Balancing Chemical Equations
• We’ve done it all – counted atoms,
examined trends, bonded atoms….
• But now, we’re going to string it all
together in a sort of equation.
– Like math…but not math.
• Consider the law of conservation of
mass:
“Matter cannot be created or destroyed.”
• CHEMICAL EQUATIONS are
representations of chemical reactions.
– REACTANTS are the chemicals that go into
the reaction.
• These are listed on the left side of the
equation.
– PRODUCTS are the chemicals that come out
of the reaction.
• These are listed on the right side of the
equation.
• Chemical equations are written much like
you would write a math problem.
– You’re basically adding things together to make a
total on the right side.
• But there are some differences, too.
– Instead of a “=“, we use an arrow to show
direction ().
– Sometimes equations aren’t as simple as
adding things up.
2*(3) + 4 = 10
2 Na + Cl2  2 NaCl
Similarities
Differences
• Let’s look at an example.
2H2 + O2  2H2O
• This is the reaction to form water from its
elements.
• What kinds of terms or features do you notice
in this equation?
• What are my reactants? What are my
products?
2H2 + O2  2H2O
• In the formation of water (H2O), we react, or
consume, 2 portions of hydrogen for every 1
portion of oxygen.
• As a result, we produce 2 portions of water.
• Let’s look at the atoms themselves.
– Is this equation BALANCED? Are all the atoms
that start the reaction present at the end?
– 4 hydrogen go in, 4 hydrogen come out
– 2 oxygen go in, 2 oxygen come out
• Try another example:
CH4 + 2O2  CO2 + 2H2O
• This is the reaction for burning methane.
Notice that one of the products is carbon
dioxide, a greenhouse gas.
CH4 + 2O2  CO2 + 2H2O
• How many carbons do we start with?
– How many do we end with?
• How many hydrogens do we start with?
– How many do we end with?
• How many oxygens do we start with?
– How many do we end with?
• This equation has already been balanced for
you. Let’s see if you can balance one on your
own.
___Na + ___H2O  ___NaOH + ___H2
• This is the reaction for sodium metal
with water.
• How can you balance this reaction?
• Now I will provide an equation that is
balanced, but is missing the product’s
formula.
• Can you predict the product of this
reaction?
C6H12O6 + 6 O2  6 CO2 + 6 _______
Predict the product
• Given the reactants, predict the product in
these reactions.
2 P + 2 O2  __________
Ca + S  _________
2 C + 2 H2  __________
2 Fe + Cl2  _________
• Are these products ionic or covalent?
• Can you diagram the products?
Sodium hydroxide (NaOH) is a potent base that
draws water and carbon dioxide out of the air if
left in the open. It is also a base used to
neutralize certain acids.
___ NaOH + ___ CO2  ___ Na2CO3 + ___ H2O
___ NaOH + ___ HCl  ___ NaCl + ___ H2O
Balancing Equations: Steps for Success
1. Set every coefficient to 1.
2. Check to see if the reaction is balanced by drawing a
table with “in” and “out” columns.
a. Count the atoms of each type going in and coming out.
3. If atoms in = atoms out for each type of atom, then
you’re done!
4. If not, then go down the chart, balancing each type of
atom as you go.
a. Must be whole numbers! You can’t split atoms!
b. You may have to revisit types of atoms more than once –
keep at it!
___ Mg(OH)2 + ___ HCl  ___ MgCl2 + ___ H2O
1. Balance Mg
2. Balance (OH)
– The nice thing about polyatomic ions
(“multiple atoms”) is that they react in
groups – they don’t usually break up.
3. Balance Cl, H
4. Check your work!
Indiana is well known for its limestone,
harvested around Bloomington. Limestone is a
hard deposit (mostly calcium carbonate) that
was likely deposited by marine life. Limestone is
used in building, but has stopped being used for
monuments in densely-populated areas.
Humans have a large impact on the
environment, with waste producing phenomena
like smog and acid rain.
A reaction for dissolving calcium carbonate is:
___CaCO3 + ___HCl  ___H2O + ___CO2 + ___CaCl2
Why might limestone be a poor building material
choice for densely-populated cities like Los Angeles or
New York City?
Acid rain eats away at the faces of limestone
buildings, eroding the strength and beauty of the
building.
FYI - The product, calcium chloride (CaCl2), is used as
salt for roads (rock salt) and water hardeners.
Individual/Small group Practice
• Practice with a partner or individually on
balancing some chemical equations. For each
of the problems, find ALL of the coefficients to
balance the equation.
– Aim for 10 basic and 6 advanced @ 100 %
– For extra practice, go for the mastery problems
– I know they won’t all fit on the worksheet. Just fill
in the first 13, then work on scrap paper.
– Write down your problems on the worksheet and
turn it in at the end of class.
• Basic problems
CH4 + O2  CO2 + H2O
KClO3 → KCl + O2
H2 + O2 → H2O
P4 + O2 → P2O5
Al + Br2 → AlBr3
CCl4+O2  CO2+Cl2
C3H8 + O2  CO2 + H2O
SnO2 + H2 → Sn + H2O
C6H6 + O2 → CO2 + H2O
AgI + Na2S → Ag2S + NaI
• Advanced problems
Na + C2Cl6 → NaCl + C2Cl2
C2H6 + O2  CO2 + H2O
C2H3Br3+O2 CO2+HBr
P4 + HCl + O2 → PCl3 + H2O
AlBr3 + Cl2 → AlCl3 + Br2
C6H5F+O2 CO2+H2O+F2
CaCN2+H2O  CaCO3+NH3
TiCl4 + H2O → TiO2 + HCl
C6O6Cr + Cl2 → CrCl3 + CO
• Mastery problems
AlI3 + HgCl2 → AlCl3 + HgI2
HSiCl3 + H2O  H10Si10O15 + HCl
KOH + H3PO4 → K3PO4 + H2O
Ba3N2 + H2O → Ba(OH)2 + NH3
Ca(OH)2 + H3PO4 → Ca3(PO4)2 + H2O
NH3 + CO → CH4 + N2 + O2
AgNO3 + K3PO4 → Ag3PO4 + KNO3
C7H9 + HNO3  C7H6(NO2)3 + H2O