Transcript Balancing Equations

Balancing Equations
CHEM
Chemical equations tell you the
following
 The substances that react together.
 The substances that are formed.
 The amounts of each substance involved.
 The arrow  is read as "yields".
HCl + NaOH  NaCl + H2O
The arrow always points from
reactants to products

Most chemical reactions are written with the arrow pointing
from left to right like this .

There are special situations that call for the arrow to be
written pointing from right to left like this. Since the arrow
always points from reactants to products, the reactants
would be on the right if the arrow is pointing this way.

There are chemical reactions that begin to form products
and the products break down into the original reactants.
These reactions are known as "reversible reactions" and
are indicated with arrows pointing in both directions like
this.
Balanced Equations:
The equation below is "balanced".
S8 + 12O2  8SO3
 Balanced equations have the same
number of each type of atom on both
sides of the arrow.
Equations must be balanced
because:
Atoms can be neither created nor destroyed
by ordinary chemical means, so there must
be the same number of atoms on both sides
of the equation.
What law is this?
Why do you think this law is so important to
chemistry?
These numbers are found in a
chemical equation
 Subscripts
The small numbers to the lower right of chemical symbols.
Subscripts represent the number of atoms of each element in the
molecule.
 Coefficients
 The large numbers in front of chemical formulas. Coefficients
represent the number of molecules of the substance in the
reaction.

 Just as subscripts of 1 are never written, coefficients of 1 are not
written either. Both are "understood".
 The balanced equation S8 + 12O2  8SO3 tells us that one
molecule of S8 reacts with twelve molecules of O2 to produce eight
molecules of SO3.
Using coefficients and subscripts to count
atoms in equations:
 Multiply the coefficient in front of the chemical formula by the subscript
after the atom.
number of atoms = coefficient X subscript
Example:
How many atoms of hydrogen and oxygen are represented in
2H2O?
•# of H atoms = coefficient 2 X subscript 2 = 4
•# of O atoms = coefficient 2 X subscript 1 = 2
Using coefficients and subscripts to
count atoms in equations cont:
 Atoms found inside parenthesis in a formula have two subscripts. The
subscript to the right of the parenthesis goes to all atoms inside.
# of atoms = coefficient X subscript inside ( ) X subscript outside (
)
 Example:
How many of each type of atom are represented by: 2Al2(SO4)3
 # of Al atoms = 2 X 2 = 4
 # of S atoms = 2 X 1 X 3 = 6
 # of O atoms = 2 X 4 X 3 = 24
7 steps to balance equations by
inspection
The order in which these steps are performed is
important. While there are some shortcuts
that can be used, following these steps in the
order given below is the best way to be sure
your equation is correct.
1. Check for Diatomic Molecules - H2 - N2 O2 - F2 - Cl2 - Br2 - I2

If these elements appear by themselves in an
equation,
they must be written with a subscript of 2
Balance equations by changing coefficients, never
by changing subscripts in formulas.
2. Balance Metals
3. Balance Nonmetals
4. Balance Oxygen
5. Balance Hydrogen
6. Recount All Atoms
If the atoms are not balanced at this point, there
is a problem somewhere. Work your way
back up the steps until you find the problem,
and correct it.
7. If every coefficient will reduce, rewrite in the
simplest whole-number ratio.
An equation is not properly balanced if it is not
written in its lowest whole-number ratio.
Use the 7 steps to write the balanced equation for each of
the following:

NaOH Na2O + H2O


Fe + O Fe2O3
CO2 + H2O C6H12O6 + O2

FeS + HCl FeCl2 + H2S

O + H H2O

Cl + NaI NaCl + I

Al(NO3)3 + H2SO4 Al2(SO4)3 + HNO3
Check your Answers:

2 NaOH Na2O + H2O

4 Fe + 3O2 2 Fe2O3

6 CO2 + 6 H2O C6H12O6 + 6 O2

FeS + 2 HCl FeCl2 + H2S

O2 + 2 H2 2 H2O

Cl2 + 2 NaI 2 NaCl + I2

2 Al(NO3)3 + 3 H2SO4 Al2(SO4)3 + 6 HNO3