Go to our home page,
www.st-annes.org, as it will
be updated regularly with
worksheets and the old tests.
St. Anne’s Episcopal School
Eighth Grade Science
Chapter 12
Chemical Reactions
TESTS
Feb 8th, 20th, and 28th.
plus pop quizzes
Overview: This chapter is the 'Big Kahuna' of middle school science. The length, four pages, is what
makes it hard. The material will make sense to you. There are some awesome labs. It is a chapter that
builds on itself. If you don't get page one, you’ll be lost on pages two and three. For that reason,
nightly study is essential. I will give pop quizzes that count. Challenge yourself to 'ace' this entire
chapter. If you do, you can handle anything in high school.
Review: In the past two chapters, you learned about atoms, elements, bonds, OEL, and the relationship
between elements in the periodic table. Now we will see how elements combine in chemical reactions.
We will be balancing both formulas and equations. Enjoy!
This week I am scheduled for some surgery. I tried to schedule it later, but the surgeon didn’t want me
waiting. I do not have cancer, and I’ll make a full recovery. But I’ll miss next Wednesday (only 1/2
class due to testing), Thursday, and Friday (1/2 class) when you’ll take the test on coefficients and
diatomics. The formula test on the 20th. The ‘Big-Deal Test’ will be the last day of the trimester (Feb.
28th). There are nine pages to the Ch 12 outline. That’s why it’s called the ‘Big Kahuna.’
OXIDATION NUMBERS:
An atom's oxidation number is the number of electrons
it loses or gains when reacting with another element in
forming a compound.
Look above at ION of calcium. Then, in the box
to the right labeled Common Oxidation Numbers, find
calcium and its oxidation #. (see calcium? Ca
2+
)
See how they get their oxidation numbers ?
You do not have to memorize the oxidation numbers. I
will always provide them.
1+
Hydrogen
Lithium
Potassium
Silver
Sodium
Ammonium
Common Oxidation Numbers
2+
3+
H
Barium
Ba
Aluminum Al
Li
Calcium
Ca
Chromium Cr
K
Cobalt
Co
Iron (III)
Fe
Ag
Copper
Cu
Na
Iron (II)
Fe
NH4
Magnesium Mg
1Bromine Br
Chloride Cl
Iodide
I
Acetate C2H3O2
Hydroxide OH
Nitrate
NO3
2Oxide (oxygen) O
Sulfide (sulfur) S
Carbonate CO4
Sulfate
SO4
3Nitride (nitrogen) N
Phosphate
PO4
* Oxide = oxygen
** Sulfide = sulfur
*** Nitride = nitrogen
Challenge: Write the electron configuration for magnesium (#12) and then predict its oxidation number.
Mg is the symbol
1s 2s 2p 3s
What is your prediction of Magnesium’s oxidation number?
After predicting above, see answer upside down below.
Page 2 of 9
CHEMICAL FORMULAS: Ok, you want to make a compound like calcium chloride or iron oxide.
To do that you need a recipe, just like making brownies (2 eggs, 1 cup milk, etc). A chemical
formula is a recipe of how many atoms you need to make a molecule or compound. When two
elements combine in a chemical reaction they form compounds. If the compound is held together
by a covalent bond it’s called a molecule. That's the difference between a molecule and compound.
RULE: When writing a formula, the element or ion with the positive charge always goes first, followed
by the element or ion with the negative charge. The formula for water is H2O, hydrogen is positive.
RULE: When naming the compound or molecule you change the ending to ' ide.' So, table salt would
be sodium chloride, not sodium chlorine. Compounds of oxygen are oxides.
Example #1: Na+ + Cl- --> NaCl or Sodium mixed with Chlorine forms a compound of table salt
-
oxidation # 
Sodium 1+
Chlorine 1
(one + and one -)
In this case, the formula NaCl is a balanced formula. In making table salt, you need only one
-
atom of sodium for every one atom of chlorine (1+, 1 ).
Thus the formula is ----> NaCl (the charges are balanced...Mother Nature loves balance)
Example #2: H2 + O2 --> H2O or Hydrogen mixed with Oxygen forms a molecule of water.
What is the formula for water? How do we figure it out? We simply look at the oxidation
Numbers in the oxidation table.
oxidation #s

Hydrogen 1+
Oxygen 2
-
(two + and one -) not balanced!
-
To have a balanced formula for water, you need 2 hydrogen for every oxygen (2+, 2 ).
Therefore the formula is ----> H2O (balanced charges.....Mother Nature smiles)
We always use subscripts when we need more than one atom.
The formula for methane is CH4: 1 atom of carbon and 4 of hydrogen. Figure out the oxidation # of C.
On Worksheet #2: Chemical Formula section, write balanced formulas and name the 14 examples.
DIATOMICS: Some elements never occur alone in nature, they are always in pairs. They are called
DIATOMICS, di meaning two and atomics meaning atoms or two atoms.
They are: H2
N2
O2
F2
Cl2
Br2
I2. (memorize)
In a chemical equation when they are alone, always write them like Cl2, etc. Remember the ever famous
equation for ______________ ________________
C6H12O6 + O2 ---> CO2 + H2O + energy
See the diatomic oxygen (O2), but when oxygen is not alone and in a compound like CO2 or CO it may
have any subscript.
Page 3 of 9
Ok, you now know how to balance formulas. Lets take some of these balance formulas or compounds
and mix them together with other compounds or individual elements in a chemical reaction.
My favorite equation is for cellular respiration: You could say it this way "inside every cell your body
mixes glucose (C6H12O6) and oxygen (O2) to give you water, carbon dioxide and energy in the form of
__________." Scientist being the shy types and not very wordy, would rather write a chemical equation.
C6H12O6 + O2 ---> CO2 + H2O + Energy
When we write an equation, we must be sure we don't break Mother Nature’s Law of Conservation of
Matter. You remember…."matter is neither created or destroyed...."
C6H12O6 + O2 ---> CO2 + H2O + Energy
If you look at the equation, we broke the law. There are 6 carbons on the left side and only 1 on the
right. The equation broke the law by destroying 5 carbons. Also, we lost hydrogens and oxygens. How
many hydrogen did we lose? _____, How many oxygens did we lose? ______ (two in CO2 and one in
H2O)
That is why we need to balance equations to keep Mother Nature happy.
Before we learn that, we must understand coefficients.
COEFFICIENTS: It is the number in front of the symbols. Eg 3CO2 Three is the coefficient and this
means there are three molecules of Carbon Dioxide. Also, like in math, 3(AB) is the same as 3
times A and 3 times B.
In 3CO2 there are 3 atoms of carbon and 6 atoms of oxygen
In the Molecule of glucose ..... C6H12O6 ..., there are 3 elements ___, ___, ___; and there are
twenty-four atoms......six atoms of Carbon, 12 atoms of Hydrogen, and ___ atoms of Oxygen.
Now think...if we have 3 molecules of glucose, we would write it with a coefficient....3 C6H12O6.
Then, we would have 18 atoms of Carbon, ____ atoms of Hydrogen, and ____ atoms of Oxygen.
How many Oxygen in 5Ca(NO3)2 ? Take your time. Answer ______oxygen. (3x2x5)
RULE: You can never, never, never place a coefficient inside a formula, always in front.
Example: not Na2Cl
but rather 2NaCl
So, when balancing an equation, you always use coefficients and they always go in front.
On the Worksheet #1 Coefficients, page 6, figure out how many atoms and elements.
Test tomorrow on Coefficients and Diatomics.
We will have only half a period in the period before the Activity Period. Be prompt.
Page 6 of 9
Worksheet #1
Coefficients:
RULE: You can never, never, never place a coefficient _________________________________
How many: elements hydrogens oxygens carbons nitrogen cobalt aluminum
4 H2O2
2
8
8
x
x
x
_
8C6H12O6
x
x
x
_
8Co(C2H3O2) 2
x
x
________
x
_
9NH4OH
5Al(NO3) 3
x
x
total atoms
x
x
x
x
4C12H22O12
16CH4
X
8AlCl3
X
X
X
________
x
x
x
________
x
x
x
________
X
x
Diatomics: Name the seven diatomics:
__H2__
16___
______ ______ ______ ______ ______ ______
_______