Lesson 28 Writing &
Balancing Equations
Objectives:
The student will write chemical
equations representing reactions.
The student will balance chemical
equations.
PA Science and Technology Standards: 3.4.10.A; 3.4.12.A;
3.2.10.B
PA Mathematics Standards: 2.2.11.A, 2.4.11.E,
2.5.11.A
I.
Atoms and Mass are conserved in chemical
reactions
a. There are word equations and chemical
equations for a reaction
i. Hydrogen + Oxygen  Water
ii. H2 + O2  H2O
b. The only problem with the chemical
equation we have written is that there is more
oxygen on the left than on the right – this is not
possible considering the law of conservation of
mass.
c. Also, when we measure the masses of the
reactants and the products, there is no
difference.
d.
We need to find some way to show that
the reaction has occurred, but following this
information
II. Coefficients indicate amounts of reactants and
products
a. We can place numbers in front of each of
the reactants and products to indicate the
amount of that reactant or product that is
present in the reaction.
b.
We call these numbers coefficients
c. We cannot change the subscripts
associated with any of the elements involved in
the reaction.
d. Coefficients can only go in front of
compounds or single elements present –
not in the middle of a chemical formula.
e.
Example
i.
Unbalanced --- H2 + O2  H2O
ii.
Balanced --- 2H2 + O2  2H2O
f. The process of inserting
coefficients to make sure that the law
of conservation of mass is followed is
called “balancing an equation”.
g.
Steps for balancing an equation (start
where you need to in order to solve the
problem you are given – you don’t always
need to start at the beginning)
i. Read the problem
1.
In a gas hot water heater,
methane gas is combined with
oxygen, and in the process,
produces carbon dioxide and
water vapor.
ii. Write out the words in equation
form.
1.
Methane + Oxygen  Carbon Dioxide +
Water
iii. Write the correct formulas for the
different elements and compounds
present.
1. CH4 + O2  CO2 + H2O
iv. Count the number of elements on
each side and make a chart. Make sure
the elements are in the same order on
both sides to keep from making the
problem any more confusing.
1.
Left side:
Right side:
C atoms – 1
C atoms – 1
H atoms – 4
H atoms – 2
O atoms – 2
O atoms – 3
CH4 + O2  CO2 + H2O
v.
Insert coefficients for atoms of one element at
a time so that the law of conservation of mass
is satisfied
1. Tips for balancing equations
a.
First, balance the types of atoms
that appear in only one reactant and one
product
b. Balance the remaining types of
atoms one at a time
of
c.
Balance H atoms and O after most
the other elements have been balanced
d. If the same polyatomic ions appear
on both sides of the equation, treat them
as if they were single units, like
monatomic ions.
e.
If you get stuck – sometimes it is a
good idea to start over from scratch.
vi. Repeat steps iii. and iv. until the law of
conservation of mass holds for all of the
elements in the equation.
vii. Go back and check your math – simple
mistakes can cause large problems.
viii.Make sure that there are equal numbers
of atoms of each element on both sides
of the equation.
ix. Finally, check to see that all of the
coefficients are in lowest terms.
i.
Common Pitfalls
Many students forget one or more of the final three steps.
These are the places where mistakes made during the
earlier stages of the problem are caught and corrected.
Since it is relatively easy to make these mistakes, and just
as easy to complete these three steps, always remember to
apply them to every problem you complete!
1.
2.
3.
Check your math!
Check for equal #’s of elements!
Check for lowest terms!
h.
Second example
I. Write the word equation, showing
all of the reactants and products
Phosphorus + potassium chlorate 
potassium chloride + phosphorus (V) oxide
i.
More examples
i.
CaSi2 + SbCl3  Si + Sb + CaCl2
ii.
C2H2 + O2  CO2 + H2O
iii.
Al + CH3OH  (CH3O)3Al + H2
iv.
Cellular respiration is the process that
your body uses to get energy from the food
you eat. In cellular respiration, sugars such
as glucose, C6H12O6, react with oxygen. The
net result is an increase of energy and the
production of carbon dioxide and water.
Write the balanced equation for cellular
respiration.
v.
When calcium is added to water, calcium
hydroxide and hydrogen gas are formed.
When calcium hydroxide is heated, water and
calcium oxide are the products. Write
balanced chemical equations that describe
this series of changes.
1.Balance each of the following equations.
Level 1
a.Zn + H2SO4  ZnSO4 + H2 
b.Na + Br2  NaBr
c.H2O + energy  H2  + O2 
d.Cl2 + KI  KCl + I2
e.HNO3 + LiOH  H2O + LiNO3
f.N2 + O2  N2O
g.NH4Cl + NaOH  NH4OH + NaCl
h.HCl + K2CO3  KCl + H2O + CO2 
2. Balance each of the following equations.
•Sodium + iodine  sodium iodide
•Zinc + hydrobromic acid  zinc bromide + hydrogen 
•Potassium hydroxide + heat  potassium oxide + water
•Magnesium + water  magnesium hydroxide + hydrogen 
Level 2
1.
a.
b.
c.
d.
Balance each of the following equations.
K3PO4 + HCl  KCl + H3PO4
Na + HNO3  NaNO3 + H2 
S + O2  SO3 
Ca(ClO3)2 + heat  CaCl2 + O2 
2.
a.
Balance each of the following equations.
Potassium iodide + lead (II) nitrate  potassium nitrate
+ lead (II) iodide
Iron (III) oxide + carbon  carbon monoxide  + iron
Mercury (II) oxide + heat  mercury + oxygen 
Calcium + aluminum chloride  calcium chloride +
aluminum
Mercury (I) nitrate + sodium carbonate  sodium nitrate
+ mercury (I) carbonate.
Potassium bromide + aluminum nitrate  potassium
nitrate + aluminum bromide
Calcium phosphate + aluminum sulfate  calcium
sulfate + aluminum phosphate
Rubidium + acetic acid  rubidium acetate + hydrogen 
b.
c.
d.
e.
f.
g.
h.
Level 3
Write and balance chemical equations for each of the
following reactions.
1.
2.
Nitrogen + hydrogen  ammonia 
butane (C4H10) + oxygen + heat  carbon dioxide  +
water 
3.
aluminum oxide  aluminum + oxygen 
4.
ethyl alcohol (C2H5OH) + oxygen + heat  carbon
monoxide  + water 
5.
nitrogen + oxygen  dinitrogen pentoxide 
6.
octane (C8H18) + oxygen + heat  carbon dioxide  +
water 
7.
aluminum sulfate + phosphoric acid  aluminum
phosphate + sulfuric acid
8.
diphosphorus pentoxide + water  phosphoric acid
9.
ammonia + nitric oxide  nitrogen  + water
10. iron (III) oxide + carbon monoxide  iron + carbon
dioxide 
11. copper + nitric acid  copper (II) nitrate + nitric oxide 
+ water
12. iron (II) sulfide + oxygen  iron (III) oxide + sulfur
dioxide 