Init: 12/14/2011 by Daniel R. Barnes
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SWBAT . . .
. . . balance chemical equations
Prentice Hall’s Chemistry textbook, page 326, says that the
reaction pictured above violates the law of conservation of mass.
Why is this?
The following stuff comes from Mr. Barnes’
Chemical Equation Balancing Practice Worksheet.
1.a. H2O
H
H
O
One water molecule
1.b. 2O2
O
O
O
O
Two oxygen gas
molecules
1.c. 3HCl
H
Cl
Cl
H
H
Cl
Three hydrogen chloride
molecules
1.d. 8NaCl
Na+
Cl
Na+
Cl
Cl
Na+
Cl
Na+
Na+
Cl
Na+
Cl
Cl
Na+
Cl
Na+
Eight sodium chloride formula units
1.e. 4CO2
O
C
O
O
C
O
O
C
O
O
C
O
Four carbon dioxide
molecules
1.f. 3Cl2
Cl
Cl
Cl
Cl
Cl
Cl
Three chlorine gas
molecules
Think-pair-share:
Compare and contrast
4O, 2O2, and O4.
(If you haven’t seen this question recently.)
O
O
O
O
O
O
O
O O
O O
O
4O
2O2
O4
Four separate,
unbonded,
monatomic
oxygen atoms
Two molecules
made of
two oxygen atoms
each
One molecule
made of
four oxygen atoms
stuck together
5.a.
H2 +
O2 
H2O
Notice all the space in front of each formula.
You’re going to need it sometimes, but you never know when,
so you better always leave about an inch of space in front of
each formula.
5.a.
H2 +
O2 
H= 2
O= 2
H2O
H= 2
O= 1
Once you’ve copied the equation, you need to do an initial
atom count.
The first step in doing that is to list the elements on both sides
of the arrow, in the same order.
Then, count the number of atoms of each element on the left
and on the right.
Your initial atom count is now finished.
5.a.
H2 +
H= 2
O= 2
O2 
H2O
H= 2
O= 1
The whole point of balancing an equation is to make sure that
the numbers on the left equal the numbers on the right.
1. Copy the equation onto your scratch paper. Leave plenty
of space (about an inch) in front of each formula for
coefficients. Yes, you might need about an inch because
the first coefficient you put down might be wrong, and you’ll
need to cross it out and put a new one to the left of it,
repeating that process several times for some formulas.
Don’t erase old coefficients. Cross them out. I want to see
all your “footprints” on your scratch paper.
2. Break down the equation into its elements. List the
elements in the same order on the left as you do on the
right. Trust me. That’ll make it easier.
3. Do an atom count for each element on both sides of the
arrow.
4. Is the equation balanced? In other words, are the
atom counts on the left equal to the atom counts on
the right? If so, you’re done. Put a happy face on it.
If not, you’re going to have to do a bit of adjusting.
5. If the equation isn’t balanced yet, change one of the
coefficients so that you end up with more of
whatever element you need more of on the side of the
arrow where you need it.
6. As soon as you change a coefficient, immediately
update all the atom counts that were affected by that
coefficient. Remember that a coefficient affects the
amount of every element in the whole formula that the
coefficient is in front of.
Let’s see those steps again, but more abbreviated.
1. Copy equation.
2. Elemental breakdown
3. Atom count.
4. Equation balanced?
5. Change a coefficient.
6. Update atom count.
Q1: What do you call the big numbers out in front of the
formulas in a chemical equation?
A: coefficients
Q2: What do you call the little numbers found inside of a
formula?
A: subscripts
Q3: What do coefficients tell you?
A: how many molecules take part in a reaction
Q4: What do subscripts tell you?
A: how many atoms of an element are in each molecule of a
substance
Q5: If a quantity does not change during a process, we say
that the quantity is . . .
A: “conserved”.
Q6: What quantities are conserved during a chemical
reaction?
A: number of atoms (both overall and of each element), mass
(amount of matter), electric charge.
Q7: How do you know if a chemical equation is balanced or
not?
A: If the number of atoms of each element is the same on the
left of the arrow as it is on the right of the arrow, the equation
is balanced.
Q8: What is the only thing you are allowed to change about
an equation when you balance it?
A: the coefficients
Q9: If there is no coefficient in front of a formula, how many
molecules are indicated?
A: one molecule
Q10: Coeffcients usually have to be integers/whole numbers.
Name one exception to this rule and a specific example of
that exception.
A: You many use fraction made of an odd number over two
as a coefficient in front of a diatomic element. For instance,
“½ O2” is acceptable.
DIRECTIONS: Balance the following equations. Show your steps.
1. Ba + N2  Ba3N2
2. Sr + P  Sr3P2
3. Al + O2  Al2O3
4. O3  O2
5. Na + F2  NaF
DIRECTIONS: Balance the following equations. Show your steps.
6. H2 + O2  H2O
7. H2O2  H2O + O2
8. Ca(OH)2 + HCl  CaCl2 + H2O
9. B + Cl2  BCl3
10. Fe(OH)3 + HNO3  Fe(NO3)3 + H2O
DIRECTIONS: Balance the following equations. Show your steps.
11. Na + O2  Na2O
12. C4H10 + O2  CO2 + H2O
13. C6H14 + O2  CO2 + H2O
14. C12H22O11 + O2  CO2 + H2O
15. CH3CH2OH + O2  CO2 + H2O
“The Monster”
(from Steve Wallis)
___K2Mn2O8 + ___H2SO4 + ___H2C2O4

___CO2 + ___MnSO4 + ___K2SO4 + ___H2O
NOTE TO INSTRUCTOR: The equation balancing quiz slides
have been broken off to form a separate presentation so that this
one might be postable on the web without compromising test
security.