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Balancing Chemical Reactions
Balancing Chemical
Equations
Balancing Chemical Reactions
Reactants: Zn + I2
Product: Zn I2
Balancing Equations:
At the conclusion of our time together,
you should be able to:
1. List the various parts of a chemical
equation and define each
2. Count the number of atoms on each side of
an equation
3. Use conservation of matter to balance a
chemical equation.
A Balanced Equation




Atoms can’t be created or
destroyed
Law of Conservation of Mass
All the atoms we start with we
must end up with
A balanced equation has the same
number of each element on both
sides of the equation (reactant and
product sides).
Counting Atoms




2H2 + O2 
Reactants 
2H2O
Product
Subscript: (numbers below)
 indicates how many atoms of an
element.
Coefficient (numbers in front)
 Indicates how many molecules/moles
of a compound
 (multiplies the number of atoms of
each element in the compound)
Chemical Formula
Counting up Atoms
Let’s Practice


NH3
1
 _____
3
 _____
1
 _____
NaCl
1
 _____
1
 _____
1
 _____
atoms of Nitrogen
atoms of Hydrogen
molecules/moles of NH3
atoms of Sodium
atoms of Chlorine
molecule/moles of NaCl
Practice Again



8NaCl
 _____
8 atoms of Sodium
8 atoms of Chlorine
 _____
 _____
8 molecule/moles of NaCl
3NH3
 ____
3 atoms of Nitrogen
 ____
9 atoms of Hydrogen
 ____
3 molecule/moles of NH3
Ca(NO3)2
1
 _____
atoms of Calcium
 _____
atoms of Nitrogen
2
 _____
atoms of Oxygen
6
 _____
molecule/moles of Ca(NO3)2
1
More Practice

SO2 + 3H2O
1
 _____
atoms of Sulfur
6
 _____
atoms of Hydrogen
5
 _____
atoms of Oxygen
1
 _____
molecule/moles of SO2
3
 _____
molecules of H2O
Let’s Step It Up a Little

2NH4NO3 + 3H2S
4
 _____
atoms of Nitrogen
14 atoms of Hydrogen
 _____
6
 _____
atoms of Oxygen
3
 _____
atoms of Sulfur
2
 _____
molecule/moles of NH4NO3
3
 _____
molecule/moles of H2S
You Must Be Joking!

2Na2CO3 + 3CO2 + 4NaCl + Mg(NO3)2
 _____
atoms of Sodium
8
 _____
atoms of Carbon
5
 _____
18 atoms of Oxygen
4
 _____
atoms of Chlorine
2
 _____
atoms of Nitrogen
1
 _____
atoms of Magnesium
2
 _____
molecules/moles of Na2CO3
 _____
molecule/moles of CO2
3
 _____
molecule/moles of NaCl
4
1
 _____
molecule/moles of Mg(NO3)2
C



+
O
O

O C
C + O2  CO2
This equation is already balanced
What if it isn’t already?
O
C




+
O
O

C + O2  CO
We need one more oxygen in the
products.
Can’t change the formula, because it
describes what is produced.
Remember, oxygen is a diatomic
molecule.
C
O
C


+
O
O

The other Oxygen must be used to
make another CO
But where did the other C come
from?
C
O
C
O
C
+
C


O
O

Must have started with two C’s
2 C + O2  2 CO
C
O
C
O
Rules for balancing
 Write the correct formulas for all the



reactants and products
Count the number of atoms of each
type appearing on both sides
Balance the elements (make them
same amount on reactant and
product sides) one at a time by
adding coefficients (the numbers in
front).
Check to make sure it is balanced.
Never!


Change a subscript to balance an
equation.
 If you change the formula you
are describing a different
reaction.
 H2O is a different compound than
H2O2
Never put a coefficient in the
middle of a formula
 2 NaCl is okay, Na2Cl is not.
Example
H2 + O2  H2O
Make a table to keep track of
where you are
Example
H2 + O2  H2O
R
P
2 H 2
2 O 1
Need twice as much O in the product
Example
H2 + O2 
R
P
2 H 2
2 O 1
Changes the O
2 H2O
Example
H2 + O2 
2 H2O
R
P
2 H 2
2 O 1 2
Also changes the H
Example
H2 + O2 
2 H2O
R
P
2 H 2 4
2 O 1 2
Need twice as much H in the reactant
Example
2 H2 + O2 
2 H2O
R
P
2 H 2 4
2 O 1 2
Recount
Example
2 H2 + O2 
2 H2O
R
P
4 2 H 2 4
2 O 1 2
The equation is balanced, has the same
number of each kind of atom on both
sides
Example
2 H2 + O2 
2 H2O
R
P
4 2 H 2 4
2 O 1 2
This is the answer
Not this
Examples





CH4 + O2  CO2 + H2O
AgNO3 + Cu  Cu(NO3)2 + Ag
Mg + N2  Mg3N2
P + O2  P4O10
Na + H2O  H2 + NaOH
Example Answers





CH4 + 2O2  CO2 + 2H2O
2AgNO3 + Cu  Cu(NO3)2 + 2Ag
3Mg + N2  Mg3N2
4P + 5O2  P4O10
2Na + 2H2O  H2 + 2NaOH
Balancing Chemical Reactions
Reactants: Zn + I2
Product: Zn I2
Balancing Equations:
At the conclusion of our time together,
you should be able to:
1. List the various parts of a chemical
equation and define each
2. Count the number of atoms on each side of
an equation
3. Use conservation of matter to balance a
chemical equation.
7th Grade Science Answers
"The body consists of three parts - the brainium, the
borax and the abominable cavity. The brainium
contains the brain, the borax contains the heart and
lungs, and the abominable cavity contains the bowls,
of which there are five - a, e, i, o, and u."
Introduction
Chemical reactions occur when bonds
between the outermost parts of atoms are
formed or broken
Chemical reactions involve changes in matter,
the making of new materials with new
properties, and energy changes.
Symbols represent elements, formulas
describe compounds, chemical equations
describe a chemical reaction
Parts of a Reaction Equation




Chemical equations show the conversion
of reactants (the molecules shown on the
left of the arrow) into products (the
molecules shown on the right of the
arrow).
A + sign separates molecules on the
same side
The arrow is read as “yields”
Example
2H2 (g) + O2 (g)  2H2O (l)
This reads “hydrogen gas plus
oxygen gas reacts to yield dihydrogen
monoxide liquid.”

The charcoal used in a grill is basically carbon. The
carbon reacts with oxygen to yield carbon dioxide.
The chemical equation for this reaction, C + O2 
CO2, contains the same information as the English
sentence but has quantitative meaning as well.
Chemical Equations
Because of the principle of the
conservation of matter,
an equation must be
balanced.
It must have the same
number of atoms of the
same kind on both sides.
Lavoisier, 1788
Symbols Used in Equations
 Solid
(s)
 Liquid (l)
 Gas (g)
 Aqueous solution (aq)
H2SO4
 Catalyst
 Escaping gas ()
 Change of temperature ()
Balancing Equations


When balancing a chemical reaction you
may add coefficients in front of the
compounds to balance the reaction, but
you may not change the subscripts.
Changing the subscripts changes the
compound. Subscripts are determined by
the valence electrons (charges for ionic or
sharing for covalent)
Subscripts vs. Coefficients

The subscripts
tell you how
many atoms of a
particular
element are in a
compound. The
coefficient tells
you about the
quantity, or
number, of
molecules of the
compound.
Balancing Equations:
Let’s see if you can:
1. List the various parts of a chemical
equation and define each
2. Count the number of atoms on each side of
an equation
All You Really Need To Know
You Can Learn From Noah's Ark


1. Don't miss the boat.
2. Don't forget that we're all in
the same boat.
Chemical Equations
4 4 Al (s) + 3 O2 (g) ---> 2 Al2O3 (s)
Black and Gray?
Depict the kind of reactants and products and
their relative amounts in a reaction.
The blue numbers are called
subscripts.
The numbers in the front are called
coefficients.
The letters (s) and (g) are the
physical states of compounds.
Balancing Chemical Reactions II
Balancing Equations
Balancing Equations:
At the conclusion of our time together,
you should be able to:
1. Use conservation of matter to
balance a chemical equation.
Chemical Equations
4 Al(s) + 3 O2(g)
---> 2 Al2O3(s)
This equation means
4 Al atoms + 3 O2 molecules
---produces--->
2 molecules of Al2O3
AND/OR
4 moles of Al + 3 moles of O2
---produces--->
2 moles of Al2O3
Familiar Saying
Freedom from guile or fraud
constitutes the most excellent
principle of procedure.
Honesty is the best policy!
Ag + Cl2
AgCl
1
1 1
2
Both sides must be equal
for ALL atoms.
Ag + Cl2
1
2
2 AgCl
1
2
1
2
Both sides must be equal
for ALL atoms.
2 Ag + Cl2
1
2
2
2 AgCl
1 1
2 2
Both sides must be equal
for ALL atoms.
CH4 + O2 CO2+ H2O
1 4
2
1 2
2
1
CH4 + O2 CO2+2H2O
1 4
2
1 2
2
1
4
2
CH4 +2 O2 CO2+2H2O
1 4
2
4
1 2
2
1
4
2
Steps to Balancing Equations
There are four basic steps to balancing a
chemical equation.
1. Write the correct formula for the reactants and
the products. DO NOT TRY TO BALANCE IT
YET! You must write the correct formulas first.
And most importantly, once you write them
correctly DO NOT CHANGE THE FORMULAS!
2. Find the number of atoms for each element on
the left side. Compare those against the number
of the atoms of the same element on the right
side.
Steps to Balancing Equations
3. Determine where to place coefficients in front of
formulas so that the left side has the same
number of atoms as the right side for EACH
element in order to balance the equation.
4. Check your answer to see if:
 The numbers of atoms on both sides of the
equation are now balanced.
 The coefficients are in the lowest possible
whole number ratios. (reduced)
Check Out This Actual Sign!!
Balancing Equations
2 H2(g) + ___ O2(g) ---> ___
2 H2O(l)
___
This equation is not balanced!
Two hydrogen atoms from a
hydrogen molecule (H2)
combines with one of the oxygen
atoms from an oxygen molecule
(O2) to form H2O. Then, the
remaining oxygen atom
combines with two more
hydrogen atoms (from another H2
molecule) to make a second H2O
molecule.
Balancing
Equations
2___ Al(s)
2 AlBr3(s)
+ 3
___ Br2(l) ---> ___
Some Suggestions to Help You
 Look
for singleton elements in
compounds and determine the
relationships.
 Do the hardest element to
balance first keeping in mind
the relationships from #1.
 Balance the lone elements last.
Balancing
Equations
Page 12:13
2
15
____C
6H6(g) + _____ O2(g) ---->
A
12
6
_____CO
2(g) + _____ H2O(g)
6A
3A
2
11
____B
4H10(g) + _____ O2(g) ---->
A
4 B O (g) + _____
10 H O(g)
___
2 3
2
2A
5A
Balancing Practice Problems
Challenges Page 10:1
2
15
____C
7H6O2 + _____ O2 ---->
A
14
6
_____CO
2(g) + _____ H2O(g)
7A
3A
Balancing Equations:
Let’s see if you can:
1. Use conservation of matter to
balance a chemical equation.
Balance Each of the Following Equations
Page 11:6
1
12 H2O ---->
____Al
_____
4C3 +
A
3
4 Al(OH)3
_____CH
_____
4 +
3A
4A
Balance Each of the Following Equations
Page 11:3
2
____H
---->
3AsO4
2A
1
_____As
2O5 +
A
3
_____
H 2O
3A
15 Helpful Hints On The Lab Report from
Mr. T’s Vast Lab Experience!!!
Hint #11. Given the most inappropriate
time for something to go wrong, that's
when it will occur.
Balancing Chemical Reactions from Words
Balancing Equations:
At the conclusion of our time together,
you should be able to:
1. List the various parts of a chemical
equation and define each
2. Count the number of atoms on each side of
an equation
3. Use conservation of matter to balance a
chemical equation.
4. Go from a word equation to a balanced
chemical equation.
MAGIC NUMBERS
1. Take your house number and double it.
2. Add 5.
3. Multiply by 50.
4. Add your age.
5. Add the number of days in a non-leap year.
6. Subtract 615.
MAGIC NUMBERS
What number do you have??
Look familiar????
_________________ - ________
Your house number
Your age
Converting and Balancing Word
Equations
Sodium phosphate + iron (III) oxide 
sodium oxide + iron (III) phosphate
2 Na3PO4 +
Fe2O3 ---->
3 Na2O +
2 FePO4
Converting Word Equations into
Chemical Equations 14:10
Strontium iodide + Lead (II) phosphate 
Strontium phosphate + lead (II) iodide
3 SrI2 +
Pb3(PO4)2 ---->
Sr3(PO4)2 + 3 PbI2
I Wouldn’t Throw That!!!