# Balancing Chemical Equations POGIL key

```Balancing Chemical Equations
NAME
HOUR
STANDARD 1 LEARNING MODULE 10
Background Information and Why: Chemical reactions can be described by chemical equations. Equations
means equal. One each side of the equal side, also known as “yields”, the number AND type of atoms are
exactly the same. The Law of Conservation of Mass states that the atoms cannot be created or destroyed. The
atoms can only be rearranged into different molecules/substances. Atoms are the building blocks of
molecules. Therefore, in order to obey The Law of Conservation of Mass, the number of atoms for each
element must be the same in the reactants (left side) as it is in the products (right side). In order to balance
equations we cannot split or add to existing molecules. We can change the coefficient, adding molecules. We
cannot change the subscripts, the small numbers, because they determine how many atoms are in a molecule.
REACTION A
REACTANTS
CuO
Chemical Equation
Atomic Representation
Cu
PRODUCTS
+
O
+
H
Cu +
+
Cu
H
H
H
O
REACTANTS
CO
Chemical Equation
H2O
Atom
Molecule
REACTION B
H2
PRODUCTS
+
O2
CO2
O
Atomic Representation
Molecule
Questions
C
h
h
s
s
s
O
+
C
O
O
Atom
h
h
s
s
s
O
0.) Label one more molecule and one more atom in EACH reaction. Use the ones done above to help you.
Pick another atom draw an arrow to it. Label it “atom”. Then circle or “bracket” another molecule and label
it “molecule”.
1.) How many of each type of atom are represented in each side of reaction A? Count them up and fill out the table
below.
# Atoms in Reactants
Atoms in Products
Cu- ONE
Cu- ONE
O- ONE
O- ONE
H- TWO
H- TWO
2.) Does equation (A) obey the Law of Conservation of Mass? Explain. Yes for each type of atom in the reactants there
is an equal number in the products.
3.) How many of each type of atom are represented in each side of reaction B? Count them up and fill out the table
below.
# Atoms in Reactants
Atoms in Products
C- ONE
C- ONE
O- THREE
O- TWO
4.) Does equation (B) obey the Law of Conservation of Mass? Explain. NO there are 3 atoms of C (carbon) in the
reactants and only 2 in the products. It does not balance. They need to be equal on both sides.
5.) If either reaction/equation A or B does not obey the Law of Conservation of Mass, draw additional molecules on the
reactant and/or product side(s) in order to make the number of all atoms equal on both sides of the equation. HINT:
recall you cannot split existing molecules or add atoms to existing molecules. In other words EXISTING MOLECULES
MUST REMAIN INTACT AND YOU CAN ONLY USE REPLICAS OF WHAT YOU STARTED WITH.???????????????
You would need to draw one of these in the reactants.
C
O
And one of these in the products.
O
C
O
Now you would have 2 atoms of carbons(C) on each side (products and reactants) and 4 atoms of oxygen (O). AND we
only used the molecules we started with.
6.) In reaction (A) how many H2O molecules are produced for every H2 molecule that is needed in the reactants?
One. The coefficient is actually a 1 even though it is not written. Just like X in algebra is really 1X.
7.) In corrected reaction (B) how many CO2 molecules are produced for every O2 molecule that is needed in the
reactants?
After we corrected it should be 2 CO2 molecules
8.) In corrected reaction (B) how many CO molecules react with every O2 molecule?
After we corrected it should be 2 CO molecules for each O2 molecule
9.) In order to answer question 5 you should see that reaction (a) is already drawn correctly, but reaction B does not
have the same number of oxygen atoms on each side. It should be drawn like this….. GO BACK AND CHECK YOURS.
CORRECTED
REACTANTS
REACTION B
C
PRODUCTS
O
O
+
C
O
O
O
O
C
C
O
We needed to add this carbon monoxide because we
O
We needed to
dioxide because
carbon dioxide. A
total of 2 carbon
dioxide.
10.) Recall we can only change the coefficients in a chemical equation. Below are the blanks where coefficients can go.
Look at the new atomic representation for Reaction (B) above and write the correct coefficient in the correct blank.
REACTANTS
PRODUCTS
2 CO
+
1 O2
2 CO2
11.) This drawing and equation now show how many oxygens in the reactants? How many oxygens in the products?
FOUR
FOUR
12.) This drawing and equation now show how many carbons in the reactants? How many carbons in the products?
TWO
TWO
13.) Is this equation balanced? Explain? Yes ALL atoms are equal on both sides of the equation. And we only used the
original molecules.
Strategy to Balance A Chemical Equation
Background: Now you understand that the atoms in the reactants and products have to be the same for each element.
This is because of the Law of Conservation of Mass. You should also recall that we cannot split any given atoms or add
any new ones to those already given. We do this by changing ONLY the coefficients and NEVER the subscripts. We will go
through an example that will show you how to make a prediction, change a coefficient, recount the number of atoms,
update a table, and determine next step.
Reaction (C)
Na +
O2
Na2O2
Step 1: Draw a table and make a list of the atoms in the reactants and products, count them up and update your table.
Reactants
Na-1
O-2
Products
Na-2
O-2
Step 2: Do the Na’s (sodium) and O’s (oxygen) balance? Are they the same in as out? No!! Determine what coefficient
(only positive integers) could be put in front of atoms and molecules to get them to balance. Let’s try a 2 in front of Na.
Step 3: Write the coefficient in the correct spot and update the table. Because the Na and O are separated by a + te 2
affects only the Na. Notice the change in the table.
2 Na
Reactants
Na-1 2
O-2
+
O2
Na2O2
Products
Na-2
O-2
Step 4: Continue placing coefficients in the correct places until the equation is balanced. You may be wrong occasionally
and have to cross out and try again. Check out below. This person put a 2 for the coefficient in the product. Because
Na2O2 is all one molecule the Na atoms AND the O atoms will go up. 2 times 2 s 4 for each. But this gets us further from
balanced! Erase as try another.
2 Na
+
O2
2 Na2O2
Reactants
Na-1 2
O-2
Products
Na-2 4
O-2 4
Step 5: Continue placing coefficients in the correct places until the equation is balanced. A coefficient of 2 in front of the
Na does the trick. Update your table EVERY TIME!!!!! Then DRAW the balanced equation model.
2 Na
+
+
Na
O2
balanced equation. CIRCLE IT
WHEN YOU GO IT!!
Na2O2
O
Na
O
Na
O
O
Na
Reactants
Na-1 2
O-2
Products
Na-2 4 2
O-2 4 2
14.) Why do we have to balance chemical equations? The law of Conservation of Mass states that what goes in must
come out. The atoms that go into a chemical reaction must be the same type and same number as are produced. This
is true even when we create new substances (new molecules) during a chemical reaction.
15.) Balance these equations. USE EXTRA PAPER IF YOU NEED THEN DRAW THE BALANCED MODEL
a.) 2 H2
H
H
+
O2
H
H
+
Reactants
H-4
O-2
2 H2O
O
Products
H-4
O-2
O
H
H
H
O
H
O
b.) 2 H2 O2
2 H2O +
H
H
Reactants
H-4
O-4
H
H
O
H
H
O2
O
O
O
+
H
H
Products
H-4
O-4
O
O
c.)
N2
+
3 H2
H
+
Reactants
N-2
H-6
2 NH3
H
H
H
H
H
H
H
H
Products
N-2
H-6
H
H
H
d.) 2 Al2 O3
3 O2
+
+
O
O
O
Reactants
Al-4
O-6
4 Al
O
O
O
Products
Al-4
O-6
e.) 3 Fe
+
2 H2O
+
Fe3 O2
+
2 H2
H
H
O
Products
Fe-3
H-4
O-2
H
H
+
H
H
Reactants
Fe-3
H-4
O-2
H
H
O
O
O
f.)
Na2SO4 +
CaCl2
CaSO4 + 2 NaCl
+
Reactants
Na-2
S-1
O-4
Ca-1
Cl-2
Products
Na-2
S-1
O-4
Ca-1
Cl-2
S
S
+
O
O
O
O
O
O
O
O
BALANCE THESE EQUATIONS: make your own table and start from scratch.
g.) C2H6 + O2
CO2
C2H6 + 5O2
2CO2
h.) Na + O2
Na2O2
2Na + O2
Na2O2
+ H2O2
+ 3H2O2
Reactants
C-2
H-6
O-10
Reactants
Na-2
O-2
Products
C-2
H-6
O-10
Products
Na-2
O-2
EXTRA CREDIT BALANCE THESE EQUATIONS THEN DRAW THE
MODEL. GET TEACHER SIGNATURE FOR EXTRA CREDIT-USE SEPARATE PAPER IF NEEDED
EC1.)
__ Ca(OH) + __ H SO __HOH + __ CaSO
EC2.)
__ Cu + __ AgNO
2
2
3
4
4
__ Cu(NO ) + __ Ag
3 2
```

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