Friday, Aug. 30th: “A” Day
Tuesday, Sept. 3rd: “B” Day
Agenda
Go over quiz/review worksheet
Section 8.2: “Balancing Chemical Equations”
Homework:
1. Sec. 8.2 review, pg. 274: #1-6
2. Chemical Formulas/Equations Worksheet
a. Pg. 329: #15 – 24/ Pg. 330: # 1- 20
3. “Writing Chemical Formulas” worksheet
4. Concept Review: “Balancing Chemical
Equations”
Reactions Conserve Mass
A basic law of science is the conservation of
mass.
Law of conservation of mass: in ordinary
chemical or physical changes, mass is neither
created nor destroyed.
If the products of a chemical reaction are all
collected, their total mass will be the same as
the total mass of the reactants.
Reactions Rearrange Atoms
The products and reactants of a chemical
reaction are made up of the same number
and kinds of atoms.
The atoms are just rearranged and connected
differently.
? Na + ? H2O ? NaOH + ? H2
To show that a reaction satisfies the
law of conservation of mass, the
equation must be BALANCED!
Balancing Equations
To balance an equation, the number of atoms
of each element needs to be the same on
each side of the arrow.
But, the formulas cannot be changed.
CO cannot be changed to CO2 to balance the
oxygen atoms – these are two different
molecules with different properties.
Place numbers, called coefficients, in
FRONT of the formulas…
Coefficients
 Coefficient: a small, whole number that appears as a
factor in front of a formula in a chemical reaction.
 A coefficient multiplies the number of atoms of each
element in the formula.
 For example:
H2O: 1 water molecule contains 2
hydrogen atoms and 1 oxygen atom
2 H2O: 2 water molecules contain 4
hydrogen atoms and 2 oxygen atoms
Diatomic Elements
For this section, you will need to remember that
the following elements are diatomic and exist as
molecules:
H2, O2, N2, F2, Cl2, Br2, I2
**Write these at the top of
your notes**
How to Balance Chemical Equations
Skills Toolkit (pg. 268)
1. Identify reactants and products.
a. Write an unbalanced equation.
2. Count atoms.
a. Balance the atoms that appear in only 1 reactant and
1 product first.
b. Balance O2 and H2 last.
c. Treat polyatomic ions as a unit if they appear on both
sides of arrow.
3. Insert coefficients to balance 1 element at a time.
4. Check to make sure you did it right!
Balancing an Equation
Sample Problem A, pg. 269
Balance the equation for the reaction of iron(III)
oxide with hydrogen to form iron and water.
1. Identify reactants and products.
Iron(III) oxide + hydrogen
iron and water
?
H2
Fe
H2O
We need to figure out the formulas for our
reactants and products.
Balancing an Equation
Sample Problem A, pg. 269 cont.
What is the formula for iron(III) oxide?
Iron(III) means that the charge on the iron cation
is 3+
Oxide is oxygen, whose ion charge is always 2-
Fe3+ O2Fe2O3
Fe2O3 + H2
Fe + H2O (unbalanced)
Sample Problem A, cont.
Fe2O3 + H2
Fe + H2O
2. Count atoms. Balance Fe (Iron) 1st because it
appears in only 1 reactant and 1 product.
Fe2O3 + H2
2 Fe + H2O
Balance O2 (Oxygen) next:
Fe2O3 + H2
2 Fe + 3 H2O
Lastly, balance H2 (Hydrogen):
Fe2O3 + 3 H2
2 Fe + 3 H2O
Is everything balanced?
YES
Additional Example
Balance the following equation:
C3H8 + O2
CO2 + H2O
Balance C (carbon) first:
C3H8 + O2
3 CO2 + H2O
Next, balance H (hydrogen):
C3H8 + O2
3 CO2 + 4 H2O
Lastly, balance O (oxygen):
C3H8 + 5 O2
3 CO2 + 4 H2O
10 oxygen atoms = 6 + 4 oxygen atoms
Never Change Subscripts
to Balance an Equation
Unbalanced Equation: H2 + O2  H2O
Incorrect: H2 + O2  H2O2
H2O  H2O2
H2O is water; H2O2 is hydrogen peroxide
They are NOT the same thing!
Instead, add coefficients to balance…
Correct: 2 H2 + O2  2 H2O
Sample Problem B
Pg. 271
The reaction of ammonia with oxygen produces
nitrogen monoxide and water vapor. Write a
balanced equation for this reaction.
1. Identify reactants and products.
Reactants: NH3 and O2
Products: NO and H2O
Write unbalanced equation:
NH3 + O2
NO + H2O
Sample Problem B, Cont.
NH3 + O2
NO + H2O
2. Count atoms.
 Balance N (nitrogen) first:
It’s already balanced – yeah!
 Balance H (hydrogen) next:
2 NH3 + O2
NO + 3 H2O
 Recheck the N, it is now unbalanced, fix it:
2 NH3 + O2
2 NO + 3 H2O
 Lastly, try to balance the O (oxygen):
2 NH3 + O2
2 NO + 3 H2O
2 oxygen
5 oxygen atoms
Sample Problem B, Cont.
2 NH3 + 2.5 O2
2 NO + 3 H2O
5 oxygen
5 oxygen atoms
In order to get the oxygen atoms to be equal, on
both sides, the O2 coefficient needs to be 2.5
But, coefficients need to be small, whole
numbers so this is incorrect.
To solve, simply double ALL of the coefficients:
4 NH3 + 5 O2
4 NO + 6 H2O
Is everything balanced?
YES
Sample Problem C, Pg. 273
Aluminum reacts with arsenic acid, HAsO3, to
form H2 and aluminum arsenate. Write a
balanced equation for this reaction.
1. Identify reactants and products.
Reactants: Al and HAsO3
Products: H2 and Al(AsO3)3
Write unbalanced equation:
Al + HAsO3
H2 + Al(AsO3)3
Sample Problem C, Cont.
Al + HAsO3
H2 + Al(AsO3)3
2. Count atoms.
First, balance the Al
It’s already balanced – yeah!
Next, balance the arsenate polyatomic ion:
Al + 3 HAsO3
H2 + Al(AsO3)3
Next, balance the H (hydrogen):
Al + 6 H AsO3
3 H2 + Al(AsO3)3
This messes up the AsO3 ion, fix it:
Al + 6 H AsO3
3 H2 + 2 Al(AsO3)3
Sample Problem C, Cont.
Al + 6 H AsO3
3 H2 + 2 Al(AsO3)3
Are we balanced yet?
NO!
Now the Al (aluminum) is unbalanced, fix it:
2 Al + 6 H AsO3
3 H2 + 2 Al(AsO3)3
How about now?
Yes, we’ve finally balanced the equation!
Practice Makes Perfect
The best way to get better at balancing
equations is to practice, a lot!
You don’t have to balance polyatomic ions as a
unit if you don’t want to. You can balance the
individual elements if you find that easier.
Sometimes, balancing equations is tricky and
you need to use trial and error…
Homework
1. Sec. 8.2 review, pg. 274: #1-6
2. Chemical Formulas/Equations WS
a. Pg. 329: #15 – 24
b. Pg. 330: # 1- 20
3. “Writing Chemical Formulas” worksheet
4. Concept Review: “Balancing Chemical Equations”
Next time:
Quiz over section 8.2: “Balancing
Chemical Equations”
High School Open House:
Wednesday, Sept. 4th: 6:30 – 8:00 pm