Introduction to balancing chemical reactions for middle school science

1. Balancing Chemical Reactions
Making new substances

2. Main Ideas
Chemical Reactions are represented by Chemical
Equations.
Chemical Equations are balanced to show the same
number of atoms of each element on each side.
The Law of Conservation of Mass says that atoms
won’t be created or destroyed in a chemical reaction.

3. Today you will
learn how to
balance a chemical
equation.

4. Why?
• All recipes are balanced
chemical reactions.
• The difference between a
campfire and an explosion
is a balanced chemical
reaction.
• Knowing how many antacid
tablets to take after eating
Taki’s is a balanced
chemical reaction

5. And
The difference between:
CH4 + O2 → CO2 + H2O
is
CH4 + 2 O2 → CO2 + 2 H2O

6. Law of Conservation of
Mass
In a chemical reaction, matter is neither created nor
destroyed.
Atoms won’t change their identity (e.g. a Carbon
atom can’t become an Iron atom)
This means that you have to have the same number
of each type of atom on each side of the chemical
equation.
Conservation of Mass Video

7. Let’s learn how to do this…

8. Oh No… Vocabulary (but wait,
you already know this)
Reactants:
Products:
substance that takes part in
and undergoes change
during a reaction.
substance that is formed as
the result of a
chemical reaction.
CH4 + 2 O2
CO2 + 2 H2O
Subscript
Coefficient
Indicates a reaction has
: tells
taken place. Read as “yields”
:
you how many atoms you
tells in a molecule. molecules
have you how many If no
you have. means you have
subscript itIf no coefficient it
means you
one atom. have one molecule.

9. Balancing Equations
After you write a chemical equation you have to balance it to
make sure that the same number of atoms of each element
are on each side.
How would you balance this equation?
Li + H2O  H2 + LiOH

10. Steps to Balancing a Chemical
Equation
Get yourself an unbalanced
equation.
Li + H2O  H2 + LiOH

11. Steps to Balancing a Chemical
Equation
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.
Li + H2O  H2 + LiOH

12. Steps to Balancing a Chemical
Equation
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.
How? Use the skills you learned on counting atoms

13. Steps to Balancing a Chemical
Equation
Li + H2O  H2 + LiOH
Li
H
O
1
2
1
Li
H
O
1
3
1

14. Steps to Balancing a Chemical
Equation
Write coefficients in front of each
of the boxes (molecules) until the
inventory for each element is the
same both before (reactants) and
after products) the reaction.

15. Steps to Balancing a Chemical
Equation
Li + H2O  H2 + LiOH
Li
H
O
1
2
1
Li
H
O
I need more hydrogens in the reactant side…
1
3
1

16. Steps to Balancing a Chemical
Equation
Li + 2 H2O  H2 + LiOH
Keep the inventory current…
Li
H
O
1
2
1
1
4
2
Li
H
O
1
3
1
Oh snap, now I need more hydrogens and oxygens in the product
side…

17. Steps to Balancing a Chemical
Equation
Li + 2 H2O  H2 +2 LiOH
Keep the inventory current…
Li
H
O
1
2
1
1
4
2
Li
H
O
1
3
1
2
4
2
Looks like we are getting somewhere, but I need more lithiums in
the reactant side…

18. Steps to Balancing a Chemical
Equation
2 Li + 2 H2O  H2 + 2 LiOH
Keep the inventory current…
Li
H
O
1
2
1
2
4
2
Yay! A balanced equation.
Li
H
O
1
3
1
2
4
2

19. Ok, let’s do this together
Using the set of cards you have, replicate this
equation
H2 + O2
H2 O

20. Inventory the atoms
Write this in your sheet
H2 + O2
2
2
H2 O
2
1
What do I need?

21. Add a coefficient
More oxygen.
Inventory!
H2 + O2
2 H2 O
2
1
2
2
NOT DONE! I need more
hydrogen in the reactants side!
4
2

22. Add another coefficient
Add more hydrogen in the reactant side.
2 H2 + O2
2
2
4
2
2 H2 + O2 → 2 H2O
Inventory!
2 H2 O
2
1
4
2
NOT DONE! Check your work
and write the final balanced
equation

23. Your Turn.
At your tables, use the cards to correctly balance the
equations. Write down your answers in your in your
sheets.

24. Super Saver Tips
Remember to simplify. If ALL THE COEFFICIENTS share a
divisor, simplify. Just like when working with fractions.
If you see repeating entities such as (SO4), (PO4), (MnO4),
(OH) in BOTH sides , treat them as one atom.
Na3PO4 + KOH → NaOH + K3PO4
Na
PO4
K
OH
3
1
1
1
3
1
3
3
Na
PO4
K
OH
1
1
3
1
3
1
3
3
Na3PO4 + 3 KOH → 3 NaOH + K3PO4
If you have H2O in one side and OH in the other, rewrite
the water as H-OH and treat the OH (hydroxide) as one
atom.