Writing and Balancing Chemical Equations
H/Chemistry
Chemical Reactions Unit – Overview
the core of chemistry – how chemicals react with one another
Two parts of this unit:
1. Language of chemical reactions
a) formulas (words/vocabulary) (from Bonding and Naming Unit)
b) chemical equations (sentences)
Sequence:
i)
balance skeleton formula equations,
ii)
word equations  balanced formula equations,
iii)
sentences  balanced formula equations
iv)
write complete, balanced formula equations to describe chemical reactions
observed in lab or as demos
2. Different types of reactions
a) classify
b) predict products
c) acids and bases, pH
Writing Chemical Equations
Example: Iron and chlorine gas react to produce the salt iron(III) chloride.
Word equation: describes the reaction in words, using equation symbols
Iron(s) + chlorine(g)  iron(III) chloride(s)
Skeleton formula equation: describes the reaction using equation symbols and correct
chemical formulas; unbalanced
Fe(s) + Cl2(g)  FeCl3(s)
Balanced formula equation: describes the reaction using equation symbols and chemical
formulas; balanced - coefficients are used to equalize numbers of each atom on each side of
the reaction
2 Fe(s) + 3 Cl2(g)  2 FeCl3(s)
Iron atoms on left (2)
Chlorine atoms on left (3 x 2 = 6)
iron atoms on right (2)
chlorine atoms on right (2 x 3 = 6)
Total and net ionic equations: describe the reaction using equation symbols and chemical
formulas, shown as ions if the compound is soluble in water (notes later)
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Writing and Balancing Chemical Equations
H/Chemistry
Steps for Writing and Balancing Chemical Equations
1. Write a word equation with the names for all reactants to the left and all products to
the right.
Reactants  Products
2. Convert the word equation into a skeleton formula equation by writing the correct
formulas for all reactants and products.
Be sure that your formulas correctly represent the particles in the reaction!
3. Use coefficients in front of formulas to balance the equation.
Do NOT change the formulas!
4. Begin balancing with an element that occurs only once on each side of the arrow.
5. Multiply the coefficient by the subscript to determine the # of atoms of a specific
element in one “term” of the equation:
e.g.
4 H2O molecules:
4 x 2 H = 8 H atoms 4 x 1 O = 4 O atoms
6. Balance one type of atom at a time.
7. Balance O and H atoms last, especially for combustion reactions.
8. Even/Odd rule
e.g. Fe + O2  Fe2O3
Since the number of oxygen atoms on the left must be even, start by making the
number of oxygen atoms on the right even, then balance the Fe.
4 Fe + 3 O2  2 Fe2O3
9. Intact polyatomic ions
e.g.
Fe + Pb(C2H3O2)2  Fe(C2H3O2)3 + Pb
Balance the atoms in acetate, C2H3O2, as a group, because they stay together as a
group.
2 Fe + 3 Pb(C2H3O2)2  2 Fe(C2H3O2)3 + 3 Pb
10. Combustion reactions
e.g. C2H2 + O2  CO2 + H2O
a) make the number of hydrogen atoms in the reactants divisible by four.
b) begin by making 4 H atoms on the left by putting 2 in front of C2H2.
c) balance the carbon atoms next, and, lastly, the oxygen atoms.
2 C2H2 + 5 O2  4 CO2 + 2 H2O
We will focus on these at a later time.
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Writing and Balancing Chemical Equations
H/Chemistry
Symbols used in Chemical Equations
Symbol
Explanation

“yields”, indicates result of reaction.

Used in place of a single arrow for reversible reaction
(s)
A reactant or product in the solid state
(l)
A reactant or product in the liquid state
(g)
A reactant or product in the gas state
(aq)
A reactant or product dissolved in water
 or heat

Reactants are heated
pressure

Pressure exceeding normal atmospheric pressure
o
0C

Temperature at which reaction is carried out, e.g. 0oC
MnO2

Formula of catalyst used to alter the rate of the reaction, e.g. manganese
dioxide
Practice – Writing and Balancing Chemical Equations – identify errors
For each of the following, explain why the equation is not properly balanced, then write the
correctly balanced equation.
1.
Mg(NO3)2(aq) + 2 K(s)  Mg(s) + K2NO6(aq)
2.
AlCl3(aq) + AgNO3(aq)  AgCl(s) + Al(NO3)3(aq)
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Writing and Balancing Chemical Equations
H/Chemistry
Practice – Writing and Balancing Chemical Equations
1.
Solid magnesium metal and solid silver sulfide react to form solid magnesium sulfide
and solid metallic silver.
Word equation:
Skeleton formula equation:
Balanced Formula equation:
2.
Aqueous nitric acid and calcium hydroxide solutions react to form water and aqueous
calcium nitrate.
Word equation:
Skeleton formula equation:
Balanced Formula equation:
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Writing and Balancing Chemical Equations
H/Chemistry
More Practice - Writing and Balancing Chemical Equations
3.
Aluminum metal reacts with oxygen in the air to form aluminum oxide.
Word equation:
Skeleton formula equation:
Balanced Formula equation:
4.
When solid mercury (II) sulfide is heated with oxygen, liquid mercury metal and
gaseous sulfur dioxide are produced.
Word equation:
Skeleton formula equation:
Balanced Formula equation:
5.
Oxygen gas can be made by heating potassium chlorate in the presence of the
catalyst manganese dioxide. Potassium chloride is left as a solid residue.
Word equation:
Skeleton formula equation:
Balanced Formula equation:
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Writing and Balancing Chemical Equations
H/Chemistry
Total and Net Ionic Equations
Purpose: Show the actual reacting species
Example: When solutions of potassium iodide and lead(II) nitrate are mixed, they react
together to form the precipitate (solid) lead(II) iodide and aqueous potassium nitrate.
Word equation:
Potassium iodide(aq) + lead (II) nitrate(aq)  lead (II) iodide(s) + potassium nitrate(aq)
Formula (Skeleton) Equation
KI(aq) + Pb(NO3)2(aq)  PbI2(s) + KNO3(aq)
Balanced formula equation:
2 KI(aq) + Pb(NO3)2(aq)  PbI2(s) + 2 KNO3(aq)
Total ionic equation:
2 K+(aq) + 2 I-(aq) + Pb2+(aq) + 2 NO3-(aq)  PbI2(s) + 2 K+(aq) + 2 NO3-(aq)
Ions that show up on both sides of the equation as ions are called “spectator ions” since
they do not participate in the reaction. In the above example, the spectator ions are 2 K+(aq)
and 2 NO3-(aq).
Net ionic equation:
Spectator ions are not included in the net ionic equation.
Pb2+(aq) + 2 I-(aq)  PbI2(s)
Practice – Net Ionic Equations
When solutions of potassium hydroxide and aluminum chloride are mixed, they produce solid
aluminum hydroxide and aqueous potassium chloride.
On binder paper, write word, skeleton, balanced formula, total ionic and net ionic equations
for this reaction.
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