Chemical
Equations
A Chemist’s Shorthand way
of writing a Chemical Reaction.
SHS
©2005, Douglas Gilliland
Honors Physical Science
Sarasota High, Sarasota, FL
Chemical Reaction
A process in
which substances
undergo physical
and chemical
changes that
result in the
formation of new
substances with
different physical
and chemical
Fire is energy released in an
properties.
exothermic reaction
between substances in wood
and oxygen in the air.
Chemical Equation
A chemical
equation is a
shorthand way of
writing a chemical
reaction using
chemical symbols,
formulas and
coefficients.
Rusting is a chemical reaction in which
iron and
oxygen
oxide
coefficient
yieldsto yield iron(III)
chemicalcombine
chemical
formula
symbol
sign
2
Fe
O
4 Fe + 3 O2 →
2 3
diatomic
REACTANTS
element
PRODUCT
The Law of
Conservation of Mass
Matter cannot be created
or destroyed
in a chemical reaction.
Atoms are simply rearranged
to form new substances.
Atoms and ions on
the reactant side
=
Atoms and ions on
the product side
Coefficients
Coefficients are whole numbers
placed in front of reactants and
products. They are used to
balance an equation to show
matter is conserved.
Hydrogen
+
Oxygen
4 hydrogen atoms
2H2
+
Yields
Water
4 hydrogen atoms
O2
→2H 2O
2 oxygen atoms
2 oxygen atoms
Counting Atoms
How many atoms/ions are present?
3 Ba3(PO4)2
Ba= 9 P= 6 O= 24
4(NH4)2SO4
N= 8 H=32 S= 4 O=16
Steps in Writing
an Equation
1. Write the chemical symbols and or
formulas for the reactants and products.
2. Check for diatomic elements.
3. Balance the equation using coefficients.
Make sure atoms are equal on both sides.
aluminum + oxygen yields aluminum oxide
4 Al + 3 O2 → 2 Al 2O3
4 atoms of aluminum + 3 diatomic molecules of oxygen
yeilds 2 unit cells of aluminum oxide.
#1 reason why
students get an
equation wrong:
diatomic elements
H
N O F
When found as elements
Cl
(not in compounds), always use
a subscript 2 to show these that
Br
these elements are always
found in pairs.
I
Diatomic molecules
H H
O
N
O
N
single
covalent
bond
H H
O
H
H
double
covalent
bond
O
O
O
triple
covalent
bond
N N
N
N
Diatomic Elements
in Equations.
What is wrong with these equations?
2Na + 2Cl
✗ → 2NaCl
2Na + Cl2 → 2NaCl
4N + ✗
2O → 2N2O
✗
2N2 + O2 → 2N2O
Al + ✗
3F → AlF3
2Al + 3F2 → 2AlF3
Reducing Coefficients
You must simplify coefficients when there is a
coefficient in front of EVERY reactant & product
and they are ALL divisible by the same number.
Can these coefficients be reduced?
✗4K + 2F → 4KF
2K + F → 2KF
✓2Ba + O → 2BaO
✗4Fe O → 8Fe + 6O
2
2
2
2 3
2
2Fe2O3→ 4Fe + 3O2
Equations Review
SHS
A chemical equation is a
shorthand way of writing a
chemical reaction using
chemical symbols and
formulas.
Coefficients are used in
front of reactants and
products to balance the
equation to show that
matter is conserved.