CHEMISTRY YEAR 9-10
Writing and Balancing Equations
Writing and Balancing Equations

A chemical equation is a shorthand
expression for a chemical change or
reaction.
word - equation :
states in words
mercury (II) oxide + heat
mercury + oxygen
bulky and cumbersome
2 HgO
2Hg + O2
Writing and Balancing Equations

General Format for Writing Equations
1. The reactants are separated from
the products by an arrow
that
indicates the direction of the reaction.
2. The reactants are placed to the left
and the products to the right of the
arrow. A plus sign (+) is placed
between reactants and between
products when needed.
Writing and Balancing Equations
3. Conditions required to carry out the
reaction may be placed above or below
the arrow. A delta indicates heat is
applied.
4. Coefficients 2 H2O are placed in front to
balance the equation. One is never
placed there, it is just understood.
Writing and Balancing Equations
5. The physical state of a substance is
indicated by the following symbols:
(s) solid
(l) liquid (g) gaseous (aq)
aqueous: means water solution
yields
reversible
H2
gas evolved
reaction
solid precipitate
Writing and Balancing Equations
 Balancing
1.
Equations
First you must have the equation in
symbol format, you cannot
balance a
word equation.
2.
Balancing:
a. Count and compare the number of
atoms of each element on each
side of the equation and determine
those that must be balanced.
Writing and Balancing Equations

Balancing Equations
b. Balance each element one at a time,
by placing whole numbers (coefficients)
in front of the formulas containing the
unbalanced element.
It is usually better to balance in this order:
metals, nonmetals, hydrogen, oxygen.
Writing and Balancing Equations

A coefficient placed before a formula
multiplies every atom in the formula by that
number!
2H2SO4 =
4H’s
2S’s
8O’s
c. re-check each time you balance an
element to see if anything else has
become unbalanced. Make
adjustments as needed.
Writing and Balancing Equations
d. Do a final check making sure that
each element is balanced and that
the smallest possible set of whole
number coefficients has been used:
4HgO
2HgO
4Hg + 2O2
2Hg + O2
incorrect
correct
Writing and Balancing Equations

Equations for practice:
Magnesium + oxygen
Magnesium Oxide
Potassium Chlorate
Potassium Choride +
Oxygen
Aluminum hydroxide + sulfuric acid
Aluminum sulfate + water
C4H10 + O2
CO2 + H2O
Writing and Balancing Equations

Types of Chemical Reactions
1. Combination or Synthesis Reaction:
Two reactants combine to give one
product.
A+B
2Mg + O2
AB
2MgO
Writing and Balancing Equations

Types of Chemical Reactions
2. Decomposition:
A single substance is decomposed or
broken down to give two or more
different substances:
AB
2PbO2
2 Na HCO3
A + B
2PbO + O2
Na2CO3 + H2O + CO2
Writing and Balancing Equations

Types of Chemical Reactions
3. Single - Displacement Reaction:
One element reacts with a compound
to take the place of one of the elements
of that compound.
A + BC
B + AC
Zn + 2HCl
H2 + ZnCl2
Note: Activity series table helps to predict
which substances (elements) will be able to
replace various other substances.
Writing and Balancing Equations

Types of Chemical Reactions
4. Double - Displacement or Metathesis:
Two compounds exchange partners
with each other to produce two different
compounds.
AB + CD
NaCl + KNO3
AD + CB
NaNo3 + KCl
Writing and Balancing Equations

Heat in Chemical Reactions
Energy changes always accompany
chemical reactions. One reason why
reactions occur is that the products attain
a lower, more stable energy state than the
reactants.
Writing and Balancing Equations

Heat in Chemical Reactions
Reactions are either exothermic or
endothermic.
Exothermic reactions liberate heat:
H2 + Cl2
2HCl + 185kJ (exothermic)
Endothermic reactions absorb heat:
N2 + O2 + 181kJ
2NO (endothermic)