Question of the Day Rules
 You
must work alone
 You may use your textbook today
 You have 10 minutes to complete
the QOD
Good luck!!
QOD
 Read
page 136-137 of your
textbook and answer question
1 & 2 at the bottom of the
page.
2


Why is it difficult to protect people from the
radiation given off by radioactive elements? Why
is this radiation harmful to human health?
Answer: Some radiation, such as gamma
rays, is stopped only by high-density
materials, like concrete or lead, so it
passes easily through the human body.
Radiation can be harmful to health
because of its ability to alter a cell’s DNA,
which can lead to the development of
cancer cells.
3


Cesium-137 has a half-life of about 30 years. Is
it normal that after more than 20 years, levels of
radioactivity in the ground near Chernobyl are
still very high? Explain your answer.
Answer: Yes, because a half-life of 30
years means that it takes 30 years for half
of the amount of cesium to decay.
Therefore, after 20 years, more than half
of the cesium still remains in the ground.
4
What are we doing Today?
Chapter 4
 QOD
 Acid Base Lab Nov. 17
 Don’t forget your test, Nov 24
 Ch. 4  Balancing Equations
 Balancing Equations Worksheet
6
If a chemical equation does not obey the law of
conservation of mass the equation is said to be what?
NOT BALANCED
So Let’s look at the steps we need to take to
BALANCE chemical equations
Let’s work with the following equation:
Fe +
O2

Fe2O3
7
Step 1. Create a RAP table
(what’s a RAP table ??)
A table that shows us what atoms are present in this
reaction, how many there are and are they reactants
or products?
For example:
#R atom #P
1
2
Fe
O
2
3
Fe +
O2

Fe2O3
8
Rule 2. Go to the first atom that’s not balanced and
balance it!
Since Fe atoms are not balanced what do we need to
do to balance it?
Right! Multiply it by 2 (Only multiply)
#R atom #P
2x 1
2
Fe
O
2
3
9
In step 2 we balanced the number of Fe atoms by
multiplying the reactant side by 2. This now becomes
the new coefficient in the chemical equation.
#R atom #P
2x 1
2
Fe
O
2
3
Modify the equation to reflect the change
2Fe +
O2

Fe2O3
Are all atoms balanced?
10
3. Move to the next unbalanced atom. What is it?
#R atom #P
2
2
Fe
O
2
3
How can we balance the
Oxygen?
Multiply Reactants by 3 and Products by 2
#R atom #P
2
3x2
Fe
O
2
3x2
Adjust the equation to reflect your
changes
2Fe +
3O2  2Fe2O3
But notice that by changing Oxygen we also
Changed Iron. We need to go back and fix this.
11
4. Write out the updated RAP table. How can we
Balance the Iron?
#R atom #P
2x 2
6
Fe
O
4
6
Sure! Multiply the # of
Reactant Fe atoms by 2 !
Re-write the equation reflecting
The new changes you’ve made.
4Fe +
3O2  2Fe2O3
Do we have a balanced Chemical Equation now?
Yes we do!
12
Polyatomics
When an equation has Polyatomics in it, such as in this
Balanced chemical equation
2AgNO3 + MgCl2  2AgCl + Mg(NO3)2
And the polyatomic appears on BOTH the reactant and product
Side of the equation Count the polyatomic as an “ATOM”
So the above reactant atoms would be:
#R atom #P
If the same polyatomic does not
Appear on both sides break the
Polyatomic down into atoms!
2
2
1
2
Ag
NO3
Mg
Cl
2
2
1
2
13
14
Is this equation balanced?
NaOH + CaBr2  Ca(OH)2 + NaBr
What atoms do we have in
This equation?
1) Count atoms & Start
the RAP table
#R atom #P
1
1
1
2
NA
OH
Ca
Br
1
2
1
1
2) Do the #Reactant atoms = the # of Product atoms?
3) So pick the 1st unbalanced atom & begin balancing
15
We’ll start with balancing Hydroxide
NaOH + CaBr2  Ca(OH)2 +NaBr
How can we make both Hydroxides equal?
Sure we’ll multiply #R OH by 2
#R atom #P
1
Next step> rewrite the modified eqn. 2x 1
1
2NaOH + CaBr2  Ca(OH)2 + NaBr
2
Na
OH
Ca
Br
1
2
1
1
Hydroxide is now balanced so let’s move to the next
Unbalanced atom, which is? …
16
What can we do to balance the Bromine?
Sure! Multiply the #P Bromine by 2
Now adjust the table to reflect
#R atom #P
The changes and then rewrite the
2
Na
1
Eqn.
2
OH 2
1
Ca
1
2
Br 1 x2
2NaOH + CaBr2  Ca(OH)2 + 2NaBr
17
Let’s update the RAP table with the new #’s
Based on our updated equation.
2NaOH + CaBr2  Ca(OH)2 + 2NaBr
Are we now balanced?
Sure!
#R atom #P
2
2
1
2
Na
OH
Ca
Br
2
2
1
2
18
Ok Try Balancing this equation:
C2H6 + O2  CO2 + H2O
Step 1. Total up the atoms
Step 2. Balance the #P Carbon
#R atom #P
2
6
2
C
H
O
2
2
5
#R atom #P
2
6
2
C
H
O
1
2
3
& Re-write the equation
C2H6 + O2  2CO2 + H2O
Are we done?
19
Step 3. Carbons are balanced now but Hydrogen isn’t.
So, balance Hydrogen atoms next
Multiply #P Hydrogen by 3
Step 4. Re-write the eqn. &
Retotal the number of atoms
#R atom #P
2
6
2
C
H
O
2
6
7
#R atom #P
2
6
2
C
H
O
2
2 x3
5
C2H6 + O2  2CO2 + 3H2O
Carbon and Hydrogen are now
balanced but oxygen isn’t.
20
Step 5. To balance Oxygen multiply O by 3½
#R atom #P
Step 6. Re-write the eqn. &
2
Retotal the number
6
of atoms
3 ½ x2
C
H
O
2
6
7
C2H6 + 3½ O2  2CO2 + 3H2O
It looks like we’re balanced. But, are we?
No! We can’t have 3 ½ Oxygen molecules! Only whole
Numbers are allowed.
So what do we need to do to fix this?
21
Step 7. Let’s clean this up by Multiply everything by 2
C2H6 + 3½ O2  2CO2 + 3H2O
x2
2C2H6 + 7 O2  4CO2 + 6H2O
#R atom #P
Step 8. Retotal #R and the
#P atoms
Are we balanced?
4
12
14
C
H
O
4
12
14
YES!
22
Try this problem
NH4OH + FeCl3  Fe(OH)3 + NH4Cl
Start here. Recognize we
Have polyatomics but they
Appear on both sides of the
Equation.
#R atom #P
1
1
1
3
NH4
OH
Fe
Cl
1
3
1
1
OK … Now finish it up
23
Answer to previous problem
3NH4OH + FeCl3  Fe(OH)3 + 3NH4Cl
24
Class Work
1) Assignment Chapter 4 (Law
of Conservation of Mass and
Balancing Equations)
2) Complete the PreLab 33 (first
page) for the Neutralization Lab
on Monday
25