go!
Step 1: Write formula and draw boxes
2
N2 +5 O2
2 N2O5
Step 2: Make an element inventory
R
P
N
2 4
O
2 10 5 10
2
4
HINT: Find the
lowest common
multiple of the
Oxygen molecules
Step 3: Add coefficients to make the
inventory balance. Don’t forget to change the
inventory as you go!
Step 1: Write formula and draw boxes
2
Na + O2
Na2O2
Step 2: Make an element inventory
R
P
Na
1 2 2
O
2
2
How do I get the
Na’s in the
reactants to equal
2?
Step 3: Add coefficients to make the
inventory balance. Don’t forget to change the
inventory as you go!
Step 1: Write formula and draw boxes
6K
3 K2O +2 B
+ B2O3
Step 2: Make an element inventory
K
B
O
Reactants Products
1
2
6
6
1
2
2
3
1
3
HINT: Start on the
products side- and
start with the
element with the
lowest number of
molecules
Step 3: Add coefficients to make the
inventory balance. Don’t forget to change the
inventory as you go!
Step 1: Write formula and draw boxes
N2 +3 H2
2 NH3
Step 2: Make an element inventory
R
P
N
2
1
2
H
2 6 3
6
Step 3: Add coefficients to make the
inventory balance. Don’t forget to change the
inventory as you go!
Step 1: Write formula and draw boxes
6
Cs + N2
2 Cs3N
Step 2: Make an element inventory
R
P
Cs
1 6 3
6
N
2
2
1
Step 3: Add coefficients to make the
inventory balance. Don’t forget to change the
inventory as you go!
Step 1: Write formula and draw boxes
3 Li + AlCl3
3 LiCl
`
+ Al
Step 2: Make an element inventory
R
Li
1
Al
1
3
Cl
P
3
Step 3: Add coefficients to make the
inventory balance. Don’t forget to change the
inventory as you go!
1 3
1
1
3
Step 1: Write formula and draw boxes
2NaOH + H2CO3
Na2CO3 +2H2O
Step 2: Make an element inventory
Hint: We are going to start
with the Na on the reactant
side because it is not
balanced and has the
lowest number of
molecules
R
Na
O
H
C
1
4
3
P
2
5
4
1
Step 3: Add coefficients to make the
inventory balance. Don’t forget to change the
inventory as you go!
2
4
2
1
5
4
Step 1: Write formula and draw boxes
2 C2H6 +7 O2
4 CO2 +6H2O
Step 2: Make an element inventory
Hint: We are going to look
for the common factor of
H’s
R
P
C
2
H
6 12
O
4
2 14
1
4
2 12
3
Step 3: Add coefficients to make the
inventory balance. Don’t forget to change the
inventory as you go!
8 14
Step 1: Write formula and draw boxes
K3PO4 +3H2O
H3PO4 +3 KOH
Step 2: Make an element inventory
Hint: We are going to look
for the lowest number of
molecules that are not
balanced.
R
H
P
O
K
4
P
6
1
2
6
1
5
7
5
1
3
3
Step 3: Add coefficients to make the
inventory balance. Don’t forget to change the
inventory as you go!
7
Step 1: Write formula and draw boxes
10 Na +2 NaNO
3
6 Na2O
+ N2
Step 2: Make an element inventory
Hint: We are going to look
for the next common factor
of 1 and 3
R
P
Na
2
3 12
2
N
1
2
2
O
3
6
1
Step 3: Add coefficients to make the
inventory balance. Don’t forget to change the
inventory as you go!
12
6
Step 1: Write formula and draw boxes
2H3PO4
3
6
+ Mg(OH)2
Mg3(PO4)2 + H2O
Step 2: Make an element inventory
R
H
P
O
Mg
P
5
912
2
1
2
2
6
1
1014
3
9
12
14
3
Step 3: Add coefficients to make the
inventory balance. Don’t forget to change the
inventory as you go!